With all this talk in the media about our recent record wet winters in the UK, I thought I would take a look at the recent daily regional precipitation figures that are available for the UK and see if I could find any cycles or periodicity in them.
By the way, the regional data extends back to 1931 and is free to download from the Met Office.
Because rainfall at the daily level produces far too much information to visualise by just looking at a moving average overlaid on the top of a bar chart of daily values, I though I would display the daily totals as a 365 running total in an effort to try to identify any trends or cycles that may be lurking in the unseen data, and lo and behold there they are!
As you can see the drought of 1975/76 is clearly visible , the moving average describes a perfect parabola. Looking at a bit more detail for the last few years:
The record wet years of 2000 and 2012 stand out very clearly. Although 2012 was marginally wetter than 2000, the highest 365 day accumulation since these records began in 1931 occurred between the 11 April 2000 and 10 April 2001 when 1363.4 mm of rain fell, which was 46.7% above the 1931-2014 long-term average.
Is there any periodicity in the annual rainfall in England Wales? Well rainfall is not like temperature, heavy rain can happen on any day of the year as can a dry day. With temperature it’s not possible to get a July day maximum in January or a December minima in August, but with rainfall anything goes. To clear matters up I decided to plot a graph of mean rainfall totals for each day through the year from the 83 years records I have, and this is the result.
As you can see there is no sine wave-like curve you get with temperature. All that you can really say is that spring is the driest season, followed by summer, winter and autumn in that order. The wettest day of the year is the 2nd of November, in fact from the end of October through to the 2nd week in November is probably the wettest period of the year.