Climatological averages for the Reading University climatological stationWhile the weather is what we observe over a short period of time, the climate is how the atmosphere "behaves" over relatively long periods of time.
Meteorologists typically calculate the climate, by means of climatological averages, over a period of about 30-years for individual weather stations. In recent years these 30-year periods have included 1961-1990 and 1971-2000, and now include 1981-2010. A 30-year period is used in order that the contribution from one or two extreme months does not impact too much on the results - thus after a cold month in the UK like December 2010 any 10-year average would be changed by over 0.5C in places by the inclusion of data from such a month, although some of this would be cancelled out by warmer Decembers.
Why might we be interested in different 30-year periods? Well, the climate is always changing - no two 30-year periods are the same - and as meteorologists are interested in the departures from the normal (i.e. how hot or cold is it compared to the average) then a recent long-term average should be used. This is particularly important if we believe that the climate is beginning to change relatively quickly.
In this note I look at the 30-year data for Reading Universioty and relate it to the 30-year period 1971-2000 to look for signs of warming/wetting, etc.
The following table lists the climatological averages for the 30-year period 1981-2010.
|Month||Mean max temp||Mean min temp||Mean temp||Mean precipitation||Mean sunshine|
Precipitation includes any fall of water in the form of rain/drizzle, sleet/snow, hail, fog and dew/hoar frost - with any solid precipitation being melted to give a water-equivalent depth.
Note that the values of air temperature are average daily values - daily values can differ considerably from the averages.
Likewise, the precipitation and sunshine amounts are average monthly values and it is not unusual for precipitation amounts to be in the range 200% to 25% of the values shown, which sunshine can sometimes vary between 150% to 50% of the amounts shown.
A table showing extreme values on individual days is shown here that illustrates how variable daily values can be when compared to the monthly mean.
|Month||Highest daily temp||Lowest daily temp||Wettest day||Sunniest day|
|Jan||14.7 (1998)||-14.5 (1982)||21.6 (1999)||8.1 (1987)|
|Feb||17.0 (1990)||-11.6 (1986)||26.8 (2009)||9.3 (1995)|
|Mar||20.4 (1990)||-6.4 (2001)||23.8 (1984)||12.1 (1997)|
|Apr||25.7 (2003)||-3.5 (2010)||30.6 (1991)||13.7 (1999)|
|May||28.5 (2005)||-1.0 (2010)||31.8 (1985)||15.5 (1985)|
|June||31.4 (1995)||1.5 (1991)||42.6 (1998)||15.3 (1994)|
|July||35.3 (2006)||5.2 (1993)||42.5 (2007)||15.3 (1986)|
|Aug||36.4 (2003)||5.1 (1983)||44.2 (2004)||14.5 (1981)|
|Sep||29.6 (2006)||1.2 (1987)||76.3 (1992)||12.6 (1982)|
|Oct||25.5 (1985)||-4.4 (1997)||49.3 (2000)||10.2 (1997)|
|Nov||18.1 (2010)||-8.3 (1983)||28.7 (2003)||8.2 (1986)|
|Dec||15.8 (1985)||-13.4 (1981)||31.3 (1995)||6.9 91986)|
|Year||36.4 (2003)||-14.5 (1982)||76.3 (1992)||15.5 (1985)|
Monthly extremesOne indication as to the way in which climate is changing is to look at the distibution of occurrences of monthly extremes. Thus in an unchanging climate, if we were to examine when the highest temperature of each of the calendar months (12) occurred in a 30-year period we might expect to find 4 per decade. Clearly, as with the example of tossing a coin, this distribution might need more than 12 samples to approach the statistically-expected result.
With this caveat in mind, consider the following table:
|Highest mean max. temp.||2||4||6|
|Highest mean min. temp.||3||2||7|
|Total of above 3 numbers||7 (19%)||10 (28%)||19 (53%)|
|Lowest mean max. temp.||7||4||1|
|Lowest mean min. temp.||8||4||0|
|Total of above 3 numbers||22 (61%)||11 (31%)||3 (8%)|
How is the climatology evolving?Finally, how do the 1981-2010 averages compare to those for 1971-2000?
|Month||Mean max temp||Mean min temp||Mean precipitation||Mean sunshine|
Changes in sunshine amounts must be judged with caution as the location of the sunshine recorder has moved during the 30-year period and there is a suggestion that tree growth may be reducing the record a little during winter months.
Rainfall has increased by about 1% between the two periods. In particular the late autumn appears to have become wetter, although some of this is offset by a driest September.
Last updated 15 February 2011.