There’s been a decline in the days with thunder over parts of southern Britain in recent years. Studies of the number of days with thunder heard at different locations in southern England going back as far as the 1800s in some cases shows a drop in frequency in recent years.
For example, according to this reasearch article in theRoyal Meteorological Society Journal the Radcliffe observatory in Oxford has a record of thunderstorm occurrence between 1828 and 2019, forming a record of almost 200 years of thunderstorm frequency for the city. Probably the longest record of thunder days anywhere in the world.
Monthly averages of days with thunder heard in Oxford. Source: Royal Meteorological Society article: Two hundred years of thunderstorms in Oxford by Stephen Burt
The average number of days with thunder heard in Oxford over the complete period of record (1828–2019) was 11.9 per annum, annual totals varying from zero in 1829 and 2 days in six other years (most recently 1990) to 28 days in 1925.
From the counts of the number of days with thunder in the article, there has been a definite decline in frequency in recent years:
an irregular rise in annual frequency to the 1920s, resulting in an annual average of 20.0 days with thunder heard for the 10 years ending 1933. Since the 1920s and 1930s, very thundery years have become much less common, with just a few years, 1983 (24 days) and 2006 (22 days), that oppose this trend. Low annual totals of thunder days have become much more frequent within the last two decades or so. The 10-year mean for the period 2010–2019 stands at just 8.1 days, the lowest such value on Oxford's records since the 1860s.
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Oxford in comparison with long multi-site thunder records from London have a similar relationship showing a decline in thunder days in recent years. Both Oxford and London tend to have the same synoptic setups that are more likely to bring thunder over much of inland southern England.
The graph below from theRMets article: Two hundred years of thunderstorms in Oxford shows the 10-year running means (plotted at year ending) for Oxford from the commencement of the record in 1828, together with those for Kew Observatory (1877–1980), Heathrow (1949–2005) and Northolt (since 1984). Since at least the 1880s, the peaks and troughs (if not always the relative amplitudes) of decadal thunderstorm incidence in London and Oxford largely coincide.
The Met Office recently conducted a Days of Thunder (DoTs) climatology study to show how the frequency of thunderstorms varied over a 30-year period for the UK from 1990 to 2019 per month, season and year. Their study showed an overall reduction in the number of days of thunder in the UK per year between 1989 and 2019. Though the study does suggest thunderstorms have increased in the north of the UK and decreased in the south in that period.
Contrary perhaps to popular belief, there is little evidence of a clear relationship between thunder occurrence and high temperatures during the summer months. Although perhaps not the sole cause of the observed reduction in frequency of thunderstorms in recent decades, a decline in synoptic weather types conducive to thunderstorm development in southern England is likely to account for the reduction The occurrence of certain weather setups that tend to be more conducive for thunder to occur, particularly Spanish plume events, along with their decline of frequency, is perhaps a main reason for the fall.
So the evidence suggests a recent decline in thunder days, but what might be the cause in a reduction in setups that bring thunder?
Thunder is most likely to be heard in the warmer months (April-September) and the synoptic setup that is most likely to bring thunder to southern Britan is Spanish plume events.
The synoptic setup for these events is an amplifying trough over the North Atlantic to the west of western Europe, with a plume of warm and humid air at the surface from the south or southeast, a layer of drier air above this pulled north off the Spanish plateau and cooling above this with approach of the upper trough combining to make a deeply unstable airmass, conducive to thunderstorms, that moves north across western Europe towards southern Britain and sometimes northern Britain too.
One of the reasons perhaps for the decline in these Spanish plume synoptic setups could be climate change which is reducing Arctic sea ice. This means there is less depth of cold air from the arctic pushing south over the North Atlantic in the warmer months. Upper troughs that move across the Atlantic are essentially pools of cold air from the polar and arctic regions that push south. With the air less cold over the arctic, the cold air is not pushing as far south as it used to. So the Atlantic troughs moving west to east aren’t amplifying south as much as they used to.
The generally flatter, less amplified upper troughs, such as we’ve seen this summer, aren’t digging far enough south over the North Atlantic to pull the unstable Spanish plumes all the day to the UK. Rather, when the upper trough arrives from the Atlantic there is more of westerly component to the flow rather than southely component across the UK, meaning instability that develops ahead of the approaching upper trough is pushed further east across France and into Belgium and Germany. Much of mainland Europe has had a very thundery summer so far, while the UK has been rather quiet, simply because of the total absence of Spanish plumes.
However, the reduction in arctic sea ice may not be the only reason for the decline in thunderstorms in the UK in recent years, there could be some other factors. But for me, the reduction in cold air leaking south from polar regions over the North Atlantic and carving out deeper troughs to the west makes most sense. This contradicts a study published in Science in 2014 that predicted that every degree rise in global temperature would increase the frequency of lightning strikes by about 12%. This may be the case for other parts of Europe, but for the UK, it apppears the warming of the arctic, which is warming more rapidly than other parts of the world, may be having a detrimental effect on days of thunder by reducing the synoptic setups that typically bring thunder in the more active warmer months.
