With the subject of climate change continuingly being discussed and a major conference being held this month it is interesting to look at one aspect that affects us as autumn advances into winter.
Air frosts can mean attractive landscapes and frozen car windscreens when they occur. The severity of them tends to deepen as the winter moves into January and February.
Extracting and analysing data since my records began in 1984 reveals a changing pattern. I will take the period from 1985 for this article and use the data when the air temperature drops to or below -0.1C.
There is a distinct trend over the period for the occurrence of air frosts to occur later and later into autumn. Using 1st September as a datum and averaging the days since that date when an air frost occurs, I find the following trend. The first and last periods are for five years whilst the central periods cover ten years.
Year(Starting in) 1985 30
1990 33
2000 57
2015 66
Days since 1st September
We all realise that the mild weather experienced in autumn tends to last longer and delay the more severe weather of winter. Looking at the overall trend, these figures show the quite dramatic shift in the advent of an air frost each year, whilst recognizing that there are considerable variations from year to year.
September has been frost-free since 2004 whilst it occurred three times in 2003 with five in 1986.
During October an air frost always occurred up to 1999 when there were five days with the coldest recorded at -1.7C. The last three consecutive years have been without an air frost in October.
November is quite a different month in that the overall trend is flat for the number of air frosts. However, there are wide variations. We experienced an air frost on 15 days in 2005 contrasting with 4, 9, 8 and 4 over the last four years. This year the first air frost occurred on 21st November.
December provides quite a contrast in that the trend for the occurrence of an air frost has increased over the years from around 8 a month to 12 a month. There were 25 days with an air frost in 2005 and only 2 in 1988.
Details of the first air frost to occur each autumn since 2000 are in 2013 (11th November), 2000/04/05 (13th November), 2011 (23rd November) and this year the first air frost was on the 22nd November (-2.6C), which almost equaled the record in 2011.
The coldest night occurred on 20th December 1999 with a minimum temperature of -14.1C with the slightly lower figure of -13.8C on 11th February 2012.
The data above provides evidence that the climate in Marlborough has changed over the past thirty years or so but is this short-term change or evidence of more permanent climate change?
