[Climate, Hydrology, Sea Level and Air Pollution]

1. Air Temperature in Central England
M Hulme

Air Temperature in Central England

(Top) Annual-mean Central England Temperature
(Middle) Annual number of 'hot' days (mean temperature >= 20C) in the Central England Temperature record
(Bottom) Annual number of 'cold' days (mean temperature <0C) in the Central England Temperature record.

Numbers are cumulated over the period July to June and dated by the January.

Specific indicators
The longest continuous record of measured surface air temperatures in the world exists for a region representative of the English Midlands - known as the Central England Temperature record. Daily records extend back to 1772 and monthly records to 1659. Annual temperature fluctuations in this region are representative of those in most of the UK.

The data are quality controlled and updated monthly. The specific indicators chosen are the annual-mean temperature and the number of hot and cold days each year. A hot day is defined as when the daily mean temperature (i.e., the average of minimum and maximum) is above 20C; a cold day is when the daily mean temperature is below 0C.
[Source: The Climatic Research Unit, University of East Anglia, Norwich NR4 7TJ]

Relevance
Air temperature is obviously one of the most fundamental indicators of climate. Fluctuations or changes in climate will almost always be reflected in variations in surface air temperature. Since global warming is anticipated to lead to changes in the frequencies of extreme events - such as hot summer days and cold winter days - these indicators are potentially useful as well.

On average (1961-90) only three to four days above 20C occur per year in the Central England Temperature record, so this definition is suitable to highlight extreme hot temperature events. The hottest day occurred on 29 July 1948 with a mean temperature of 25.2C. On average (1961-90) just under 12 days occur below 0C per year. The coldest day occurred on 20 January 1838 with a mean temperature of -11.9C.

Sensitivity to climatic and other factors
The Central England Temperature is perhaps the single most important and representative measure of the surface climate of the UK. It is also quite well correlated with land temperatures over the entire Northern Hemisphere. At an annual level this correlation is about 0.4, but when average values over 10-year periods are compared this correlation rises to about 0.75. The number of hot days each year correlates fairly closely with mean summer Central England Temperature (correlation about 0.73). There are clearly some years, however, when mean summer temperature and the number of hot days are not well correlated. The number of cold days each year correlates very closely with mean winter Central England Temperature (correlation about -0.89).

Change over time
Over the twentieth century the annual-mean Central England Temperature warmed by about 0.67°C (mean linear trend). The warmest years in the entire 340-year record occurred in 1990 and 1999, and five of the ten warmest years occurred in the last decade. This has made the 10-year period 1993-2002 the warmest such period in the record, 0.7°C above the 1961-90 mean.

There has also been an increase in recent years in the number of summers with large numbers of hot days. The summer of 1995 recorded 26 such days - easily the largest number this century - and 1976, 1983 and 1997 also recorded many hot days. The average number of hot days per year over the last decade, 1993 - 2002, has been 7.4 days, more than twice the long-term average. It is noticeable that the period 1962 to 1966 inclusive did not record any hot days in the UK.

The number of cold days shows little long-term trend over the twentieth century. The period from about 1940 to 1970 recorded generally average or above average numbers of cold days, with the record of 57 cold days occurring in the winter of 1962/63. The most recent decade, 1993 - 2002, has recorded on average only about six such days per winter, well below the long-term mean of nearly 11.5 days. The early decades of this century, however, also recorded relatively few cold days. The winters of 1922/3 and 1924/5 for example, did not record any cold days at all, a situation repeated only twice this century: in 1974/5 and 1997/8.
In the last four years annual CET has remained well above the 1961-90 average, and indeed only slipped below 10oC in 2001. The year 1999 matched the highest value, which was also recorded in 1990, and 2002 had the fourth highest CET annual temperature in the record. Despite this, the number of hot days has not been exceptionally high in the last four years. The number of cold days has, however, been well below average in every year since 1998 except the 2000/2001 winter.

Anticipating continued global warming, it is expected that the annual-mean Central England Temperature will continue to warm, with higher numbers of hot days and fewer cold days. Such trends, however, will only be manifest when averaged over periods of 10 years or more. Individual years will experience large fluctuations and some, like 1996, will record temperatures below the average. It remains quite possible, for example, that in the next few years we will experience a very cold winter with a large number of cold days.