State of the UK climate 2018

This report provides a summary of the UK weather and climate through the calendar year 2018, alongside the historical context for a number of essential climate variables. This is the fifth in a series of annual “State of the UK climate” publications and an update to the 2017 report (Kendon et al., 2018). It provides an accessible, authoritative and up‐to‐date assessment of UK climate trends, variations and extremes based on the most up to date observational datasets of climate quality.


INTRODUCTION
This report provides a summary of the UK weather and climate through the calendar year 2018, alongside the historical context for a number of essential climate variables. This is the fifth in a series of annual "State of the UK climate" publications and an update to the 2017 report . It provides an accessible, authoritative and up-to-date assessment of UK climate trends, variations and extremes based on the most up to date observational datasets of climate quality.
The majority of this report is based on observations of temperature, precipitation, sunshine and wind speed from the UK land weather station network as managed by the Met Office and a number of key partners and co-operating volunteers. The observations are carefully managed such that they conform to current best practice observational standards as defined by the World Meteorological Organization (WMO). The observations also pass through a range of quality assurance procedures at the Met Office before application for climate monitoring. In addition, time series of near-coast sea-surface temperature (SST) and sea-level rise are also presented. The process for generating national and regional statistics from these observations has been updated since . This report makes use of a new dataset, HadUK-Grid, which provides improved quality and traceability for these national statistics along with temperature and rainfall series that extend back into the 19th Century. Differences with previous data are described in the relevant sections and appendices.
The report presents summary statistics for year 2018 and the most recent decade (2009-2018) against 1961-1990 and 1981-2010 averages. Year 2009-2018 is a non-standard reference period, but it provides a 10-year "snapshot" of the most recent experience of the UK's climate and how that compares to historical records. This means differences between 2009 and 2018 and the baseline reference averages may reflect shorter-term decadal variations as well as long-term trends. These data are presented to show what has happened in recent years, not necessarily what is expected to happen in a changing climate.
This article is published with the permission of the Controller of HMSO and the Queen's Printer for Scotland.
The majority of maps in this report show year 2018 against the 1981-2010 baseline reference averaging period-that is, they are anomaly maps which show the spatial variation in this difference from average. Maps of actual values are in most cases not displayed because these are dominated by the underlying climatology, which for this report is of a lesser interest than the year-to-year variability. Throughout the report's text the terms "above normal" and "above average," etc.
refer to the 1981-2010 baseline reference averaging period unless otherwise stated.
Values quoted in tables throughout this report are rounded, but where the difference between two such values is quoted in the text (for example, comparing the most recent decade with 1981-2010), this difference is calculated from the original unrounded values.

EXECUTIVE SUMMARY
Land temperature • 2018 was the seventh warmest year for the UK in a series from 1884, and fourth warmest year for Central England in a series from 1659. • Summer 2018 was the equal-warmest summer for the UK in a series from 1884, and the warmest in the series for England. • All the top 10 warmest years for the UK in the series from 1884 have occurred since 2002. • The most recent decade (2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018) has been on average 0.3 C warmer than the 1981-2010 average and 0.9 C warmer than 1961-1990. • The Central England Temperature series provides evidence that the 21st century so far has overall been warmer than the previous three centuries.
Air and ground frost • The number of air frosts in 2018 was slightly below average for the year overall. The number of ground frosts was equal-11th lowest in a series from 1961. • The most recent decade (2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018) has had 5% fewer days of air frost and 9% fewer days of ground frost compared to the 1981-2010 average, and both 15% fewer compared to 1961-1990. Energy demand and growing conditions indices • Heating degree days in 2018 were below average, and cooling and growing degree days (GDDs) were each thirdhighest in series from 1960. • The most recent decade (2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018) has had 4% fewer heating degree days per year on average compared to 1981-2010 and 9% fewer compared to . • The most recent decade (2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018) has had 5% more GDDs per year on average compared to 1981-2010 and 15% more compared to 1961-1990. Near-coast SST • Year 2018 was the equal-11th warmest year for UK nearcoastal SST in a series from 1870. wetter than 1961-1990. UK winters have been 5% wetter than 1981-2010and 12% wetter than 1961-1990 Snow • From late February to early March the UK experienced the most significant spell of widespread snow since December 2010. • Year 2018 was a relatively snowy year in the context of the last two decades, but near average compared to the last 60-years. • Widespread and substantial snow events have occurred in 2018, 2013, 2010 and 2009, but their number and severity have generally declined since the 1960s. Sunshine • Year 2018 sunshine for the UK overall was 114% of the 1981-2010 average and the third sunniest year in a series from 1929. • The 3-month period May-July 2018 was the sunniest 3-month period for the UK on record. • The most recent decade (2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018) has had for the UK on average 4% more hours of bright sunshine than the 1981-2010 average and 7% more than the 1961-1990 average. • Winter and spring for the most recent decade (2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018) have had for the UK on average 3/9% more sunshine than the 1981-2010 average and 11/14% more than 1961-1990. Wind • Ten named storms affected the UK in year 2018 (including storms David and Emma, named by Meteo France and the Portuguese Met Service, respectively). The overall number and severity of these storms were not unusual compared to recent decades. • There are no compelling trends in storminess as determined by maximum gust speeds from the UK wind network over the last five decades.
Sea-level rise • The UK mean sea level index for 2018 was equal-highest (with 2015) in the series from 1901, although uncertainties in the series mean caution is needed when comparing individual years. • Mean sea level around the UK has risen by approximately 1.4 mm/year from the start of the 20th century, when excluding the effect of vertical land movement. • The 99th percentile water level (exceeded 1% of the time) at Newlyn, Cornwall for year 2018 was the second highest in the series from 1916. Year 2014 was highest in this series.
Significant weather • A spell of snow and low temperatures from late February to early March 2018 was the most significant severe winter weather to affect the UK since December 2010. • Summer 2018 was among the most warm, dry and sunny summers experienced by the UK for over 100 years. • Storm Callum on October 11-12, 2018 was one of the most notable extreme rainfall events across South Wales in the last 50 years.
1 | SYNOPTIC SITUATION Figure 1 shows seasonal mean sea-level pressure anomalies for the four seasons of 2018 relative to the 1981-2010 average, using the NCEP/NCAR reanalysis (Kalnay et al., 1996). This provides an indication of atmospheric circulation patterns for each season overall. Sea-level pressure was slightly lower than normal across the UK and central Europe in winter 2017/2018, and to a greater extent spring, whereas it was above normal for UK and northern Europe in summer and autumn. December 2017 and January 2018 saw a mix of weather types. In January, a changeable westerly weather type brought Atlantic low-pressure systems including four of the named storms of the year and the windiest days of the year. In February, high pressure over Scandinavia often extended its influence to the UK, with low pressure over the Mediterranean. The weather was relatively dry, cold and sunny, and a bitterly cold east wind established in late February extending into early March. During March, low pressure again tended to dominate the UK's weather bringing often cold, wet and dull conditions, particularly outside the north-west. By April, the low-pressure anomaly moved to the west of the UK. The weather remained often relatively dull and unsettled, but warmer with a more southerly flow and a brief hot spell from 18th to 22nd.
May, June and July were dominated by high pressure near or over the UK, bringing plenty of warm, dry, sunny weather, hot at times, with an airflow from the near continent. However, by August, the high pressure slipped to the south allowing low-pressure systems to influence more northerly areas with a cooler and more unsettled regime after the first week. In September and October, the UK was again influenced by high pressure, but with unsettled spells in mid-September (coinciding with storms Ali and Bronagh) and October (storm Callum). In November and to a lesser extent December, the low-pressure anomaly was to the west of the UK, with high pressure over Scandinavia in November and Iberia in December, bringing mostly milder but sometimes unsettled weather and a mainly south-east or south-westerly flow.  (Note here and throughout the report winter refers to the year in which January and February fall.) This index is a measure of the large-scale surface pressure gradient in the North Atlantic between Iceland and the Azores, which determines the strength of westerly winds across the Atlantic, and is thus the principal mode of spatial variability of atmospheric patterns in this region. When the pressure difference is large, the WNAO is positive and westerly winds dominate with stronger and more frequent storms. When the pressure difference is small, the WNAO is negative with an increased tendency for blocked weather patterns, reducing the influence of Atlantic weather systems.
The WNAO index for 2018 was near-zero, and the winter overall was fairly near average for both temperature and rainfall.
Overall, the WNAO index shows a large annual variability but also decadal variability with periods of mainly positive phase (e.g., 1910-1920s and 1990s) and negative phases (e.g., 1960s). Hanna et al. (2015) discusses recent changes in the NAO index and notes an increase in variability of WNAO within the last two decades. The most negative winter in the index is 2010 which was characterized by prolonged periods of blocked weather patterns and this was the UK's coldest winter since 1979. Figure 3 shows the summer North Atlantic Oscillation (SNAO) index from 1850 to 2018 inclusive. Similar to the WNAO index, this is a measure of large-scale climate variability in the North Atlantic based on the surface pressure gradient, but based on a more northerly location and smaller spatial scale than the winter counterpart, reflecting the more northerly location of the Atlantic storm track.
The UK's summer in 2018 was much warmer, drier and sunnier than average. The 2018 SNAO index was the highest since 1955 and fourth highest in the series. F I G U R E 4 Year 2018 annual average temperature anomalies ( C) relative to 1981-2010 average for mean, maximum and minimum temperature. Bulls-eye features present in the T min map are likely to be due to localized micro-climate features, such as frost hollow effects, at individual weather stations which the gridding process is unable to fully represent Other summers in the second half of the series with a high SNAO index include 2013SNAO index include , 2006SNAO index include , 1995SNAO index include , 1983SNAO index include , 1976SNAO index include , 1975SNAO index include and 1955, and these were all warm, dry and sunny summers for the UK overall. As with its winter counterpart, the SNAO shows periods of mainly positive phase (e.g., 1970-1990s) and negative phase (e.g., 1880s and 1890s), with Hanna et al. (2015) noting a striking recent decrease in SNAO since the 1990s, which includes the run of recent wet summers from 2007 to 2012. The summers of 2013 and 2018 are in marked contrast to this recent sequence.

| TEMPERATURE
The UK mean temperature (T mean ) for 2018 was 9.5 C, which is 0.6 C above the 1981-2010 long-term average, making this the seventh warmest year in the UK series from 1884. Year 2018 was ranked fourth warmest in the Central England temperature (CET) series from 1659. The annual mean temperature was around 0.5-1.0 C above normal across England and Wales, with anomalies exceeding 1.0 C for a few locations in England, whereas anomalies across Scotland and Northern Ireland were generally less than 0.5 C (Figure 4, Table 1).
The UK annual mean maximum temperature (T max ) for 2018 was 13.2 C, which is 0.8 C above average. The highest T max anomalies of over 1.0 C were across central southern England, whereas anomalies were lower across the northern half of the UK-particularly Northern Ireland and Scotland's Central Belt. The UK annual mean minimum temperature (T min ) for 2018 was 5.7 C, which is 0.4 C above average, with T min anomalies generally lower than T max anomalies across the UK, and near-zero across parts of Northern Ireland and northern Scotland ( Figure 4, Table 1). The UK seasonal T mean for winter 2018 (December 2017 to February 2018) was 3.5 C, which is 0.2 C below the 1981-2010 average and this was the coldest winter for the UK since 2013. For the UK overall, temperatures were nearnormal in December 2017 and January 2018, but 1.3 C below normal in February (Figures 5 and 6, Table 1). A spell of severe winter weather with easterly winds became established across the UK from late-February to early March, with temperatures as much as 10 C below average for the time of year (for more details see the significant weather events section).
The UK seasonal T mean for spring was 8.1 C, which is 0.3 C above the 1981-2010 average. Temperature anomalies were 1.7 C below normal in March and on the first Tredegar, Blaenau Gwent recorded a daily maximum temperature of −4.7 C, the lowest March maximum on record for the UK at F I G U R E 6 Year 2018 monthly average temperature anomalies ( C) relative to 1981-2010 average a low-level station. Even so, March 2018 was not as cold as March 2013 overall. In contrast, temperatures were 1.7 C above normal in May. It was the third-warmest May for the UK in a series from 1884 (although not quite as warm as the previous May of 2017) and for T max the warmest May in the series. Remarkably, both the early and late May bank holidays were dominated by dry, warm, settled weather across most of the UK. There was also a significant warm spell in mid-April; temperatures reached 25-27 C widely across central and eastern England on the 19th, with 29.1 C at London St James's Park the UK's highest April temperature since 1949 (see the significant weather events section).
The UK seasonal T mean for summer was 15.8 C, 1.4 C above normal, largely due to fine, warm weather in June and July. For the UK, this was the equal-warmest summer in a series from 1884, with 2006, and marginally warmer than the summers of 2003 and 1976, while England had its warmest summer in the series. Temperatures were widely more than 2.0 C above normal in June (for Wales the warmest June in the series from 1884) and more than 2.5 C above normal across central southern England in July. Temperatures exceeded 30 C across parts of Highland Scotland, North Wales and Northern Ireland on 28th June, and fairly widely across England on 15 days during July and early August, although T mean for August was near normal overall (for more details see the significant weather events section).
The UK seasonal T mean for autumn was 9.7 C, 0.3 C above normal, with anomalies near-normal in September and October but somewhat above in November. December 2018 was another mild month overall. In summary, for year 2018 overall, temperatures in February and March were well below average, with May, June, July and December well above ( Figure 6, Table 1). Figure 7 shows time series of annual T mean anomalies for the UK and countries from 1884 to 2018 inclusive, and Figure 8 the seasonal UK T mean anomaly series. There has been an increase in temperature from 1970s to 2000s with the most recent decade (2009-2018) being on average 0.9 C warmer than the 1961-1990 average and 0.3 C above 1981-2010. All the top 10 warmest years in the UK T mean series have occurred since 2002, and year 2018 is ranked seventh (Figure 7). Year 2018 was warmer than any year in the series from 1884 to 2002. In contrast, none of the top 10 coldest years in the UK T mean series have occurred this century. The coldest year this century (2010)  the smallest for winter at 0.6 C ( Figure 8). As with the annual series, the seasonal series show large inter-annual variability and some decadal variability, with an increase in temperature across all four seasons from 1970s to 2000s. Warming has been slightly greater for T max than T min (Figure 9) resulting in a small increase in the average daily temperature range but to levels similar to those observed prior to the mid 20th century.
The uncertainty in these statistics is principally a function of the number and distribution of stations in the observing network which varies through time. For monthly, seasonal and annual averages this uncertainty is less than 0.1 C and consequently much smaller than the year-to-year variability. For simplicity of presentation, all the temperature data are presented in the tables to the nearest 0.1 C. More information relating to the uncertainties and how they are estimated is provided in Annex 2. Figure 10 shows annual T mean for England from 1884 to 2018 and CET series from 1659. The series are highly correlated for the period of overlap (R 2 value 0.98) and have a root mean square difference of 0.1 C which is comparable to the estimated series uncertainty as described in Annex 2. The CET series could effectively be considered a proxy for an England series from 1659, although because these are different datasets produced in different ways, some differences are inevitable. The CET series provides evidence that the 21st century so far has overall been warmer than any period of equivalent length in the previous three centuries, and that all seasons are also warmer (Figures 10 and 11). When comparing the early 21st century (2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018) to previous centennial averages, the difference is typically 0.5-1.0 C compared to 1901-2000 and 0.5-1.5 C compared to 1801-1900 and 1701-1800 with the greatest difference in autumn and the least in summer (Table 2).

| Days of air and ground frost
The average number of days of air frost for the UK for 2018 was 54 days, which is 3 days below the 1981-2010 average. The number of air frosts was near average across much of the UK, but below normal across central and south-east England and above normal across parts of north-east Scotland. The number of days of ground frost for 2018 was 96 days, 14 days below the 1981-2010 average and equal-11th lowest in a series from 1961. Some locations recorded at least 30 fewer days of ground frost for the year overall compared to normal but with considerable spatial variation across the UK ( Figure 12). The number of air and ground frosts was well below average in January (except across Scotland), April, November and December but above in February, March and October. The annual numbers of days of air and ground frost for the UK overall for 2018 was close to or slightly below the average for the most recent decade, with the series showing a reduction through the 1980s and 1990s. The most F I G U R E 1 2 Days of air frost and days of ground frost anomaly for year 2018 relative to 1981-2010. See Annex 1 for definitions. Bulls-eye features in these and the T min maps are likely to be due to localized factors such as frost hollow effects at individual weather stations (present in the actual or long-term average grids) which the gridding process is unable to fully represent, particularly for ground frost  Figure 13). Annex 1 explains how these areal-series are calculated.

| Degree days
A degree day is an integration of temperature over time and is commonly used to relate temperature to particular impacts. It is typically estimated as the sum of degrees above or below a defined threshold each day over a fixed period of time. The standard degree days monitored by the Met Office are heating, cooling and GDDss which relate to the requirement for heating or cooling of buildings to maintain comfortable temperatures, or the conditions suitable for plant growth, respectively. These indices are useful metrics, but as they are derived from temperature only, users should be aware that other relevant factors such as solar gain, day length, wind and rain will also influence the actual responses of, for example, plant growth. The definitions and thresholds used are described in Annex 1.
Heating degree days (HDD) for 2018 were between 90 and 95% of average across most of England but near average across much of Wales, Scotland and Northern Ireland ( Figure 14). Averaged across the UK, HDD for 2018 were 95% of the 1981-2010 average. The lowest 10 HDD  Figure 15). For the UK, the most recent decade has had an annual average HDD 9% lower than 1961-1990 and 4% lower than 1981-2010. Nevertheless, recent years such as 2010 and 2013 demonstrate it is still possible for UK climate to experience well above average HDD values. The UK experienced a fine, warm summer, with daily maximum temperatures widely exceeding 28 C (at 20 or more observing stations) on 28 days from late June to early August across England. As a result, cooling degree days (CDD) were well above average, with the highest values across the east Midlands, East Anglia and Greater London (the latter due in part to the urban heat island effect). In these areas, there were between 60 and 80 CDD widely, compared with a 1981-2010 average of around 30-40 CDD in these areas, and in some locations more than 50 CDD above average ( Figure 16). The UK recorded 32 CDD compared to a 1981-2010 long-term average of 13 CDD, and all countries recorded between two and three times the long-term average.
The years with high CDD in the time-series across England and Wales are those when major summer heatwaves occurred. This sequence of years-1976, 1995, 2003 and 2006-now also includes 2018. For both the UK and England, CDD in 2018 were the third highest in the series F I G U R E 1 6 Cooling degree days for 2018, (left) actual and (right) anomaly. The anomaly is presented as a difference from, rather than percentage of, average. This is because CDD are close to zero over much of Highland Scotland behind 1995 and 1976, and for Wales fifth highest. The cooler climate of Scotland and Northern Ireland means that CDD are much lower, each with long-term averages of less than 5 CDD, but nevertheless CDD in 2018 were equalfourth highest on record for each country. Although there has been a general increase in CDD across England, significant peaks are dependent on when major heat-waves happen to occur. Prior to 2018, the most recent major summer heatwave for the UK affecting CDD was in July 2006, with a run of notably cool summers with low CDD from 2007 to 2012 ( Figure 17). CDD for the most recent decade are comparable with 1981-2010, largely because 2013 and 2018 are the only years with well above-average CDD in the period 2009-2018. GDD for 2018 were between 110 and 115% of average across most of the UK, but higher than this for some areas ( Figure 18). UK GDD overall were 112% of the 1981-2010 average, the third highest in a series from 1960, behind 2006 and 2014. GDD for 2018 were also above the average for the most recent decade across all regions.
The most recent decade has had an annual GDD 15% higher than 1961-1990 and 5% higher than 1981-2010, and the similar (downward) trend in HDD and (upward) trend in GDD from 1960 to date each reflect the underlying warming of the UK's climate ( Figure 19). The recent cold years of 2010 and 2013 still recorded GDD above the 1961-1990 average.

| Coastal waters
The annual mean SST for 2018 for near-coast waters around the UK was 11.8 C, 0.3 C above the 1981-2010 long-term average. This ranked equal-11th warmest in the series from 1870 and warmer than any year in the series before 1959 ( Figure 20). Near-coast SST data is highly correlated with the land observations (R 2 value 0.82, see Annex 2) with a root mean square difference of less than 0.3 C. Some differences between historical trends in these series are apparent, notably the 1960s and 1970s and the period pre-1900. However, these differences are also apparent in the Central England temperature series, also shown in Figure 20, which closely follows the UK series. Uncertainties in the SST dataset will generally be larger at smaller scales (such as UK near-coast) and can include uncertainty in the bias adjustments applied to minimize the effect of instrumentation changes. For UK near-coast SST, the most recent decade, 2009-2018, is 0.6 C warmer than the 1961-1990 average and 0.3 C above 1981-2010. Eight of the 10 warmest years in the series have occurred since 2002. Every year this century has fallen within the warmest third of the series.

| PRECIPITATION
The UK rainfall total for 2018 was 1,056 mm, 92% of the 1981-2010 average and the driest year for the UK since 2010. Most of the country was drier than averageespecially parts of north-east Scotland which recorded less than 75% of normal. In contrast, some coastal fringesparticularly west Wales and much of the south coast-had above average rainfall ( Figure 21).
The wettest and driest observed locations for the year generally reflected the long-term climatology. Several rain-gauges across the Lake District and West Highlands recorded over 3,000 mm (with the highest annual total 4,972 mm at Styhead, Cumbria)-although totals in these areas were more typically 2000-3000 mm. In contrast, the driest locations in parts of Essex, Suffolk Cambridgeshire and Moray recorded less than 500 mm-with a few locations less than 450 mm. Overall this represents a fairly typical range for the UK. Figures 22 and 23 and Table 3 show seasonal and monthly rainfall anomalies across the UK for 2018. Inevitably, as is always the case, the annual map conceals the detail behind significant monthly and seasonal variations which occurred in rainfall patterns over the course of the year.
The UK rainfall anomalies for winter (98%), spring (99%) and autumn (96%) were near-normal overall but with large spatial variation, while summer was relatively dry (72%). The winter rainfall pattern was variable but it was notably wet across parts of East Anglia and Northern Ireland, and dry across much of central and eastern Scotland. December 2017 was rather drier than average in some north-eastern areas, but central and south-east England, especially East Anglia, were rather wet (not shown, see . January 2018 was wet in some western areas, dry in Aberdeenshire, but totals were near-normal across much of the UK. February was relatively dry across all areas. Spring 2018 was relatively wet across much of England, Wales and eastern Scotland with more than 150% of average fairly widely-but in contrast dry across north-west England, Northern Ireland and western Scotland with less than 50% in a few locations. This north-west/south-east gradient is a relatively frequent feature of UK spring rainfall, with a similar pattern seen in spring 2012, and a reversed wet north-west/dry south-east gradient during springs 2011 and 2015. More than 200% of average rainfall fell in a swathe from Cornwall to Aberdeenshire during March 2018, while much of western Scotland received less than 50% of average. April was somewhat wetter than average across most of the UK, while May was relatively dry for most of the UK except central southern England where showers, thunderstorms and longer spells of rain broke out toward the end of the month in the wake of the hot weather. Summer 2018 was relatively dry across the UK (the driest summer since 1995). It was particualry dry across central England with less than 50% of normal rainfall widely-for England this was the eighth driest summer in a series from 1862-although not as dry as the other notable heatwave summers of 1995 and 1976. High pressure brought dry, settled conditions through much of May, June and July and some locations across southern England recorded a run of over 50 dry days from the end of May until late July. June was exceptionally dry across parts of central and southern England with less than 5 mm of rain widely-for southern England only June 1925 was drier in a series from 1862. July was also much drier than average until the last few days of the month-especially toward the south-east. August rainfall totals were broadly near-average across the UK overall.
The autumn rainfall pattern was variable but rather dry across central England. September was wetter than average across many western and northern areas but drier elsewhere; rainfall in October was near or slightly below average across many areas, especially the south coast of England; rainfall totals in November and December were above average in certain areas but below in others; northern Scotland was notably dry during both months.
Various flood events occurred during 2018. Some flooding was reported in south-east England, Wales and southern Scotland in early January, associated with heavy rainfall from storm Eleanor. Further localized flooding from heavy rain and snowmelt affected parts of west Wales, south-west England and southern Scotland late in the month. In mid-February, flooding affected roads in the south-west including the M5, causing two serious accidents. In mid-March, a low-pressure system to the west brought flooding to parts of the south-west, the West Midlands and Northern Ireland. There was further flooding in parts of Derbyshire and Yorkshire on first and second April due to heavy rain; the Foss Barrier in York was activated as levels rose on the River Ouse. Heavy rain also affected parts of Devon and north-east England on 9-10th April, while thunderstorms and heavy rain caused flash flooding in the Southampton area on 1-2ndand across southeast England on 2 9-30th. In early May, there was further localized flooding in Kent, Norfolk and south Wales.
Heavy showers and thunderstorms associated with a breakdown of the hot weather at the end of May caused further flooding problem, with the M5 shut in the Birmingham area and there was widespread flooding in Wales. Toward the end of the month, localized flooding affected parts of Kent and the Southampton area. In early June, further heavy rainfall caused widespread flooding problems across Scotland, Northern Ireland, Norfolk, Lincolnshire and Nottinghamshire.
Thunderstorms again caused surface water flooding problems in Kent on 5th July and more widely in the London area, parts of eastern England and North Wales around mid-month. Parts of Lincolnshire, Northern Ireland and Gloucestershire were affected by flooding late in the month. Torrential rain affected Merseyside, Lancashire and parts of south-east England in mid-August.
Storms Ali and Bronagh brought some heavy rain to Scotland, northern England and Wales in mid-September, and there was localized flooding in the West Highlands in early October. A very significant flood event for the year resulted from persistent rain from Storm Callum on 12th October, causing significant flooding problems across parts of south Wales and south-west England. The wettest area was south Wales, with much of the Brecon Beacons National Park recording 100-150 mm of rainfall over a 2-day period, and up to 200 mm across the higher ground (for more details see the significant weather events section). Further flooding affected Northern Ireland and south-west England in early November, and parts of Wales late in the month, and Storm Deirdre caused some flooding in Northern Ireland and south-west England in mid-December. However, in comparison with storms Ali, Bronagh and Callum, the remainder of the year was mainly relatively quiet. The precipitation data show large annual variability, with a slight increase from the 1970s onwards ( Figure 24). The most recent decade (2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018) has been on average 5% wetter than 1961-1990 although only 1% wetter than 1981-2010; this increase is most pronounced for Scotland being 8% wetter than . The wettest year for the UK overall is 1872 (127% of average) and the driest 1887 (72%). Year 2018 was ranked in the middle third of the UK series from 1862. Six of the 10 wettest years in the UK series from 1862 have occurred since 1998, including 2014, 2012 and 2008. Figure 25 shows seasonal rainfall series for the UK from 1862 to 2018 (for winter 1863-2018). The two recent winters of 2013-2014 and 2015-2016 stand out, each with over 150% of the 1981-2010 average UK rainfall overall. As with the annual series, the seasonal series typically show a large inter-annual variability about a relatively stable long-term mean, although with some decadal variability.
The annual rainfall total for 2018 in the long running England and Wales precipitation (EWP) series was 912 mm (Figure 26), which is 96% of the 1981-2010 average, making this an unremarkable year in the EWP series overall. Figure 26 shows there are some notable decadal fluctuations in the series such as a wet period through the 1870s, and the "Long Drought" from 1890-1910 (Marsh et al., 2007) highlighting the value of rainfall series before the 20th Century for understanding the full historical context of UK rainfall. The most recent decade is a relatively wet decade in this series, being 2% wetter than 1981-2010 and 5% wetter than . The England and Wales areal rainfall series based on 1 km resolution gridded data is highly correlated to EWP for the period of overlap, with an R 2 value of 0.98 and root mean square difference of 1.9%. Minor differences between the series are inevitable due to the more limited sampling of stations used for the EWP series and the gridding method used for the England and Wales areal series. Figure 27 shows trends in seasonal EWP rainfall amounts from 1766 to date. While there is little change in the longterm mean for the annual EWP series, this is certainly not the case for the seasonal series. EWP shows a marked increase in winter rainfall (winter 2014 is the wettest winter in this series and 2016 ranked eighth wettest). Before 1900, EWP winter rainfall was substantially lower than autumn rainfall, but the increase in winter rainfall has meant that during the 20th century autumn and winter rainfall were roughly equal on average. However, there are potential issues with the estimation of early winter rainfall in the series relating to the treatment of snow before systematic meteorological observing networks were established which could be associated with an underestimation of early winter rainfall (Murphy et al., submitted).
The increasing winter rainfall has been offset by a slightly smaller reduction in summer rainfall, although a run of recent wet summers from 2007 to 2012 demonstrates that these trends are very sensitive to the choice of start and end dates, and summer rainfall trends in the 18th and early 19th Century are also subject to some uncertainty and possibly over estimated (Murphy et al., 2019). Spring/autumn rainfall have each remained fairly steady with only a slight increase/decrease, respectively. The rainfall statistics throughout are presented to the nearest whole mm, but the uncertainties of the areal statistics relating to changes in the observing network over time can approach 1-4% depending on region in early decades, but less than 1 or 2% for the comprehensive network of rain gauges in the years since 1960. The uncertainties are therefore much smaller than the year to year variability and more detail on this can be found in Annex 2. However, it is non-trivial to determine the robustness or significance of observed trends in rainfall as they are quite sensitive to region, season and choice of start and end dates.

| Days of rain and rainfall intensity
The number of days of rain greater than or equal to 1 mm (RR1) during 2018 was mostly within 5 days of the longterm average, with a variable spatial pattern across the UK, but notably below normal across northern Scotland ( Figure 28). In general, the monthly variation was comparable to the rainfall anomaly pattern ( Figure 23), with fewer days of rain than average in May, June, July and October but more in January and March-although with some regional variation. The number of days of rain greater than or equal to 10 mm (RR10) was near-normal across much of the UK, but below normal across upland areas of the west and north-and again particularly western and northern Scotland ( Figure 28), with February, March, November and December all having much fewer than normal RR10 in this F I G U R E 2 8 Days of rain ≥1 mm (RR1) and 10 mm (RR10) for 2018, difference from 1981-2010 average F I G U R E 2 7 Seasonal rainfall trends for EWP series in mm, 1766-2018 (note winter from 1767). The figure shows a smoothing trend for each series using a weighted filter (see Annex 2) area-these being climatologically among the wettest months of the year. Figure 29 shows the annual area-average RR1 and RR10 for the UK for 1961-2018. Overall, 2018 was below the 1981-2010 average for both RR1 and RR10. For both RR1 and RR10 the year with the most days was 2000, and this year was also the third-wettest in the UK series from 1862, with only 1872 and 1903 wetter. While the RR1 series is broadly flat, the RR10 series shows a slight increase from around 31 days for 1961-1990 to 34 days for the period 1981-2010, an increase of around 10%. This suggests an increase in the number of days of widespread heavy rain across the UK in the last few decades, although caution is needed because both time-series are relatively short and with large annual variability. Figure 30 shows an estimate of the areal-average rainfall intensity (see Annex 1 for definition) across the UK for each year from 1961 to 2018. The figure is indicative of trends in rainfall intensity across the UK on wet days although, as with RR1 and RR10, it neither provides a seasonal breakdown, nor distinguishes between upland and lowland areas, Overall, 2018 was also below the 1981-2010 average for this metric, and consistent with RR1 and RR10 also being below average. The 2 years with highest rainfall intensity in the series (2000 and 2012) also correspond to the wettest years in the UK series since 1961. There is a slight upward increase of 0.2-0.3 mm (approximately 3-5%), but again this is a short time-series dominated by year-to-year variability. The rainfall intensity series is well correlated with the RR10 series (R 2 value 0.70), as would be expected because in years with a large number of very wet days the average rainfall intensity on wet days is higher.

| Heavy rainfall
Alternative metrics for heavy rain are presented here. Heavy rainfall is a complex variable to monitor due to its potential to be highly localized. These metrics adopt two different methods: a percentile approach and an absolute threshold. The ranking of individual years is quite sensitive to the choice of definition used and the series are relatively short given the variability of rainfall. However, there are some consistent features across these different metrics-most notably, more heavy rain events have been recorded in the most recent decade than in earlier decades in the series.
The 95th and 99th percentiles of UK daily areal-average rainfall based on the 50-year period 1961-2010 inclusive are 9.4 and 13.9 mm, respectively. Figure 31 plots the number of days each year in the series from 1961 to 2018 when this percentile was exceeded (by definition we would expect on average 18 days and 3-4 days, respectively). As with rainfall intensity, this neither includes a seasonal break-down, nor does it distinguish between orographically enhanced frontal rain and convective rain. The climatologically wetter parts of the UK in the north and west will tend to have a greater influence on this metric than the drier parts in the south and east, but rainfall would need to be fairly widespread across the UK to exceed these percentiles, so this metric gives some indication of trends in widespread heavy rain events.
Fifteen days in 2018 exceeded the fifth percentile, and 4 days exceeded the ninth percentile. The 4 days were 20 September (storm Bronagh), 12 and 13 October (storm Callum) and 15 December (storm Deirdre)-see the wind section of this report. They, therefore, corresponded to heavy and widespread frontal rainfall events associated with major autumn and winter Atlantic storm systems affecting the UK. Both series show large annual variability, with some decadal variability and a slight rising trend from around 17/3 days for   (although with a subsequent small reduction for the most recent years)-that is again some evidence for a slight increase in widespread heavy rain events. Figure 32 provides a count of the number of times each year any rain gauge in the observing network below 500 m elevation has recorded a daily rainfall total greater than or equal to 50 mm. We refer to this type of metric as a count of stationdays. This metric cannot distinguish between a small number of widespread events recorded at many stations, or more frequent but localized events, but is a useful gauge of the occurrence of extreme heavy rainfall overall. This series has been adjusted to take into account the changing size of the UK raingauge network which reached over 5,000 gauges in the 1970s and has reduced to fewer than 3,000 in the 2010s (Appendix Figure A1.2a). The dense network of several thousands rain gauges across the UK means that widespread heavy rain events will tend to be well captured, although highly localized convective events may still be missed. The adjustment is made by applying a scaling factor to the station-day counts for each year, so that earlier years are scaled down and later years scaled up and the apparent number of stations in the network remains constant throughout. However, note that this adjustment does not take into account the fact that the relative proportion of rain-gauges within different parts of the UK also changes with time. Therefore we cannot rule out the possibility that the present day network while having fewer stations overall may provide better sampling of regions that experience higher frequency of heavy rain days such as western Scotland.

| Snow
The UK experienced a spell of wintry weather in mid-January 2018, although not exceptional for the time of year. Snow depths from 16th to 22nd exceeded 20 cm across parts of Highland Scotland and the east of Northern Ireland, with up to 40 cm across southern Scotland and 53 cm at Leadhills, Lanarkshire on 20th and 21st. There were numerous road traffic accidents including two fatalities in Northern Ireland, and schools were closed across Northern Ireland, northern England and Scotland. Severe transport disruption affected Scotland on the 16-17th, with a 7-mile traffic jam on the M74 due to jack-knifed lorries. Further disruption affected the east of England and the Midlands on the 21st from snow as frontal systems pushed into the colder air from the south-west; East Midlands airport was closed for a time. There was further disruption from snow across western and central parts of England, North Wales, Northern Ireland, the Highlands and Scotland's Central Belt in mid-February.
The most significant snow event of the year occurred from late February to early March. High pressure dominating Scandinavia and Northern Europe blocked milder Atlantic weather, with a bitterly cold easterly airflow affecting the UK. Initially, eastern England and eastern Scotland were affected by snow showers from the North Sea, with convergence lines running through the English Channel and other areas such as eastern Scotland. There was further heavy snow across southern and south-west England and South Wales on first March as storm Emma in the Bay of Biscay pushed mild, moist air north across the UK. Strong winds caused significant drifting of lying snow and this lay un-melted for several   This event caused major disruption to road, rail and air travel. In the worst affected areas, numerous major roads were impassable or experienced severe delays, there were large numbers of road traffic accidents and in several cases vehicles stranded overnight. Hundreds of trains and flights were cancelled or delayed widely across the UK. Thousands of schools were closed for several days, while tens of thousands of homes lost electricity supplies. There were reported to be at least 10 weather-related deaths, and in early March Chinook helicopters were used to drop supplies to some isolated communities in east Cumbria.
A brief return of cold easterly winds on 17-18th March led to further snow, with southern coastal counties and southwest England particularly affected by significant lying snow. There was further widespread transport disruption (with flights cancelled at Heathrow, Gatwick and London City Airports) although overall the extent and severity of this snow event was less than at the start of the month. Northern parts of the UK were affected by snowfalls at the start of April, and there was further freezing rain, snow and ice across the north of the UK in mid-December, associated with storm Deirdre.
The late February to early March 2018 event saw significant and widespread lying snow across lowland areas of the UK. The last time this occurred in the UK was in January and March 2013 and before that there were major snow episodes in January and December 2010. Year 2010 was the snowiest year by far for the UK in the last two decades, and was comparable to several snowy years in the 1970s and 1980s. Figure 33 shows the count of station-days where snow depth sensors recorded greater than or equal to 10 or 20 cm of lying snow. The series has not been adjusted for network size, consequently, it is indicative but not homogeneous. Year 2018 was a relatively snowy year in the context of the last two decades, but not as snowy overall as 2013, 2010 and 2009 and would be regarded as near-average for the first half of the series (with the 2018 network size roughly half that of the 1960-1990s, see Annex 1).

| SUNSHINE
The UK sunshine total for 2018 was 1,560 hours, 114% of the 1981-2010 average. This was the third sunniest year for the UK in a series from 1929, with only 2003 and 1995 sunnier. Sunshine totals were above normal across the majority of the UK, with more than 20% above average across parts of eastern and northern England, and especially northern Scotland and the Northern Isles. Northern Scotland recorded its second-sunniest year on record. The highest sunshine anomalies were at Kinloss, Moray (134% of average), Shoeburyness, Essex (127%) and Lerwick, Shetland (126%) but the majority of stations recorded sunshine anomalies of between 110 and 120% ( Figure 34). Note however the possibility that imperfect exposure at individual stations and the relatively sparse density of stations are likely to have had some influence on the detail in the sunshine anomaly pattern. The UK's highest annual sunshine total was 2,171 hours at Shoeburyness, but several other stations in Dorset, East and West Sussex and Kent also recorded more than 1900 hours. The lowest total was at Loch Glascarnoch, Ross-shire with 1,177 hours (still well above average in this location).
Winter 2018 (December 2017 to February 2018) was sunny overall across most of the UK. December and January sunshine totals were near normal across the UK, but with significant regional variation. February was sunnier than average almost everywhere, with a sunshine total exceeding December and January combined. It was the second-sunniest February in the UK series from 1929.
March and April were relatively dull across much of the UK, except for some northern areas. In contrast, May was sunny across the UK-it was the second-sunniest May in the UK series from 1929. June and July were also each sixth-sunniest in the series, and the UK's 3-month sunshine F I G U R E 3 6 Sunshine anomalies (%) for months of 2018 26 total for May-July of 709 hours (7.8 hours per day and 134% of the 1981-2010 average) represented the sunniest 3-month period for the UK in the series from 1929 by a margin of 11 hours. August was rather dull in western areas but even so the UK's summer sunshine total was fifth highest in the series from 1929 and the sunniest summer since 1995.
Southern and eastern areas, particularly East Anglia, were relatively sunny through the autumn, while November was a sunny month across the far north-west of the UK. December 2018 was bright in eastern areas but very dull in parts of the south-west; Wales had its dullest December and calendar month on record with only 18 hours of bright sunshine (Figures 35 and 36). Figures 37 and 38 show annual sunshine anomalies for the UK and countries, and seasonal sunshine anomalies for the UK, from 1929 to 2018 inclusive. The smoothed trend shows a slight increase in sunshine from a low during the 1960-1980s to a sunnier period from 2000 onwards. The most recent decade (2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018) has had for the UK on average 7% more hours of bright sunshine than the 1961-1990 average and 4% more than the 1981-2010 average. This trend is apparent across all countries but is most prominent during the winter and spring, where the most recent decade is 11 and 14% higher than 1961-1990, respectively. Of note is the run of sunny springs for the UK since 2007 (with the exception of 2013 and 2014).
The sunshine network is relatively sparse, with the 2018 network comprising around 110 stations ( Figure A1.4). This means that some parts of the UK such as Highland Scotland and central Wales have few observations. Sunshine stations may be affected by exposure issues, particularly in the winter months when the sun is at a low elevation and topographic shading may be important. The sunshine statistics throughout are presented to the nearest whole hour, but the uncertainties of the areal statistics relating to changes in the observing network over time can approach 2%. More details can be found in Annex 2.

| WIND
The windiest days of 2018 are listed in by using a single authoritative system. This scheme was introduced in autumn 2015 with storms named if they had the potential to cause medium or high impacts from wind on the UK and/or Ireland. The naming system was subsequently adjusted to take into account other weather types, so storms could be named on the basis of impacts from wind but also include impacts of rain and snow. The change in convention means that the number of named storms from year-to-year should not be used as a climate index in its own right. Storm Eleanor brought strong winds across Northern Ireland and most of England and Wales on 2 to January 3, 2018. Winds were widely 50-60 Kt (58-69 mph) around exposed coastlines but some inland locations also recorded gusts well over 50 Kt, with 63 Kt (72 mph) at Northolt, Greater London-it is fairly unusual for winds as strong as this in the London area. Storm Fionn on 16 January was named by Met Eireann and brought strong winds across much of Ireland although impacts were limited in the UK.
Storm David was named by Meteo France and tracked rapidly eastward across the southern half of the UK overnight 17-18 January. The strongest gusts were in a swathe from North Wales, through the south Pennines to Lincolnshire and Norfolk, exceeding 60 Kt even in some inland locations. Capel Curig, Conwy recorded a gust of 81 Kt (93 mph), the UK's highest gust of the year at a lowlevel station, and Tibenham, Norfolk recorded a gust of 72 Kt (83 mph), the highest gust in East Anglia since October 27, 2002. The storm brought significant disruption with over 100,000 properties without power, reports of fallen trees and rail services affected. The storm continued to intensify as it moved in to the North Sea-causing further disruption across Holland and Germany (where it was named Friederike) ( Table 5).
Storm Georgina on 24 January was a fairly typical winter storm tracking across the north of Scotland. The strongest winds were near the centre of the low with a gust of 74 Kt (85 mph The first storms of the 2018/2019 season arrived in close succession in mid-September, marking an emphatic transition from the fine summer of 2018 to autumn. Storm Ali brought widespread gusts in excess of 60 Kt (69 mph) across the north of the UK on 19 September and this was one of the most notable storms at this time of year in recent decades. Two people died in County Armagh and County Galway (Irish Republic) and there was extensive travel disruption and power outages (for more details see the weather events section). Storm Bronagh on 20-21 September brought strong winds across southern England and Wales but impacts were mainly flood-related from heavy rain. Storm Callum on 12 October brought wind gusts of 50-60 Kt across exposed locations in the north and west of the UK but this storm was also much more significant for heavy rain across south Wales (for more details see the weather events section). Storm Deirdre brought strong winds to western and northern parts of the UK in mid-December but impacts were mainly related to freezing rain.
As a measure of storminess Figure 39 counts the number of days each year on which at least 20 stations recorded gusts exceeding 40/50/60 Kt (46/58/69 mph). Most winter storms have widespread effects, so this metric will reasonably capture fairly widespread strong wind events. The metric will consider large-scale storm systems rather than localized convective gusts. There are no compelling trends in max gust speeds recorded by the UK wind network in the last five decades, particularly bearing in mind the year-to-year and decadal variations and relatively short length of this time series.
The most significant storms of 2018 were David and Ali-in mid-January and mid-September, respectively. The latter was a very significant September storm with over 30 stations recording a maximum gust speed of 60 Kt (69 mph) or higher, but this was the only day of the year where at least 20 stations reached this threshold. Overall 2018 was an unexceptional year for storminess when compared to previous decades and does not stand out in terms of the 40, 50 or 60 Kt metrics. There were many windier years than 2018, particularly in the 1980s and 1990s.
Note that the 40 Kt counts as shown in Figure 35 broadly track the positive phase of the winter NAO as shown in Figure 2. This earlier period also included among the most severe storms experienced in the UK in the observational records including the "Burns Day Storm" of January 25, 1990, the "Boxing Day Storm" of December 26, 1998 and the "Great Storm" of October 16, 1987. In the last decade, the most significant major winter storms have been on December 5, 2013, January 3, 2012 and December 8, 2011, and none of the storms of 2018 compared with these for overall severity across the UK.
Changes in instrument type, station network size, station exposure and choice of metric used mean that interpreting trends in storminess from UK wind speed data is not straightforward due to the limitations of available data, and results should be treated with caution. The wind network on which Figure 39 Figure 39 has not been adjusted to take into account this changing network but this may partly account for the higher station counts in 40 Kt gusts through the 1980s and 1990s.

| SEA LEVEL
A UK sea level index (Figure 40) for the period since 1901 provides a best-estimate trend of 1.4 ± 0.2 mm/year for sea level rise, when excluding the effect of vertical land movement (Woodworth et al., 2009). This is close to the estimate of 1.7 ± 0.2 mm/year estimated for the global sea-level rise suggested by the Fifth Assessment Report of Intergovernmental Panel on Climate Change (Church et al., 2013). However, UK sea level change is not a simple linear increase, but also includes variations on annual and decadal timescales. A number of large-scale atmospheric and ocean processes contribute to non-uniform sea-level rise around the coast of the UK. The UK sea level index for 2018 was the equal-highest on record (with 2015) in this series. However, the 2018 value is based on one station only (Newlyn) due to missing data for the other stations at various times during the year. The error bars indicate uncertainty (1 SD) in values for individual years; the method for calculating uncertainties does not currently taken into account missing stations. Unfortunately, the uncertainties in the UK sea level index for several recent years, notably 2007, 2011 and 2015, are large; it is suspected that these relate to data quality issues at the Liverpool gauge. Years 2010 and 2018 values are based on only one gauge so error bars are not available. Figure 41 presents a 100-year record of sea level at Newlyn, Cornwall showing time-series of the annual 99th percentile water level and annual maximum water levels, relative to the long-term mean for the 99th percentile. The 99th percentile is the level which is exceeded 1% of the time, or for about 88 hours in any given year. Any periods of high tides and storm surges in the year are likely to be in the 88 hours above the 99th percentile. The annual maximum water level shows greater annual variability than the 99th percentile series. Consequently, the 99th percentile timeseries is sometimes preferred because it provides a description of change in high and low water characteristics without the greater year-to-year variability inherent in the true extremes.
The 99th percentile water level at Newlyn for year 2018 was the second highest in the series, with year 2014 highest. The highest maximum water level during 2018 was seventh highest. The long-term trends in 99 percentile level and highest maximum water levels are both 2.1 mm/year for the period 1916-2018. Table 6 shows the UK weather extremes for year 2018. The highest temperature of the year, 35.6 C at Felsham, Suffolk on 27th July was the UK's highest temperature since 36.7 C at Heathrow Airport on July 1, 2015. Temperatures also reached 35.3 C on 26th July. In the last 50 years, 35 C has been reached in 2018, 2015, 2006, 2003, 1995, 1990 and 1976  , in mm. The blue and red time-series are annual 99 percentiles and maximum water levels respectively. Levels are relative to the long-term average for the 99 percentile, computed for the whole period 1916-2018. The linear trend-lines for the 99th percentile and maximum water levels each have gradients of 2.1 mm/year 30 start of the spell of severe winter weather; temperatures fell widely below −10 C across Kent and Surrey. It is comparatively unusual for the UK's lowest minimum to occur outside Scotland, and this was the only day of the year where more than 10 stations fell below −10 C. During this spell, the strong easterly wind substantially reduced the normal diurnal temperature variation, with the relatively mild influence of the North Sea preventing overnight temperatures falling further. The greatest snow depth of 57 cm at Little Rissington, Gloucestershire on fourth March also occurred during this spell but is likely to have been significantly influenced by drifting.

| EXTREMES FOR YEAR 2018
The highest daily rainfall total of 183.4 mm occurred during storm Callum and this was a major rainfall event across South Wales. This total is only 28 mm below the Wales daily rainfall record. The highest gust speed of 81 Kt at Capel Curig occurred during storm David and is fairly typical for the UK. This station, although not at a particularly high elevation, is often the windiest in the UK, being located in an exposed position in a west-east orientated valley in Snowdonia where local topography is likely to have a significant influence on gust speeds.

| SIGNIFICANT WEATHER EVENTS OF 2018
This section describes notable weather events which occurred during 2018. The choice of event is determined by the National Climate Information Centre based on our experience of monitoring the UK's climate through the year, broadly taking into account a combination of spatial extent, severity and duration and any associated impacts. It does not represent a comprehensive list of all impactful weather affecting the UK during the year. A discussion of notable and named storms for 2018 is also included in the wind section of this report.

| Snow and low temperatures late February to early March
The UK experienced a spell of severe winter weather with very low temperatures and significant snowfalls from late February to early March 2018. In terms of weather impacts, this was the most significant weather event of the year. There was severe travel disruption with roads closed, numerous road traffic collisions and cars were stranded overnight on many roads in both Scotland and England, for example, the A31 in Hampshire and M80 in Scotland. Rail series were cancelled and air transport was severely disrupted, for example, Glasgow airport closed on 28 February. Thousands of schools across England, Wales and Scotland were closed, and many areas suffered power cuts. Isolated communities and farms across the North Pennines received supplies by helicopter. Figure 42 shows analysis charts for 28 February and March 2, 2018. A large area of high pressure dominated Scandinavia and northern Europe with an easterly airflow drawing bitterly cold air originating from Finland, northwest Russia and the Barents Sea, extending across much of Europe and well out into the Atlantic. The squeeze in the isobars provides an indication of the strength of the easterly wind, leading to a wind-chill at times widely below −10 C

T A B L E 6
Annual extremes for the UK for year 2018, excluding stations above 500 m above mean sea level (masl) even at low-level. This situation was associated with a stratospheric warming event over the previous few days, blocking the Jet Stream and milder air associated with Atlantic weather systems. A discussion of the contributing factors which may have led up to this event, including the sudden stratospheric warming, is given in Greening and Hodgson, 2019. Figure 43 shows rain-radar images during this event, with the majority of this precipitation falling as snow. Initially, the main areas affected by snow were across eastern England and eastern Scotland, from persistent snow showers as the easterly airflow picked up moisture from the North Sea. These snow showers tended form in convergence lines resulting in some locations receiving relatively little snow whereas others nearby very large amounts. Examples of these convergence lines run through the English Channel and other parts of the UK such as eastern Scotland; due to persistent heavy snow a Met Office Red Warning was issued for Scotland's Central Belt for 28 February (Figure 43a).
During the afternoon of 1 March, a low-pressure system in the Bay of Biscay (named as Storm Emma by the Portuguese Met Service) pushed north, bringing mild, moist air into the colder air resulting in further heavy snow, particularly across southern and south-west England and south Wales; the Met Office issued a second Red Warning for this event (Figure 43b). Subsequently as this milder, moist air moved north, much of the UK received further snow. Behind this weather system southern areas also experienced freezing rain (on contact with cold surfaces). Figure 44 shows daily maximum temperatures from 27 February to March 3, 2018 relative to the 1981-2010 February and March long-term averages. On 28 February, maximum temperatures were typically below −2 C across central and eastern England and eastern Scotland, and below −4 C across upland areas in the north. Sheltered locations in the west and south remained nearer or above freezing, but all areas were exposed to the strong east wind, typically gusting at 20-30 Kt (23-35 mph); such conditions resulting in a "feels like" temperature of around −10 C. Even St Mary's Airport, Isles of Scilly, recorded a daily maximimum of only 0.8 C. These temperatures were widely between 8 and 10 C below the February average.
On March 1, 2018 (the first day of meteorological spring), daily maximum temperatures again remained widely below freezing. Parts of the west Midlands and south Wales recorded daily maximum temperatures more than 12 C below the March average (around −2 C compared to a March average of around 10 C). In these areas, some new records were set: −4.7 C at Tredegar, Blaenau Gwent was the lowest March daily maximum temperature for the UK and Wales; −3.7 C at Pennerley, Shropshire and Little Rissington, Gloucestershire was also the lowest March daily maximum temperature for England. A large number of longrunning weather stations recorded their lowest March daily maximum temperature on record. Figure 45 shows hourly air temperature and hourly maximum gust speed for Heathrow, Greater London, and Edinburgh, Gogarbank from 22 February to 8 March showing temperatures hovering around freezing or falling to around −5 C from 26 February to 3 March. The wind speeds picked up notably during this period, particularly on 1 and 2 March, contributing to the significant wind-chill. For example, on the morning of March 1, 2018 the air temperature at Heathrow was around −4 C with wind gusting at around 25 Kt, and by the morning of March 2, 2018 the temperature was around −1 C with wind gusting at around 40 Kt. The strong easterly wind suppressed the normal diurnal temperature cycle.
Conditions were particularly severe across upland areas. On 28 February, Cairngorm Summit recorded an air temperature of around −14 C with a wind speed gusting at around 50 Kt. By 1st March, the temperature had risen slightly to around −10 C and wind gusting at up to 87 Kt (100 mph). The wind chill temperature approached −30 C and this arguably represented some of the most severe winter weather conditions experienced anywhere in the UK in the last 25 years (Kendon and Diggins, 2019). Figure 46 shows snow depths at 09:00 UTC on March 2, 2018, with peak snow depths for this event. While almost all areas of the UK experienced some lying snow, the greatest depths were across upland areas of northern and eastern England and eastern Scotland, and subsequently to parts of south-west England and south Wales. Across northern England and southern Scotland depths were recorded of over 40 cm (49 cm at Drumalbin, Lanarkshire, 46 cm at Glasgow, Bishopton and 41 cm at Spadeadam, Cumbria), but depths were more typically 10-20 cm. Southern England and South Wales also received significant snow with 49 cm at St Athan, South Glamorgan, 25 cm at Hereford and 16 cm at Yeovilton (Somerset). Note the St Athan value is likely to have been significantly influenced by drifting. Figure 47 shows time-series of snow depth measurements at these stations. Across northern areas snow accumulated steadily from 27 February to 2 March with depths subsequently reducing steadily but slowly due to the continuing low temperatures; by 7 March (a week after the event) significant lying snow was still present in many locations. The snow event across south-west England and South Wales occurred mainly on the afternoon and evening of 1 March. Strong winds resulted in significant drifting; for example, the A39 in North Devon was blocked due to wind effects on hedges at the side of the road while adjacent fields were stripped bare.
The last time the UK experienced a flow of bitterly cold air from Siberia, with significant snowfalls, was in March 2013, but the 2018 event was more severe in terms of extent, duration and severity and this was the most significant spell of severe winter weather since December 2010. In terms of atmospheric patterns, an analysis of the arealextent and duration of the "cold pool" based on the NCEP reanalysis suggests that this was the second most severe cold pool event affecting the UK in the last 70 years, with only January 1987 more severe (Graham and Webb, 2019). It also demonstrated once again the potential for severe winter weather to still affect the UK, even in the context of a warming climate. In 2010, there were two spells of snow and freezing temperatures from late-November to early December, and from mid-December to Christmas. December 2010 was the coldest December for the UK in a series from 1884 and, remarkably, the secondcoldest December for Central England in a series from 1659. Snow depths accumulated to 50 cm or more across upland areas of northern and eastern England and eastern Scotland with the temperature falling below −20 C in the far north (Prior and Kendon, 2011).
From 17 to March 19, 2018 the UK experienced a further spell of snow and low temperatures as easterly winds briefly returned. This was not as severe as the earlier spell but nevertheless still represented a significant event with parts of the south-west (for example Exeter) recording the second period of significant lying snow in just over 2 weeks. See also the cover image of this report.

| Exceptional warmth, April
The UK experienced some unusually high temperatures for the time of year from 18 to 22 April, with high pressure over the near continent drawing very warm air from the south (Figure 48). The hottest day was 19 April with temperatures across south-east England widely above 26 C and locally above 28 C -in places more than 14 C above the long-term average for the time of year (Figure 49). St James's Park (London) recorded a maximum temperature of 29.1 C making this the UK's warmest April day since 1949. A large number of long-running stations recorded their highest April on record (Table 7).
On April 19, 2018 Kenley Airfield (Greater London) recorded a daily minimum temperature of 15.9 C, a new April record for the UK, breaking the previous record of 15.2 C at Inverailort, Highland on April 15, 2007 and at Eastbourne, East Sussex on 23 April 2011. The spell of hot weather was in marked contrast to much colder conditions earlier in the month; daily maximum temperatures on April 12, 2018 across much of central England reached only 6 to 7 C. This difference of almost 20 C in little over a week nicely illustrates the highly variable nature of the UK's spring weather.

| Warm, dry and sunny summer
Summer 2018 brought plenty of warm, dry, sunny weather with the UK often under the influence of high pressure, particularly during June and July. This was the UK's warmest summer since 2006, and the driest and sunniest since 1995. Any short, unsettled spells brought rainfall mostly of a showery and sometimes thundery nature. Temperatures were often well above average with 30 C exceeded fairly widely on 15 days during July and early August. The summer mean The hatched lines are periods of no data but have been included to provide an indication of the timing and overall rates of accumulation at these stations. These stations have been selected as showing the greatest snow depths and are likely to be significantly influenced by drifting, the large spatial variation in depths across the UK means that a 'representative' snow depth cannot realistically be defined temperature was around 2.0 C above average across much of England and Wales, largely as a result of warmth in June and July, whereas temperatures for August overall were nearer the long-term average (Figures 5 and 6). Most of England and Wales recorded less than 75% of average rainfall, with large areas of central England less than 50% (Figure 22). Summer sunshine totals were widely 125% of average or more across the UK (Figure 35).
While for the UK, this was the equal-warmest summer for the UK in a series from 1884, England had its warmest summer in the series ( Figure 50) and it was also the equal-  fourth warmest summer in the Central England Temperature (CET) series from 1659, with summer 1976 warmest in this series.
As is a feature of all the warmest summers in the UK series, mean maximum temperature anomalies relative to the long-term average were generally much higher than mean minimum temperature anomalies, as a result of generally dry air, low humidity and cloud-free skies resulting in a large diurnal cycle with long sunny days and an absence of cloudcover at night. The UK mean maximum and minimum temperature anomalies for summer 2018 were + 2.3 C and + 0.8 C, respectively, but nevertheless in both cases this was the third-warmest summer in the series, for maxtemp behind 1976 and 1995 and for mintemp behind 2003 and 2006. The highest maxtemp anomalies were across central England, but in general significantly lower than summer 1976. The highest mintemp anomalies were also mainly across the southern half of the UK (Figures 51 and 52). Figure 53 shows a count of the number of stations across the UK and England on each day during the summer with a daily maximum temperature exceeding 25 C. This shows a prolonged spell of widespread warmth from late-June to early August, with only brief interludes of cooler conditions around 11th, 17th and 29th July. The hottest weather was mostly focussed across England (particularly south-east England and East Anglia), but extended at times to northwestern parts of the UK, particularly in late June and early July.
The hottest day across north-western parts of the UK was 28 June, with 30 C exceeded across north-west England, Scotland's Central Belt, and parts of the Highlands, Northern Ireland and North Wales. In some locations, maximum temperatures were more than 14 C above the monthly average, and most of the UK except coastal areas of Yorkshire, Lincolnshire and Norfolk exceeded 25 C. Maximum temperatures on 28 June of 31.9 C at Glasgow Bishopton and 30.5 C at three stations in Northern Ireland fell within 1 C of the all-time Scotland and Northern Ireland records, while in Wales 33.0 C at Porthmadog (Gwynedd) on 28 June was part of a sequence of 6 out of 7 days more than 30 C at this station. Numerous stations recorded their hottest June day on record including Armagh-152 years, Balmoral (Aberdeenshire)-104 years and Eskdalemuir (Dumfriesshire)-also 104 years ( Figure 54).
The UK's hottest day of the summer was on 27 July, with 35.6 C recorded at Felsham (Suffolk); 32 C was exceeded June was exceptionally dry across parts of southern England with a run of over 50 dry days at some stations in the south-east lasting until late July. June rainfall totals across parts of central and southern England were widely less than 5 mm, and some areas (for example, parts of Hampshire, London and Cambridgeshire) less than 1 mmthat is, essentially no appreciable rain. The June rainfall map below shows large parts of southern England recording less than 5% of the June average rainfall, with some places less than 2% (Figure 55). Southern England recorded its driest June since 1925 (Figure 56).
In contrast to the summers of 2006 and 2003, there were no extended spells of exceptonal heat during summer 2018 (despite the fact that temperatures reached 35 C on 2 days). Nevertheless, while summer 1976 will remain as the benchmark summer for the UK in terms of exceptional and prolonged hot and dry conditions, the summer of 2018 stands out as among the most notable fine, warm, dry, sunny summers in 100+ years. In terms of global-scale atmospheric circulation patterns, the warm, dry summer, associated with high pressure established near the UK during June and July cannot be viewed in isolation, but may have been linked with other severe weather events across the northern hemisphere, both heatwaves and extreme rainfall (Kornhuber et al., 2019).

| Strong winds from storm Ali
Storm Ali brought very strong winds and heavy rain to Scotland and Northern Ireland on September 19, 2018, with widespread gusts in excess of 60 Kt (69 mph) across the north of the UK. There were extensive power outages and travel disruption in Northern Ireland and Scotland, with rail cancellations due to fallen trees on the line. Some damage to buildings and vehicles was reported alongside many fallen trees across the northwest of the UK. Five hundred cruise passengers and crew were stranded in Greenock after severe weather broke their ship's mooring lines. Deep areas of low pressure are to be expected at this time of year. However, the most powerful Atlantic storms tend to be later in autumn-more typically October than September. Figure 59 provides a count of the number of stations in the wind network which recorded gusts ≥60 Kt for the UK in either September or October. Thirty stations across the UK, and 15 in Scotland (around 20% and over 40% of the network in these areas, respectively) reached 60 Kt. This was therefore one of the most notable storms to affect the UK in recent decades, particularly in September in the context of the area affected by gusts exceeding this threshold. One of the most notable autumn storms of recent decades was October 27, 2002 (mainly affecting the south of the UK) while ex-hurricane Ophelia on October 16, 2017 also brought some very strong winds to western parts of the UK and Ireland . However, other storms, while less extensive, were much more severe-for example, October 16, 1987.

| Heavy rain across South Wales from storm Callum
Storm Callum brought persistent heavy rain across western upland areas of the UK on October 11-12,, 2018. The wettest area was South Wales, with much of the Brecon Beacons National Park recording 100-150 mm of rainfall over a 2-day period, and up to 200 mm across the higher ground. The frontal system also led to a dramatic showing storm Callum centred to the north-west of the UK with associated fronts trailing across northern England and Wales temperature contrast of more than 10 C and unseasonably high temperatures across parts of eastern England. Figure 60 shows the surface pressure chart and Figure 61 a rain-radar image from storm Callum. See also the cover image of this report.
Storm Callum brought widespread travel disruption from strong winds and flooding. Large waves battered exposed coastlines in the south and west. Power cuts affected thousands of homes and there were flight cancellations, travel disruption on roads and rail cancellations due to landslips. The most severe impacts were across south Wales. Homes and businesses were flooded in Carmarthenshire, Ceredigion and Powys. Around 100 sheep were swept away by floodwater in west Wales. Rail services were delayed or cancelled across Wales, south-west England and between Preston and Scotland.
There was very significant orographic enhancement of the rainfall across South Wales, so that upland areas recorded 3 or 4 times as much rainfall as coastal locations around Swansea, or locations to the north-east in the rainshadow-such as the Black Mountains ( Figure 62). This very pronounced contrast in the spatial pattern of rainfall is very typical of such extreme rainfall events, for example, November 19, 2009 or December 5, 2015. The extreme nature of the rainfall was due to duration rather than intensity. For example, at Libanus, Powys, the rain-rate averaged around 6 mm per hour for 34 hours (Figure 63), with a total of 202.4 mm over this duration, 115% of the October wholemonth average rainfall (and October being one of the wettest months of the year on average).
Historically, events with 2-day rainfall accumulations across upland areas of Wales widely 100-150 mm or more have occurred in 1994, 1981, 1980, 1979, 1973, 1965, 1964 and 1963. Of these, the stand-out event in terms of rainfall totals across south Wales was that of 17 to December 18, 1965 with 10 raingauges recording a 2-day total of over 200 mm and two of these over 250 mm. Figure 64 compares rainfall totals of 11 to October 12, 2018 against 17 to December 18, 1965. In terms of historical context, 11 to October 12, 2018 was undoubtedly one of the most notable ANNEX 1: DATASETS

NAO index
The Winter North Atlantic Oscillation (WNAO) index is traditionally defined as the normalized pressure difference between Iceland and the Azores. This represents the principal mode of spatial variability of atmospheric pressure patterns in the North Atlantic. The WNAO index presented in this report is an extended version of this index based on a series maintained by the University of East Anglia Climatic Research Unit, using data from stations in south-west Iceland and Gibraltar (Jones et al., 1997). These two sites are located close to the centres of action that comprise the WNAO. Data from these stations have been used to create homogeneous pressure series at the two locations which extend back to 1821. The WNAO index in this report is based on these data and presented back to 1850, with winter defined as December to February, to provide consistency with winter statistics presented elsewhere in this report. For the UK, a positive WNAO index tends to be associated with higher temperatures and higher rainfall (R 2 values of 0.55 for winter mean temperature and 0.31 for winter rainfall based on years 1885-2018 and 1863-2018, respectively, see Annex 2). This means that just over half of the annual variability for UK winter mean temperature and almost a third for rainfall may be associated with the WNAO. Importantly, however, it also implies that the WNAO is unable to fully explain the variability of UK winters because the complexity of weather types and associated temperature and rainfall patterns through the season cannot be fully accounted for by this single index. This is because other modes of spatial variability in atmospheric pressure patterns also affect the UK's weather. For example, the East Atlantic (EA) and Scandinavian (SCA) patterns-the second and third modes of spatial variability represented in their positive phases by low pressure to the west of Ireland and high pressure over Scandinavia respectively-also exert an influence (Hall and Hanna, 2018). The influence of WNAO may also differ regionally across the UK, for example, for rainfall across the north-west compared to the south-east, which overall UK rainfall statistics will tend to smooth out (West et al., 2018).
The centres of action that define the summer NAO (SNAO, Folland et al., 2009) correspond to grid-point pairs 60 N, 5 E and 80 N, 50 W-corresponding to locations to the east of the Shetland Islands and in north-west Greenland, respectively. These locations reflect the smaller spatial scale and a more northerly location of the Atlantic storm track (Folland et al., 2009). Unfortunately, due to the location of these points a station-based SNAO series cannot be used. Instead, the SNAO index was calculated from the Met Office Hadley Centre's sea-level pressure dataset, HadSLP2 (Allan and Ansell, 2006) as the difference in seasonal mean sealevel pressure between these grid-point pairs for each year from 1850 to 2018 inclusive. Summer is defined as June, July and August to provide consistency with summer statistics presented elsewhere in the report. Note this SNAO definition differs from Folland et al., 2009 which uses July and August only. For the UK, a positive SNAO tends to be associated with higher temperatures and lower rainfall (R 2 values of 0.30 for summer mean temperature and 0.45 for summer rainfall based on years 1884-2018 and 1862-2018, respectively).
HadSLP2 is a global dataset of monthly mean sea-level pressure on a 5 latitude-longitude grid from 1850 to date (Allan and Ansell, 2006). The dataset is derived from a combination of marine observations from ICOADS (International Comprehensive Ocean-Atmosphere Data Set) and land (terrestrial and island) observations from over 2000 stations around the globe. The dataset has a step change in variance in the mid-2000s, with an increased variance after this relating to when real-time updates from the NCEP reanalysis fields started.

Monthly, daily and annual grids
The principal sources of data in this report are monthly and daily gridded datasets covering the UK, released as HadUK-Grid for the Met Office UK Climate Projections (UKCP18) in November 2018 (Hollis et al., 2019). The primary purpose of these data is to facilitate monitoring of UK climate and research into climate change, impacts and adaptation.
All gridded data are at 1 km resolution. The method used for gridding follows that used for the previous set of UK gridded datasets released as part of the Met Office UK Climate Projections (UKCP09) in 2009 Hollis, 2005b andPerry et al., 2009) which were at 5 km resolution. The 5 km gridded dataset was used for the previous State of the UK Climate 2017 report , earlier State of UK Climate reports, and UK climate statistics published on the Met Office website, but these are now superseded by the HadUK-Grid dataset. The grids are based on the GB national grid, extended to cover Northern Ireland and the Isle of Man, but excluding the Channel Islands. Table A1.1 shows the monthly and daily grids from HadUK-Grid used for this report, including the year from which variables are available. Derived annual grids are also included. A key improvement in the dataset is the improved resolution (1 km rather than 5 km). This increases the level of detail, and allows the grids to better represent smallerscale local features of the UK's climate-although inevitably this will always be constrained by the limitations of the underlying station network data and so micro-climate effects may not necessarily be improved. The higher resolution can also be used to better explore uncertainties in the gridding methods, and facilitates re-gridding to new high-resolution climate model resolutions.
In addition, the dataset makes use of new data sources, allowing the grids to extend significantly further back in time. The principal source of data is the Met Office Integrated Data Archive System Land and Marine Surface Stations (MIDAS) Database containing UK station data, but this has been supplemented by further recently digitized data from multiple sources including British Rainfall and Met Office Monthly and Daily Weather Reports; in total several million additional daily and monthly observations of rainfall and temperature from up to 190 stations for the period 1862-1958. The additional historical data has allowed monthly temperature to extend back to 1884 (previously T A B L E A 1 . 1 List of monthly and daily variables presented in this report, gridded over the UK at 1 km resolution. The table also includes monthly and annual grids derived from daily grids  previously 1958). The extension of these gridded datasets back to the late 19th century provides an invaluable longer-term context for the interpretation of the time-series and variability of the UK's climate. A further advantage of the new dataset is that the data extraction and gridding process has been carried out in a single batch process, whereas the previous 5 km dataset was processed in a series of batches covering 1961-2000, 1914-1960, 1910-1913 and subsequent monthly updates from 2001 to 2018. Generating the entire dataset in a single process through a managed code base in a consistent manner eliminates the possibility of inhomogeneities being inadvertently introduced by changes in the processing chain over a period of several decades, potentially introducing nonclimatic changes to the resulting dataset.
The 1 km resolution daily maxtemp, mintemp and rainfall grids of the UK have also been generated using a similar method to that for the 5 km grids (Perry et al., 2009). Daily temperature has been gridded back to 1960, covering the same period as previously, with daily rainfall back to 1891 as described above. With daily data, there is often a weaker link between the data and the geographical factors which shape the average over a longer time-scale. Metrics in this report based on the daily rainfall grids are only presented from 1961, even though these grids extend back to 1891. This is because of the step-change in station network density in 1961. The smaller number of stations before this date means that further work is needed to determine the extent to which any trends in metrics in earlier years are influenced by the relatively low station network density.
One difference from the previous 5 km gridded datasets is that several of the monthly climate variables (days of air frost, days of rain > = 1 mm and days of rain > = 10 mm) have been derived from the daily grids (daily mintemp and daily rainfall respectively) rather than gridded from monthly station values directly. This approach has the advantage of ensuring that these monthly variables are consistent with the daily grids on which they are based (which would otherwise not be the case). Because the gridding is at a daily timescale, we also anticipate that there will be a better overall representation of spatial variation in these monthly derived variables, although this is subject to ongoing research. Annual degreeday and rainfall intensity grids have also been derived from daily temperature and daily rainfall grids, respectively.
The network used for gridding for each variable changes each month. A key aim of the gridding process is to remove the impact of these changes in the distribution of stations on the climate monitoring statistics. This could be overcome by only using stations with a complete record, but the sparseness of such stations would introduce much greater uncertainty due to the spatial interpolation required. Instead, all stations believed to have a good record in any month are used, and every effort made to compensate for missing stations during the gridding process reducing uncertainty by maximizing the number of observations used. A description of the gridding process is also given in Jenkins et al. (2008) and Prior and Perry (2014).
Figures A1.1 to A1.3 show the number of stations used for creating monthly and daily grids for each of the variables. For monthly temperature, the number of stations varies from fewer than 100 for the period 1884 to 1900, increasing to between 200 and 400 from the 1910s to 1950s and reaching a peak of over 500 stations from the 1960s to 1990s, followed by a subsequent decline to below 400 stations. The number of stations recording monthly days of ground frost (i.e., with a grass minimum thermometer) is typically around 100 fewer than air temperature from the 1960s onwards. The number of monthly sunshine stations rises from around 200 to almost 400 from the 1930s to 1970, followed by a steady decline to around 100 stations in the 2010s ( Figure A1.1). The number of stations for rainfall shows a fairly steady increase from fewer than 100 in the 1860s to over 800 in the late 1950s ( Figure A1.2a), followed by a step-change to over 4,000 stations from 1961 and a peak of over 5,500 stations in the mid-1970s, with a subsequent steady decline to fewer than 3,000 in the 2010s ( Figure A1.2b).  Figure A1.1 F I G U R E A 1 . 4 State of the UK observing network in 2018. The number of observations is indicative as these may vary on a daily basis due to data availability As would be expected the number of stations for daily temperature over the period 1960-2018 matches that for monthly temperature (Figure A1.3). However, the number of daily rainfall stations is significantly fewer than for monthly rainfall. This may be partly accounted for where some rainfall data has only been digitized at monthly timescales (for example from the British Rainfall publications), and in addition due to the presence of some monthly rain-gauges in the network, principally across upland areas of the UK ( Figure A1.2).
Overall, Figures A1.1-A1.3 also emphasize the scope for further data recovery / digitization work, since the increase in station numbers in 1961 reflects an increase in digitized data since this date, but not an increase in the underlying observation network; many records held in paper archives are yet to be recovered.
The approximate total number of station values used to generate the grids for each variable is given in Table A1.2. In total over 100 million station values have been used to generate the HadUKGrid_v1, with more than 90% of these accounted for by daily temperature and daily rainfall. Note however that for monthly variables (for example monthly maxtemp), the majority of the monthly station values will have themselves been derived from daily station values (daily maxtemp). So in practice the number of station values used to generate the grids will differ from the number of station observations extracted from the MIDAS database or the other recently digitized data sources. Figure A1.4 shows the state of the UK's observing network in 2018. The networks are designed and maintained to achieve a good spatial coverage with stations representative of all areas of the UK. Due to the high spatial variation in rainfall, the network is much denser than for other variables, but even so highly localized events may still be missed. While the majority of the UK is reasonably well covered, some areas, notably western Scotland, are more data-sparse than others, but these also tend to correspond to areas with a smaller population. Coverage for some variables (notably sunshine) may considerably reduce if data for an individual station is missing, and where surrounding stations struggle to cover the gapthere is limited redundancy in the network. Overall however, even though the number of stations in the 2010s may be fewer than in earlier decades (for example the 1970s), the spatial distribution of station is more even, and so there is an improvement in the overall network's ability to capture the spatial characteristics of climate variables over that day, month, season or year

Long-term average grids
Areal averages for the WMO standard 30-year climatological reference periods 1961-1990 and 1981-2010 presented in this report have been calculated from long-term average monthly gridded datasets at 1 km resolution covering the UK. These gridded datasets were produced for HadUK-Grid following the same general method as the previous longterm average gridded datasets used previously (Perry and Hollis, 2005a) but modified as described in more detail by Hollis et al. (2019). The process for producing these grids is outlined as follows: For the majority of variables, long-term averages for each station are calculated from monthly station data. Gaps in individual months at stations are filled with estimates obtained via regression relationships with a number of well-correlated neighbours, and long-term averages are then calculated for each site. Gridded datasets of longterm averages are created by regression against latitude, longitude, elevation, terrain shape, proximity to coast and urban extent, followed by inverse-distance weighted interpolation of residuals from the regressions. The estimation of missing values allows a dense network of stations to be used, and this along with the range of independent variables used in the regression, allows detailed and accurate long-term average datasets to be produced. These are then used to constrain the gridded analyses for individual years, seasons, months and days via the geographical interpolation of deviations from, or ratios of, the long-term average. However, this method does not work well for a number of variables, including days of air frost and ground frost, and an alternative approach is used. Here, the gridded long-term average datasets are obtained by averaging the monthly grids (Hollis et al., 2019).
Because the long-term average grids are obtained by gridding long-term average station data directly ('average then grid') rather than calculated from the monthly grids ('grid then average') the long-term averages are not exactly consistent with the monthly analyses. This is both because the order of the calculation differs, and because the station network will be very much denser for the long-term average grids than the monthly grids due to the infilling process used when calculating station long-term averages. Table A1.3 shows the approximate number of stations used to generate the HadUK-Grid long-term average grids for the period 1981-2010, compared to the average number of stations per monthly grid over the same period. For monthly maxtemp, rainfall and sunshine there are typically two to three times more stations used for the long-term average grids than for the monthly grids for these variables. Table A1.4 compares 1981-2010 long-term average annual mean temperature and rainfall as derived from 1 km long-term average grids, and from the 360 individual monthly 1 km grids. For temperature, the difference of 0.02 C for the UK overall is much smaller than the difference of 0.6 C between 1961-1990 and 1981-2010 1 km long-term averages. For rainfall, the difference of 0.5% is also much smaller than the difference of 5% between the 1961-1990 and 1981-2010 1 km long-term averages. These "order of operation" differences are generally much smaller than those seen when comparing 1 km long-term averages and those derived from 5 km monthly grids presented in previous State of UK Climate reports (see  ,  Table A1.3), largely because for the HadUK-Grid dataset the long-term average and individual monthly grids are at the same resolution (both 1 km). Overall this therefore represents a marked improvement in self-consistency of the dataset.

Annual degree days
Degree-day datasets were generated from the daily temperature grids, as indicated in Table A1.1, using formulae given in Table A1.5a and A1.5b. The daily mean temperature T mean is calculated from the daily maximum temperature T max and the daily minimum temperature Tmin as (T max + T min )/2. The degree-day value is estimated differently depending on which of T max , T mean or Tmin are above (for Cooling Degree Days and Growing Degree Days) or below (for Heating Degree Days) the defined threshold.

Consistency and quality control
Quality control of station observations held in the Met Office Integrated Data Archive System (MIDAS) database is the responsibility of the Met Office Observations Quality Management (OBQM) team. This team runs a suite of both automated and manual quality control checks on MIDAS, which is the source of the majority of the station data used in HadUK-Grid. The other digitized data sources have also had quality checks at time of digitization where possible.
T A B L E A 1 . 4 Comparison of 1981-2010 long-term average annual mean temperature and rainfall as derived from 1 km long-term average grids and individual monthly grids. For example, tables of monthly rainfall published in British Rainfall also include annual totals, so the latter can be used as a closure check on the monthly totals. The previous 5 km UK gridded datasets have been manually quality controlled since 2001, and the HadUK-Grid processing chain included a step to ensure these QC decisions were transferred by comparing the HadUK-Grid 1 km resolution grids and the associated 5 km resolution grids. Further details are beyond the scope of this report. The dataset has adopted new and open-source ancillary files for terrain elevation, proximity to coast and urban land use that are used within the interpolation scheme. This provides a more traceable dataset. The HadUK-Grid dataset is also version controlledincluding a version-controlled numbering system, so this and future State of the UK Climate publications can all be linked to a specific version of the dataset. This report uses version 1.0.1.0 of the HadUK-Grid dataset. Details of the HadUK-Grid dataset are provided in Hollis et al. (2019).

Areal series
The monthly series for the UK and countries are calculated as area-averages derived from the 1 km monthly gridded datasets. Each monthly value is an average of all the individual 1 km grid point values which fall within the UK or country. The seasonal and annual series in turn are calculated from the monthly areal series. This approach enables a single statistic to be produced for each area (UK or country) from each grid, despite the fact that the UK's climate has a very high degree of spatial variation (for example with elevation). These statistics are self-consistent through time. In the same way, long-term averages are calculated as an average of all the individual 1 km long-term average grid points which fall within the UK or country. Daily area-averages have similarly been calculated from the 1 km daily gridded datasets.
Statistics for the UK and countries are useful for monitoring annual variability, trends and extremes but inevitably may mask considerable spatial varation across the area as shown in the anomaly maps.

Central England Temperature
The Central England Temperature (CET) monthly series, beginning in 1659, is the longest continuous temperature record in the world (Manley, 1974). It comprises the mean of three observing stations covering a roughly triangular area of England from Bristol to London to Lancashire; the current stations used for this series are Pershore College (Worcestershire), Rothamsted (Hertfordshire) and Stonyhurst (Lancashire) although the stations used in this series have changed in the past. A CET daily series is also available from 1,772 (Parker et al., 1992).
Following each station change the data are adjusted to ensure consistency with the historical series by analysing periods of overlap between stations, and since 1960 the data have been adjusted to allow for any effects of warming due to the expansion of local built up areas. Parker and Horton (2005) and Parker (2010) have investigated uncertainties in the CET series.

Sea-surface temperature data
The Met Office Hadley Centre's sea ice and SST data set, HadISST1 is a global dataset of monthly SST and sea ice concentration on a 1 latitude-longitude grid from 1870 to date (Rayner et al., 2003). The dataset is derived from a combination of fixed and drifting buoys, ship bucket and engine room intake thermometers and hull sensors; and satellite data. The UK near-coast SST series in this report comprises the average of all 1 latitude-longitude grid cells adjacent to the coast of Great Britain (approximately 50 grid cells). These grid cells were selected to ensure that all the main UK landmass fell within this area ( Figure A1.5).

England and wales precipitation series
The England and Wales precipitation series (EWP) has monthly data back to 1,766, and is the longest instrumental series of this kind in the world. The daily EWP series begins in 1931. The series incorporates a selection of long-running rainfall stations to provide a homogeneity-adjusted series of areal-averaged precipitation. EWP totals are based on daily weighted totals from a network of stations within each of five England and Wales regions.
The extent to which seasonal trends apparent in the EWP series are influenced by homogeneity issues (for example: the number of stations used historically to compile the EWP F I G U R E A 1 . 5 1 Grid cells from HadISST1 used to calculate UK near-coast sea-surface temperature series, how well the network has historically captured orographically enhanced rainfall across high ground, how well the network has historically captured precipitation which has fallen as snow) remains an area of investigationsee for example Murphy et al (submitted). Various papers detail the development of the EWP series (Wigley, 1984, Alexander and Jones, 2001, Simpson and Jones, 2012 Rain gauge and snow depth data Daily rainfall data presented in this report are 0900-0900 UTC totals from either daily or tipping-bucket rain-gauges registered with the Met Office. The rain-gauge network has diminished from over 4,000 rain-gauges across the UK in the 1960s to between 2,500 and 3,000 in the 2010s. The gauges are owned and maintained by several organizations: the Met Office, the Environment Agency, Natural Resources Wales, SEPA and Northern Ireland Water. The spatial distribution of the network has changed with time but nevertheless, the high network density ensures that all but the most localized convective events are captured at a daily time-scale. Snow depth data are recorded at 0900 UTC. These are either spot observations from automatic snow depth sensors or manual observations of representative level depth in a location free from drifting or scour by wind; ideally the average of three measurements would be recorded. The network comprised over 400 stations from 1960 to 2000 but has subsequently reduced to just under 200 stations in 2018.

Sunshine data
The UK's sunshine network in 2018 comprises two instrument types: just under half the network comprises Campbell-Stokes (CS) sunshine recorders which are read manually; the remainder comprising Kipp & Zonen CSD-1 (KZ) automatic sunshine recorders. An upward adjustment of KZ totals is made to give a monthly 'CS equivalent sunshine'. This ensures that the full sunshine network (automatic and manual) is used while maintaining consistency between the two instrument types. Legg (2014) and references therein provide further details.

Sea level data
Sea-level changes around the British Isles are monitored by the UK national network of tide gauges; for 2018 this network comprises 44 sites. For more than 100 years tide gauges provide measurements of sea-level change relative to the Earth's crust. However, tide gauges are attached to the land, which can move vertically thus creating an apparent sea level change. A UK sea level index for the period since 1901 computed from sea level data from five of these sites (Aberdeen, North Shields, Sheerness, Newlyn and Liverpool) provides the best estimate for UK sea level rise, excluding the effect of this vertical land movement. The records from each site are combined after removing the long-term trend from each to account for varying vertical land movement rates across the country. After aggregating the records, the calculated country-wide average rate of 1.4 mm/yr is reintroduced (Woodworth et al., 2009, Bradley et al., 2011. As mentioned in Woodworth (2009), the network of 44 sites falls under the responsibility of the Environment Agency (although it is no longer operated by the Proudman Oceanographic Laboratory), but only five date back to the beginning of the 20th century: the other sites did not begin until the 1950s. In creating the long-term index, we follow Woodworth's approach, which only uses data from the longterm series.
Woodworth (2009), which is based on data from up to 2006, notes that throughout the course of the record, at least three of the five stations are present for all years apart from three, the last of which was 1915. Unfortunately, from 2007 onward, there have been more gaps in observations for the five stations. More information about reasons for the issues can be found in the UK Coastal Monitoring and Forecasting annual reports, which are available from https://www.bodc. ac.uk/data/hosted_data_systems/sea_level/uk_tide_gauge_ network/reports/ Newlyn, Cornwall has a century of hourly (or, since 1993, 15-minute) sea-level data from float and pressure tide gauges that have been maintained better than most around the UK. It also has a more open ocean location than stations around North or Irish Sea coasts (Araujo and Pugh, 2008).

Time-series and trends shown in this report
The time-series in this report are plotted on either actual or anomaly scales. The plots with anomaly scales often show several different areas, seasons or variables which are offset for clarity and ease of comparison; the offsets do not reflect absolute differences between the time-series.
The time-series shown throughout are plotted showing the annual series and a smooth trend. This means that both annual variability and longer-term trends (removing this short-term variability) can be viewed simultaneously. Importantly, we note that for some series there may be few individual years that fall close to this long-term trend; and many or even most years may fall well above or well below. Most time-series plots also include the 1981-2010 and 1961-1990 long-term averages.
The smooth trend-lines are constructed using a weighted kernel filter of triangular shape, with 14 terms either side of each target point. The kernel defines how much weighting the terms either side of a point in the series have in estimating the smoothed average at that point; in this case the triangular shape using 14 data points either side means that data points further away have less influence. The effect is to smooth out the year-to-year variations and estimate any longer-term variations in the data. At the ends of the time series, only the 14 points to one side of the target point are used, increasing to the full 29-year bandwidth by the 15th point from each end. Similar smoothing filters were used for the earlier trend reports and State of UK Climate reports (Jenkins et al., 2008, Prior and Perry, 2014.
Climate records at individual stations may be influenced by a variety of non-climatic factors such as changes in station exposure, instrumentation and observing practices. Issues of changing instrumentation and observing practices will tend to be of greater importance early in the series, particularly before the 20th Century. In contrast, station exposure issues related to urbanization, which may for example affect temperature-related variables, may be of greater importance in the late part of the series from the mid-20th Century. Identifying and correcting for such factors in climate monitoring is referred to as homogenisation. Some homogenisation has been undertaken for some series presented in this report, such as the Central England Temperature record, and the adjustment of sunshine records described in Annex 1. For most variables however the individual station data in this report have not been explicitly homogenized to account for these non-climatic factors.
We note that the 1961-1990 and 1981-2010 averages presented are not exactly consistent with the average of the yearly data through the same period (see previous discussion on long-term averages), although in practice any differences are small. Annex 1 Table A1.4 provides further details. We use the 1 km resolution 1961-1990 and 1981-2010 averages because these datasets contain the most comprehensive set of stations, and thus represent our best estimate of these climatologies.

Uncertainty estimates
Recent studies have considered uncertainties in the gridded data and areal-averages (Legg, 2011, Legg, 2015, based on the previous 5 km gridded dataset. The HadUKGrid_v1 1km gridded dataset, while at a different resolution, uses the same method of interpolation, and a key source of uncertainty in both datasets is associated with spatial samplingi.e., the density of the observation network, which is the same in both cases. We therefore anticipate the uncertainty estimates for HadUK-Grid associated with spatial sampling to be similar to the 5 km gridded dataset. The uncertainty estimates in these studies have been adjusted upward to acknowledge other sources of error, for example, observational errors such as random errors in instrument readings, calibration errors or structural uncertainty (the latter implying that alternative methods of analysis may produce slightly different results). Legg (2015) published uncertainty ranges for areal-averages of monthly mean temperature, rainfall and sunshine; these increase in the past as the network density reduces. Table A2.1 lists 1σ uncertainty ranges for annual mean temperature, rainfall and sunshine for different periods in the 5 km gridded dataset. Indicative date periods are presented here. These correspond to: the earliest years in the 5 km dataset where the availability of station data is generally lowest and uncertainty highest; a period in the dataset around the 1960s which for rainfall corresponds to a step increase in availability of station data and corresponding decrease in uncertainty; and a recent period in the the dataset indicating current uncertainty. More comprehensive tables covering the full date range can be found in Legg (2015). We have applied a conservative reduction factor of √2 to convert monthly uncertainty ranges to annual. Uncertainty associated with individual months of the year cannot be considered independent but it is reasonable to assume that winter half-year biases are likely to be different in nature from summer half-year biases (Parker, 2010). Uncertainties in the CET and EWP series have also been investigated elsewhere (Parker and Horton, 2005, Parker, 2010, Simpson and Jones, 2012. The summary rainfall statistics for the UK and countries presented in this report are based on an areal average of the rainfall total in mm, rather than an areal average of the rainfall anomaly field as a percentage. This is judged to be the simpler and more readily comprehensible statistic for the majority of users and is directly proportional to the total volume of rainfall across the country. However, it means that climatologically wetter area of the UK have a greater influence on the overall UK summary statistic than the drier areas, rather than all equal-sized areas having equal influence (as would be the case using an areal average of the rainfall anomaly field). This introduces uncertainty because the rank of each year relative to the others may vary depending on which of these two metrics is chosen . It may also influence any trend in overall UK rainfall if this varies spatially between climatologically wetter and drier parts of the UK.
A further source of uncertainty in the rainfall data is introduced by measurement of precipitation which has fallen as snow. At manually read rain gauges the observer will measure precipitation equivalent of fresh snow fallen at 0900 UTC, whereas at automatic rain gauges any snow collected will be recorded when it subsequently melts; quality control of these data may then re-apportion this precipitation to previous days. However, inevitably snow measurement can be problematic, for example if wind eddies may carry snow over or blow it into or out of the gauge, in many situations estimation of precipitation from snow may be either underestimated or overestimated. However, this now tends to be usually less of a problem than during colder, snowier years of earlier decades.
Comparison of areal averages with 5 km gridded dataset Table A2.2 provides a summary of grid-point differences between the HadUK-Grid and 5 km gridded dataset. The net impact of changes based on mean and root-mean-square difference across all grid points in all grids for each variable are well within the expected uncertainty range of the interpolation and overall relatively small. Figure A2.1 compares the difference in area-average monthly mean temperature (a), and rainfall (b) from 1910 to 2017 inclusive (finalized 2018 data were not available at the time of comparison), between the HadUK-Grid and 5 km gridded datasets for the UK and Scotland. A full comparison between these datasets is beyond the scope of this report, but the figure provides an indication of the size of any differences and time-periods over which they occur. Scotland is included because it contains a large proportion of mountainous terrain, so any changes in topographic factors are likely to have a greater effect here than elsewhere in the UK.
The differences for UK monthly mean temperature are generally less than 0.1 C except for the period 1961-2000 where they are up to 0.2 C, and for Scotland occasionally over 0.3 C. The difference in rainfall for the UK is generally F I G U R E A 2 . 1 Difference in area-average monthly mean temperature ( C) (a), and rainfall (%) (b) from 1910 to 2017 inclusive, between the HadUKGrid_v1 1km resolution gridded datasets and the previous 5 km gridded dataset for the UK and Scotland less than 2%, except for this same period 1961-2000 in which the UK monthly totals are typically around 2% wetter from September to March and 3% wetter from April to August. This is most apparent for Scotland where these differences are typically around 3% from September to March and 5 to 6% wetter from April to August. The differences in UK and Scotland monthly mean temperature and rainfall values from 1961 to 2000 relate to historical changes in the processing chain of the 5 km dataset and therefore reflect structural uncertainties within this 5 km dataset. These structural uncertainties have been eliminated in HadUK-Grid due to this being processed as a single batcha significant benefit of HadUK-Grid as noted in Annex 1. Although the differences are overall relatively small, they highlight an inhomogeneity in the 5 km gridded datasetin particular a dry bias over the period 1961-2000, mainly affecting upland areas of the UK. Most differences in the earlier decades are likely to relate to changes in the underlying station data. These differences are in some instances significantly larger than the uncertainties presented in Table A2.1. The focus of Legg, 2015 was primarily uncertainties associated with spatial sampling, and although upward adjustments were made to acknowledge the potential for other sources of error (such as structural uncertainty) these were not examined explicitly in this paper. The generation of the HadUK-Grid dataset and comparison with the 5 km dataset has allowed these structural uncertainties to be better understood. The key statistics and headline messages in this report based on the HadUK-Grid dataset remain consistent with earlier State of the UK Climate reports using the 5 km dataset (e.g., .

Coefficient of determination
The coefficient of determination, R 2 , is the square of the correlation coefficient, r, between an independent and a dependent variable based on linear least-squares regression. The R 2 value is a statistical measure of how closely the dependent variable can be predicted from the independent variable. An R 2 value of 1 would indicate a perfect correlation, in which the dependent variable can be predicted without error from the independent variable. An R 2 value of 0 would mean the dependent variable cannot be predicted from the independent variable. An R 2 value of 0.5 would mean that 50% of the variance in the dependent variable can be explained by variations in the independent variable. For example, in this report R 2 values which exceed 0.9 would indicate time-series are very highly correlated.

Rounding
Values quoted in tables throughout this report are rounded, but where the difference between two such values is quoted in the text (for example comparing the most recent decade with 1981-2010), this difference is calculated from the original unrounded values.