Windrush Weather

June 1, 2010November 28, 2011

Monthly summary May 2010

Another month with two distinct halves, the first being unseasonably cold and the latter produced both maxima and minima above the average for May. There were three nights with air frost, which burnt the early tender foliage of both vegetables and flowers, with a minimum of -1.4°C on the 12^th. These were the latest spring air frosts since 2003. Fortunately, from the 17^th as the pressure rose, so did the temperatures resulting in many warm days in the third week with a maximum of 27.8°C on the 24^th, the warmest May day I have recorded. Overall the mean temperature was the lowest for May since 1996 at 0.45°C below the long-term average. The rainfall, totaling 32.7mm, was just 55% of the long-term average, the bulk of which fell on the 1^st (15.1mm).

It was the coldest spring since 2001 being 0.1°C below the long-term average. The total rainfall for spring 2010 was also low, with a total of 130mm it was only 72% of the long-term average.

May 1, 2010November 28, 2011

Monthly summary April 2010

A month which was notable for the lack of rainfall and the number of dry days (21). It was the fifth driest April that I have recorded with a total of 25.9mm, just 43% of the long-tern average. The daytime temperatures at the start of the month were decidedly cool but from 6^th were average or above whereas we experienced many cool nights, eight with air frosts, the greatest number since 1997, although none were severe. The mean temperature was 0.5°C above the long-term average, principally due to the warm days in the latter half of the month. The individual day extreme statistics were – warmest day on the 28^th with 20.1°C, coldest night on the 17^th with -1.4°C and wettest day on the 2^nd with 8.0mm.

April 1, 2010November 28, 2011

Monthly summary March 2010

Consistently high pressure until the 17^th brought cold and dry weather as the anticyclone blocked fronts advancing from the Atlantic. The dry, strong mainly northerly winds produced just 1mm of rainfall over the sixteen days but gave us eight nights with air frost. The remainder of the month brought very different weather with just one dry day between the 18^th and 31^st. A maximum of 14.6°C was recorded on the 18^th with the coldest night experienced during the early hours of the 5^th when the thermometer fell to -6.7°. The wettest day was the 24^th with 14.3mm. The total rainfall for March was 70.5mm, which is 118% of the long-term average. Due to the persistent northerly winds it was a cold month with the mean temperature 0.5°C below the long-term average.

March 1, 2010October 13, 2011

‘Aquifer’ rainfall

Analyzing the ‘aquifer’ rainfall, the precipitation which falls between mid October and mid March that percolates down into the sub strata, it has been the fifth wettest period since 1984 with 16% (63mm) above the long-term average. My new equipment enables me to see when evaporation exceeds the moisture that soaks down through the earth and I find that in March, almost 60% of the total rainfall evaporated. There has been an increasing trend in the total aquifer precipitation over the last 27 years in Marlborough, from approximately 375mm in the 1980’s to almost 440mm in the 2000’s.

March 1, 2010October 23, 2011

Monthly summary February 2010

February 2010 was another cold month, which completed the statistics for the winter season. It was the coldest February since 2006 and the sixth coldest recorded here, the mean temperature being 1.1°C below the long-term average. There were seven days when snow fell as flakes or pellets and there was considerable wind chill from 8^th – 11^th with the lowest being -7° C. There were nine days with air frost, none severe; the lowest was recorded on the 21^st when the thermometer fell to -3.2°C. The total rainfall of 66.2mm was 103% of the long-term average and most fell in modest daily amounts, the wettest being the 27^th with 10mm. There were just four totally dry days in the month, the lowest since February 1955. During the night of 23^rd/24^th warm air managed to edge in from the south as the cold air mass, which had dominated our weather for most of the month, eased away.

The temperature at the Earth’s surface, as we know, is a balance between heat energy arriving from the Sun and heat energy escaping from the Earth. This loss is progressive throughout winter until strengthening energy from the sun reverses the trend. Bare soil is a poor conductor of heat, but snow is worse, thus night-time drop in temperature is accentuated when there is snow cover, especially fresh, deep snow. For many years I have kept data on the coldest night of the year and it is proving remarkably consistent. From 1984 to 1996, averaged over the winter months, the coldest night was 13^th February. In 1997 the 14^th moved into second position and in 1997 it became the coldest on average and has remained in this position ever since. It is more remarkable that although there is approximately 0.1°C or less separating the next six nights, in temperature order, the divergence between first and second is seven times greater, a significant difference.

The past winter was the coldest I have recorded, even beating the severe weather experienced in 1984 and 1985, mainly due to the frequency of sub zero nights rather than depressed day time maxima. The rainfall of 235mm was 97% of the long-term average. However, not all of this precipitation seeped into the aquifers. Due to evapotranspiration, 31mm found its way back into the atmosphere through evaporation from ground and water surfaces, with minimal transpiration from plant material at this time of year.