Monthly summary May 2009

The month was noticeable for the relatively high average air pressure with the minimum not falling below 1000.0 Mb. This resulted in 17 totally dry days and when rain fell the daily totals were not large – 8.0mm on the 14th being the wettest day. With the rainfall for the month totalling just 31.7mm, being 52% of the long-term average, it was the driest May since 1998. Most of the shower activity centered around the 14th – 19th due to low pressure to the west of the country. As pressure rose again from an Azores high, temperatures picked up and brought glorious hot, sunny weather to conclude the month although maxima were trimmed due to brisk easterly winds. There were two nights with minimal air frosts on the 4th and 10th. May 2009 produced the 14th consecutive month with above average temperatures (+0.7°C) with the hottest day being the 29th when the thermometer peaked at 24.6°C.

Monthly summary April 2009

The start to April was dry and warm but by the 6th pressure was dropping and there followed cloudy and wet days, particularly over the Easter weekend, excepting the Monday, which was glorious. The 15th gave us the warmest day with a maximum of 19.2°C but the following day gave us the wettest with a total of 21.5mm – a third of the rainfall for the whole month. However, as an anticyclone settled over the country we then experienced a dry and very warm period from the 18th to the 23rd. The remainder of the month was cloudy and wet, daytime maxima were close to or above the average but followed by cool nights with air frost on the 28th (-1.1°C) and 28th (-0.9°C).

The rainfall of 68.5mm was 112% of the long-term average. This was the third warmest April I have recorded with a mean of 9.72°C, which is 1.4°C above the long-term average – 1987 was 9.73°C and 2007 was 11.41°C.

Monthly summary March 2009

March provided us with a wide variety of weather. The first couple of weeks, with low pressure predominating, brought maxima near average but depressed night time temperatures with several frosts and the majority of rainfall for the month. The wettest day occurred on the 3rd with 16.8mm of rainfall, which was 40 % of the total for March. A spring like week started on the 15th with warm, dry days and much sunshine all due an anticyclone which also provided eight consecutive days without rainfall. The remainder of the month brought near average maxima but some sharp frosts, namely -3.8°C and the coldest of -4.6°C on the 30th and 21st respectively. There were eleven air frosts which is three above the long-term average. The mean temperature was just 0.2°C above the long-term average and the total rainfall of 41.6mm was only 69% of the long-term average. The aquifer rainfall, from 16th October – 15th March, which is generally recognized as the period when rainfall drains through to the aquifer rather than evaporating, was 45mm below the long-term average and the driest since 2005/06.

Monthly summary February 2009

The weather in February fell very distinctively into two halves. From the 1st to the 15th we endured continuous frosts, many quite severe with the lowest being on the 14th at -6.5°C. Snow fell for seven of these days, with considerable accumulations from the 2nd to the 5th. The early dry snow arrived on easterly winds but as the moist air from the south met the cold easterly plume of air, it became wetter and produced the equivalent of considerable rainfall, the heaviest of this on the 9th amounting to 22.1mm. As high pressure became established to the southwest of the country from the 14th it deflected Atlantic weather fronts to the north of the country and brought mild air from the southwest. Although much cloud was trapped beneath the anticyclone, there were several days in this second period with maxima well above the average, the maximum occurring on the 22nd at 11.9°C. Saturday 21st brought a beautifully warm and sunny day to dispel the gloom. There was only one day of minimal rainfall in this latter fortnight. Analyzing temperatures for the two parts of February I find that the mean period for the 1st – 14th was 4.4°C below the long-term average and in contrast the second half was 2.6°C above, resulting in the mean for the whole month being almost 1°C below the long-term average. We have experienced five colder Februarys since 1984, in severity order they were 2006, 1996, 1985, 1991 and 1986, the latter being a bitter 6.1°C below the long-term average. With a total equivalent rainfall of 74.2mm this was 10mm or 16% above the long-term average.

The winter of 2008/09 was the coldest since 1990 being 1.7°C below the long-term average. The total rainfall for the winter was 201mm being only 82% of the long-term average.

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°