Above average maxima and risks of ground frost

The last four days have brought much warmer weather, a little above average, but cool nights with minima close to giving a ground frost.
The UV levels have moved into the moderate category and during the 9th were rated as high.
Hours of global sunshine, as opposed to direct solar irradiance, totalled 10 hours and above on three days.

Evidence of climate change in Marlborough

Climate change is evident in the data for Marlborough over the past thirty years. It is particularly seen in the data for heavy rainfall and temperature.

Comparing the mean temperatures for both maxima and minima during the spring season, there is an almost continuous upward trend. The mean figure for spring in the 1980s was 7.7°C but by 2013 this had risen to 8.8°C. The very cold springs of 1996 and 2013 produced a blip in the trend but a difference of approximately 1.1°C is quite significant. The upward trend becomes stronger moving from March through to May.

Linked with the temperature results is the occurrence of air frosts. Breaking down the years since 1990 into five-year groups I find that the incidence of the last air frost in spring, on average, has moved successively backwards from 11th May to 5th May, 16th April and 21st April. The four-year group from 2010 to 2013 has an average of 20th April but as we are experiencing a mild start to the year this average is likely to move further backwards.

There is a similar trend to the first occurrence of an air frost in the autumn. Using the same five-year groupings the average has moved successively from 22nd September to 9th October, 18th October and 26th October. The result of the four-year group for 2010 to 2013 is now 4th November.
Rainfall has been very much in the news during the last few months, including the flooding in Marlborough. Warmer air holds more moisture so it is logical to expect rainstorms to show evidence of this fact. Over the last thirty years there is an increasing trend for the incidence of heavy rain in Marlborough. Analysing the data further and breaking it down into groups of heavy rainfall I find that the daily totals of up to 10mm have increased by 7, 20mm by 2 and 25mm by 1. Not surprisingly, the summer months of July and August show the greatest increase.

In January and February 2014 my 30-year records for monthly rainfall were broken with totals of 219.1mm and 151.6mm respectively. Daily records were also broken for both these months along with December 2013.

‘High’ level of pollution

Pollution over last two days classed as ‘High’ including light dusting of Saharan sand (not climate change related). Visibility restricted to about one mile and lowest levels of UV radiation and bright sunshine for almost ten days. No measurable precipitation to wash pollutants out out of the atmosphere.

Monthly Summary for March 2014

March followed the trend of above average temperatures for the third month in a row. With a mean temperature of 7.2C it was 0.8C above the 30-year average. It followed January and February which were +1.2C and +1.6C. The warmest day was the 30th when the thermometer touched 18.8C, the warmest day since 7th October.

It follows that the incidence of air frost was below the long-term average. There were seven days with an air frost, two when the temperature only just dropped below zero. It was quite a contrast with March 2013 when there were 19 occasions that an air frost occurred. The lowest temperature was in the early hours of the 24th, the coldest since November, when a low of -3.8C was recorded.

Unlike the previous two months when we had record rainfall, only 40.8mm of precipitation was recorded. This is 70% of the 30-year average or 17mm below the long-term average. There 16 dry days, the totals when rainfall did occur were modest with the very wet day on the 2nd producing 15.4mm.

Fog occurred during the morning of 4 days, thick fog (visibility less than 200 metres) for 3 days of those days, namely 12th, 13th and 14th.

Hail fell during rain showers that occurred on 3 days. On the 16th and 23rd it was large hail which is classed as greater than 5mm in diameter.

The monthly totals for Solar Energy and Sunshine were 5% and 6% respectively above the mean for recent years.

Warmest day for over five months

With a maximum of 18.8C. on the 30th it was the warmest day since 7th October 2013. The recent increase in air temperature, by day and night, has raised the soil temperature at the 5cm depth to 8.1C.