SHORT COMMUNICATION HOW WELL DO THE CENTRAL ENGLAND TEMPERATURE AND THE ENGLAND AND WALES PRECIPITATION SERIES REPRESENT THE CLIMATE OF THE UK?

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INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 26: 2287 2292 (2006) Published online 6 July 2006 in Wiley InterScience (www.interscience.wiley.com).

1 INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 26: (2006) Published online 6 July 2006 in Wiley InterScience ( SHORT COMMUNICATION HOW WELL DO THE CENTRAL ENGLAND TEMPERATURE AND THE ENGLAND AND WALES PRECIPITATION SERIES REPRESENT THE CLIMATE OF THE UK? P. J. CROXTON, a K. HUBER, b N. COLLINSON c andt.h.sparks a * a NERC Centre for Ecology and Hydrology, Monks Wood, Abbots Ripton, Huntingdon, Cambridgeshire PE28 2LS, UK b Fachhochschule München, Lothstrasse 34, München, Germany c Woodland Trust, Autumn Park, Dysart Road, Grantham, Lincs NG31 6LL, UK Received 24 August 2005 Accepted 20 May 2006 ABSTRACT In this paper we compare the Central England Temperature (CET) and England and Wales Precipitation (EWP) series with temperature and precipitation records from 24 individual meteorological stations throughout the UK over a 41-year period With a single exception, the correlation of CET monthly means with those of all local stations was highly significant (p <0.001). Correlations with EWP were not as large, but for the most part were highly significant; exceptions occurring increasingly in the most northerly stations. Because of their general availability, the CET and EWP series will continue to be popular, and their use in a range of broad studies of UK climate appears to be justified. Copyright 2006 Royal Meteorological Society. KEY WORDS: rainfall; correlation; Met Office; met station; comparison 1. INTRODUCTION As a relatively small island with a large oceanic area to the west and the European landmass to the east, the UK is prone to a wide variation in local weather conditions. This area is particularly prone to the influences of the Icelandic Low and the Azores High, the pressure systems that form the basis of the North Atlantic Oscillation (NAO) (Davies et al., 1997). The proximity of the sea, prevailing wind direction, altitude and topography are among the variables having an influence both on the local and regional scale. Many biological events are influenced by seasonal variables, with temperature and precipitation rates shown to have an important role (e.g. Collinson and Sparks, 2004). Phenological studies, for example, which examine the timing of natural events against weather variables, may be influenced by specific local conditions. In agriculture, the germination of seeds is temperature-related making the commencement of the growing season a movable feast dependant on local conditions. The Central England Temperature (CET) series (Parker et al., 1992), which is based on temperatures averaged from the English Midlands, i.e. within a loose triangle formed by Bristol, London and Lancashire, is frequently used as a standard measure for temperature in both local and national studies. Similarly the England and Wales Precipitation (EWP) series (Wigley et al., 1984), which gives a mean value based on measurements from 35 weather stations, is widely used as a standard rainfall measure. The Hadley Centre maintains both series. * Correspondence to: T. H. Sparks, NERC Centre for Ecology and Hydrology, Monks Wood, Abbots Ripton, Huntingdon, Cambridgeshire PE28 2LS, UK; ths@ceh.ac.uk Copyright 2006 Royal Meteorological Society

2 2288 P. J. CROXTON ET AL. Recent trends indicate an increase in winter precipitation and a decrease in summer precipitation in all regions (Jones and Conway, 1997). With devolution of the four countries and the increasing awareness of local severe weather events the use of the CET and EWP series may sometimes be less appropriate than records from local stations, particularly in the more outlying areas. There are only a few comparisons in the literature, most noticeably the Mossman 1896 data for Edinburgh (Duncan, 1991), and the monthly regional temperature series developed for the Scottish islands, the Scottish mainland and Northern Ireland (Jones and Lister, 2004). Our paper examines data for from 24 weather stations (Figure 1, Table I) from around the UK and compares them with the monthly CET and EWP series to ascertain how well the series represent the broad climate. 2. MATERIALS AND METHODS All data used in this paper were abstracted from the web site of the UK s Met Office ( The web site includes historic monthly data for 26 stations across the UK. To maximise a common set of years we excluded data from Cardiff (data available only from 1977) and Southampton (station closed down after 2000) and selected monthly temperature and precipitation data for the period for the remaining 24 stations (see Figure 1 for locations and Table I for list of names). The CET and EWP monthly series are also available from the same web site. Eight months data were missing for Yeovilton in 1964, two for Ringway in 2004 and four for Greenwich in For each month and each station, temperature and precipitation were compared to the equivalent CET and EWP values. Paired t-tests were calculated to compare whether the station mean was consistently above or Figure 1. The location of the 24 met stations used in this paper; see Table I for names

3 HOW REPRESENTATIVE ARE THE CET AND EWP? 2289 Table I. Monthly mean Central England Temperatures (CET; C) and difference from CET of monthly mean temperatures ( C) for 24 weather stations Bold = highly significantly cooler or warmer (p <0.001), bold italic = significantly cooler or warmer (p <0.05) Station Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec CET Lerwick Stornoway Airport Braemar Tiree Leuchars Paisley Armagh Newton Rigg Durham Bradford Valley Ringway Sheffield Shawbury Sutton Bonington Aberporth Cambridge Ross-on-Wye Oxford Greenwich Yeovilton St Mawgan Hurn Eastbourne below that of the CET (or EWP) and a correlation coefficient calculated to examine the association between CET, EWP and local station data. 3. RESULTS Table I presents the difference in mean monthly values between CET and the 24 weather stations and shows some distinct patterns. The mean temperatures at seven weather stations (Bradford, Braemar, Durham, Lerwick, Leuchars, Newton Rigg and Shawbury) were significantly lower than the CET across all months of the year. Six stations (Eastbourne, Greenwich, Hurn, Oxford, Ross-on-Wye and Yeovilton) were significantly warmer than the CET across all months. The mean temperatures for stations along the west coast (Tiree, Valley, Aberporth and St Mawgan) were significantly higher than CET through the winter months and generally cooler during the summer. Table II presents the correlation coefficients of CET with the temperatures of the weather stations. Temperatures at all 24 stations were significantly correlated with CET for each month of the year (p <0.001) with the exception of Lerwick in July significant at p<0.05. Correlations were lower at the Scottish sites and, to some extent, the most southerly sites. Table III presents the mean monthly values for EWP and the percentage difference in precipitation from EWP for the 24 weather stations. Variation in rainfall occurs in both seasonal and regional patterns. The mean monthly rainfall at eight weather stations (Cambridge, Durham, Greenwich, Leuchars, Oxford,

4 2290 P. J. CROXTON ET AL. Table II. Correlation coefficients of Central England Temperature with temperatures from 24 weather stations Bold p<0.001, bold italic p<0.05 Station Jan Feb Mar Apr Mar Jun Jul Aug Sep Oct Nov Dec 1 Lerwick Stornoway Airport Braemar Tiree Leuchars Paisley Armagh Newton Rigg Durham Bradford Valley Ringway Sheffield Shawbury Sutton Bonington Aberporth Cambridge Ross-on-Wye Oxford Greenwich Yeovilton St Mawgan Hurn Eastbourne Ross-on-Wye, Shawbury and Sutton Bonnington) was significantly less than the EWP mean over the 41-year period. Rainfall at the Scottish stations of Lerwick, Stornoway airport, Paisley and Tiree was significantly higher than EWP through the winter months (September March). These differences may be expected when extending EWP to examine precipitation in Scotland. St Mawgan also had significantly more precipitation from October February. Table IV presents the correlation coefficients of the monthly means of EWP with precipitation at the weather stations. Significant correlations were obtained in all months for 20 of the 24 stations, the exceptions occurring at Lerwick, Paisley, Stornoway airport and Tiree. The stations at Lerwick and Stornoway airport were only significantly correlated with EWP for 5 and 4 months, respectively. Correlations were typically larger in the southern half of the UK. 4. DISCUSSION It is clear from the results that temperatures from individual stations in the UK can differ significantly from the CET. The mediating influence of the North Atlantic sustains mild conditions all along the west coast during winter, despite the relatively northern latitude of the UK, and is generally cooler during summer months. This winter warming remains apparent even at Scottish island stations, which are far enough north to be influenced by the pattern of cooler temperatures recorded over the northern North Atlantic during the last 50 years (Jones and Hulme, 1997). Spring temperature is especially important for the germination of crops and stimulating the first growth of the native flora. The mean winter temperatures at St. Mawgan remains very close to 6 C, generally regarded

5 HOW REPRESENTATIVE ARE THE CET AND EWP? 2291 Table III. Monthly mean England and Wales Precipitation (EWP; mm) and percentage difference from EWP of mean monthly precipitation for 24 weather stations Bold = highly significantly drier or wetter (p <0.001), bold italic = significantly drier or wetter (p <0.05) Station Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec EWP Lerwick Stornoway Airport Braemar Tiree Leuchars Paisley Armagh Newton Rigg Durham Bradford Valley Ringway Sheffield Shawbury Sutton Bonington Aberporth Cambridge Ross-on-Wye Oxford Greenwich Yeovilton St Mawgan Hurn Eastbourne as sufficient for grass to grow, whereas the CET reaches 6 C during the middle of March, and in Braemar this threshold is delayed until late April into May. Although mean temperatures frequently differ significantly from the CET, correlations are generally high indicating that the pattern of temperature fluctuations in the CET is closely followed at most stations, albeit at a higher or lower level. The exceptions are the two most northerly stations at Lerwick and Stornoway airport. Alexander and Jones (2004) commented on the higher winter precipitation along the west of Scotland, and this is reinforced here by the percentage differences in precipitation shown at the Lerwick and Stornoway airport sites compared to EWP. There is a contrast in conditions in the south of England where, at several stations, there is less precipitation throughout all months of the year, with the winter months tending to be particularly dry with up to 50% less precipitation than the EWP mean. The influences of topography and geography extend this trend in reduced precipitation to the east coast sites of Leuchars and Durham. For regional studies, the use of Hadley Centre Scottish and regional versions of the precipitation series may be preferable. Correlations with EWP were generally not as high as those with CET, but still highly significant at the majority of sites for most months. The results indicate that for most inland areas the EWP and, in particular, the CET series give a representative measure of precipitation and temperature, respectively. We have not investigated whether the relationship with CET and EWP remains constant over time (Butler et al., 2005) and feel that this would be better investigated with longer time series than examined here. Notwithstanding the encouraging results shown here, there will be numerous situations where local weather records are necessary, for example, in modelling grass growth. The CET and EWP are both maintained as active series of data

6 2292 P. J. CROXTON ET AL. Table IV. Correlation coefficients of England and Wales Precipitation with precipitation from 24 weather stations Bold p<0.001, bold italic p<0.05 Station Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 1 Lerwick Stornoway Airport Braemar Tiree Leuchars Paisley Armagh Newton Rigg Durham Bradford Valley Ringway Sheffield Shawbury Sutton Bonington Aberporth Cambridge Ross-on-Wye Oxford Greenwich Yeovilton St Mawgan Hurn Eastbourne with new data available within a few days of the end of the month. Their accessibility, broad applicability (with the exceptions noted above) and up-to-date nature ensure that they will remain the preferred climatic variables for a wide range of applications. REFERENCES Alexander LV, Jones PD Updated precipitation series for the U.K. and discussion of recent extremes. Atmospheric Science Letters 1: Butler CJ, Garcia Suárez AM, Coughlin ADS, Morell C Air temperatures at Armagh observatory, Northern Ireland, from 1796 to International Journal of Climatology 25: Collinson NH, Sparks TH Nature s changing seasons results from the UK Phenology Network. British Wildlife 15: Duncan K A comparison of the temperature records of Edinburgh and central England. Weather 46: Davies T, Kelly PM, Osborn T Explaining the climate of the British Isles. In Climates of the British Isles Present, Past and Future, Hulme M, Barrow E (eds). Routledge: London, Jones PD, Conway D Precipitation in the British Isles: an analysis of area-average data updated to International Journal of Climatology 17: Jones PD, Hulme M The changing temperature of central England. In Climates of the British Isles Present, Past and Future, Hulme M, Barrow E (eds). Routledge: London, Jones PD, Lister D The development of monthly temperature series for Scotland and Northern Ireland. International Journal of Climatology 24: Parker DE, Legg TP, Folland CK A new daily central England temperature series, International Journal of Climatology 12: Wigley TML, Lough JM, Jones PD England and Wales precipitation: a discussion of recent changes in variability and an update to Journal of Climatology 4: 1 25.

THE DEVELOPMENT OF MONTHLY TEMPERATURE SERIES FOR SCOTLAND AND NORTHERN IRELAND

INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 24: 59 590 (2004) Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/joc.1017 THE DEVELOPMENT OF MONTHLY TEMPERATURE