October - where was the rain?

Written by Eric Gilbert.

Westonbirt in all it's October gloryWestonbirt in all it's October gloryThis quotation from John Burroughs (1837-1921), an American naturalist, is so apt for the last couple of weeks.  He is best known for his observations on birds, flowers and rural scenes. “October’s Party.  How beautifully leaves grow old.  How full of light and colour are their last days”.

Our weather is still breaking records.  We have just enjoyed 23 dry days, average is 13, with a total of just 31.5mm of rainfall.  This makes October 2016 a record month for my weather station with minimal rainfall of just 31.5mm, the driest since 1984.

Looking at my records I note how this month can be so different from another in that the previous driest was in 2003 with 36.8mm but the following October was a washout with 159.3mm.  The 33-year average is 88.1mm.  October was the second driest month in the year, after July, which is surprising being an autumn month.

The wettest day occurred on the October 1st with 12.0mm with the second wettest on October 15th with 7.9mm.  These two days make up two thirds of the rainfall recorded in October.

When the records for wind direction are studied the reason for this dry weather becomes clear.  The winds for the month have been predominantly from the north to east quarter.  There were several days with light drizzle brought in from the North Sea and four days with daily rainfall that was less than 1mm.

Although we did have many days with several hours of strong sunshine, the end of the month brought a number of days with thick, persistent cloud.  The high pressure trapped beneath it stagnant air that was reluctant to move due to minimal wind.  On October 25th and October 31st the maximum gust all day was just 9mph and many hours when the wind dropped out altogether.

Analysis of the data for barometric pressure shows that the average daily pressure for October was the highest I have recorded for this month with 1023.3mb – another reason for the dry month.

The latter half of October is the time when, as a rule of thumb, evaporation of moisture from plants and ground sources into the atmosphere is overtaken by rainfall totals.  At this point precipitation exceeds evaporation, which allows the rainfall to percolate down through the soil to the aquifers.  October 2016 is an exception in that the evaporation for the month of 31.6mm is greater than the rainfall total of 31.5mm.

The many dry days this month produced 95.4 hours of strong sunshine.  This total exceeds those for the last two years, 2014 and 2015 gave us 65 and 68 hours respectively, since this instrument was installed.  Even though we had many dry days and much sunshine it was not a record month for temperatures.  The beginning of the month was warm with maxima a few degrees above the mean but noteworthy was October 31st when the last of the very mild air pushed the thermometer to 17.6C. In fact the mean for October was 0.14C below the 33-year averages.

At least one air frost has occurred each October, sometimes as many as six in the 1980’s and 1990s. However no air frost occurred in the year 2000, with several years since being frost-free, but the last four consecutive years have been frost-free.

There were four days in the month when fog formed overnight with visibility down to 200 metres although when October 31st dawned this dropped to 100 metres before the sun broke through.