First and last frosts

In my report for May 2009 I included data on air frost and how the occurrence of below zero temperatures had changed over the years since my records began in 1984. I have now analysed the pattern for the first frost at this time of year. To see the trend more clearly I have analyzed the data over five-year periods and the average for each period is in the table below, both for Spring and Autumn. Although the warming in Spring is evident from 1985, this trend for Autumn didn’t start until the late 1990’s. For both periods there is now a shift of around one month, for these average figures.

1985 – 89

1990 – 94

1995 – 99

2000 – 04

2005 – 09

Last Frost of Spring

14th May

11th May

5th May

26 April

22nd April

First Frost of Autumn

29th Sept.

22nd Sept.

9th Oct.

18th Oct.

26th Oct.

The trend in the total degrees of air frost for each month since 1984, November & December 2009 excluded, also produces interesting data, as in the table below. November and particularly December during this period, show an increasing severity of air frost in contrast to the less severe monthly frost totals at the beginning of the year.

January

February

March

October

November

December

-8°

-10°

-3°

-1°

+3°

+20°

Monthly summary January 2009

January 2009 will be remembered as a truly winter month being the coldest January since 1997 and the 4th coldest I have recorded. The mean temperature was just 1.8°C, this being 2.2°C below the long-term average. There were 18 air frosts, which are 7 above the average and close to the record of 22 in January 1985 and also 21 in January 1987. Not only were these frosts frequent but also quite severe with a record low for January of -13.3°C during the night of the 7th. There were also some very cold days when the thermometer did not reach a positive figure, namely 6th and 9th with -1.0C° and -2.5°C respectively. The rainfall of 71.4mm was 81% of the long-term average. With a wet period from the 7th to 22nd, due to mild, moist southerly winds, resulting in the majority of the rainfall and the two wettest days on the 21st and 22nd with 14.8mm and 12.3mm respectively. There were five mornings that dawned with fog and four days with light snowfall. My analysis of the diurnal temperatures, the daily difference between the maximum and minimum, shows a gently increasing maximum, a rise of almost 2°C from the 1980’s to this century.