April 2020 sunniest on record – but just how do they measure it?

Courtesy of UKMO

The Met Office yesterday tweeted that new provisional information suggests that April 2020 is now the sunniest on record with 212.5 hours of sunshine have been recorded so far, beating the previous record of 211.9 hours that was set in 2015. The amount of climate stations that report daily sunshine totals is really quite small, there are only sixty SYNOP reporting stations in the whole of IONA. Here is a chart of the totals up to and including the 27th of April.

How do the Met Office measure sunshine?

Because the network of sunshine sensors across the UK is quite coarse, I did tweet that I could only assume the amount of detail in their anomaly chart suggests that it can only be based on observational data from satellites. If it is, this begs the question, are satellites the reason why sunshine totals have been increasing in recent years? Take the month of April, since 1929 and you’ll see that a linear trend reveals that sunshine has been increasing at the rate of 2.1 hours per decade, April’s are now over 19 hours sunnier than they were in 1929. The Clean Air Act that came into force in 1956 actually did little to improve the sunshine record of April, as you can see sunshine totals dropped in the early 1960’s, before rising again from the mid 1990’s.

This is all conjecture on my part, because who know the Met Office climate charts may be constructed purely using observational data from sunshine sensors. But could the increases in sunshine be due to these sensors which replaced many if not all Campbell-Stokes sunshine recorders in the 1990’s? After all these sunshine sensors are very much more sensitive than reading burns on a sunshine card were!

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