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 6th 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 28th with 20.1°C, coldest night on the 17th with -1.4°C and wettest day on the 2nd with 8.0mm.

Monthly summary March 2010

Consistently high pressure until the 17th 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 18th and 31st. A maximum of 14.6°C was recorded on the 18th with the coldest night experienced during the early hours of the 5th when the thermometer fell to -6.7°. The wettest day was the 24th 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.

‘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.

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 8th – 11th with the lowest being -7° C. There were nine days with air frost, none severe; the lowest was recorded on the 21st 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 27th with 10mm. There were just four totally dry days in the month, the lowest since February 1955. During the night of 23rd/24th 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 13th February. In 1997 the 14th 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.

Monthly summary January 2010

The very cold winter (December – February) continued into January 2010. Snow falling, in one form or another, was recorded on 10 days. The 50% snow cover at 0900 was noted on 14 days and for 12 consecutive days from 4th to 15th. The snow depth from each fall was averaged each day over several ground areas and the total for the month was 25cm. With low temperatures and strong winds the wind chill was significant on several days with -11°C and -12°C logged on the 9th and 7th respectively.  There were 22 days with air frost (equaling the record in 1985), the lowest occurring on the 7th when the thermometer dropped to -8.5°C (record low was of -13.3°C in 2009). For three days, 6th to 8th, the maximum temperature did not rise above freezing with a maximum of only -1.8°C on the 7th. To add to the picture fog was recorded at 0900 on 5 days.

This was the third coldest January I have observed with the mean temperature of 0.48°C some 3.6°C below the long-term average. Only 1985 (0.30C°) and 1987 (-0.15C°) were colder. The total rainfall, mostly falling as snow, was 81.4mm, which is 93% of the long-term average.

There has been a rising trend for January mean temperatures since the late 1980’s but the Januarys of 2009 and 2010 have reversed that trend.