‘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.

Monthly summary December 2009

December began where November finished, with disturbed windy and wet weather. With wind from a southerly quarter from 1st to the 9th, temperatures were above average accompanied by heavy rainfall. As pressure rose and winds swung into a northerly quarter, temperatures dropped and air frost occurred. However, worse was to come as snow fell on the 16th, 17th, 19th and 21st. With strong winds the wind-chill factor was significant for several days dropping to -8°C on the 18th. A snow covering at 0900 of at least 50% was evident from the 20th to the 24th. Rain showers then fell on very cold ground producing sheet ice that covered everything and was extremely dangerous to pedestrians and drivers alike.

With a mean temperature of 2.26°C it was the third coldest December I have recorded, equal to 2001, but less cold than 1996 (2.18°C) and 1995 (1.75°C) and 2°C below the long-term average. It was the wettest December since 2002 with a total of 107.9mm which is 120% of the long-term average. Although the Spring months show a rising trend in temperatures, December is quite the opposite with the frequency and severity of air frosts increasing significantly.

The mean temperature for the year was almost identical to the long-term average of 9.6°C. It is of interest that the average for my records in the 1980’s is 8.9°C. The annual rainfall of 885mm was 105% of the long-term average.

Monthly summary November 2009

With just two days that were totally dry and almost record rainfall, November was a very disturbed month. Frequent low pressure systems crossed near or over the country and delivered substantial rainfall, a total of 171.8mm was the second highest on record and 193% of the long-term average. The wettest day was the 24th with 21.2mm but the 18th brought almost as much rainfall with 18.8mm. As a consequence of the frequent cloud cover the mean temperature was 1.7°C above the long-term average. Also of note was the total absence of any air frost, which has not happened since records began here in 1984. The other feature was the frequent strong winds which reached gale force on a number of occasions. The maximum gusts on the 24th, 13th and 14th were respectively, 42, 50 and 51mph.