Out of interest I also did a graphic showing which areas of the UK have shown the biggest changes in "Manchester" Summer Index between 1961 and 2016 based on the trend increase between the start and end dates (i.e linear regression smoothing out all the peaks and troughs in the year on year data) and this is below

Again there is a SE/NW split with the biggest increases in the SE
What was weird was the exceptionally big increases in the Thames Estuary (N Kent/Sheerness by the looks of it) and also on Teesside by the coast around Hartlepool/Middlesborough/Sunderland as far as I can tell
When I looked into which of the variables (Max temp, Rain Days, Sunshine Hours) had generated the big increase it turned out to be the sunshine hours
I can only assume that this was industrial smoke from the earlier days in these areas giving way to clean air. The "trend" increases were well over 100 hours per year for summer totals
Originally Posted by: lanky