Extra - September 16, 2014
September Down The Ages
Recent warming trend stalled

By Philip Eden


September has acquired a reputation for being a wet and windy month in the UK, but the statistics show that, over a long period of years, September's rainfall is less than that of either August or October.


True, there have been a number of very wet Septembers in the past; these are usually markedly 'Atlantic' months when south-westerly winds blow without a break. The Atlantic Ocean is at its warmest in September so air which reaches us from the southwest is both warm and moist, providing the fuel for prolonged downpours. These rainstorms are occasionally supplemented by the remnants of hurricanes which get caught up in the south-westerly flow at intervals. Such Septembers are clearly autumnal months.


By contrast there have also been many Septembers which one could reasonably describe as summery. Very dry months occur more frequently in September than in August with high pressure systems controlling the weather for long periods. Such months are characterised by cool misty mornings, warm sunny afternoons, light breezes, and an absence of rain and thunder.


The long-term statistics reveal that September became appreciably warmer between 1997 and 2006, but the warming trend has since stalled. Looking further back, Septembers were markedly cold between the 1660s and 1690s and abnormally warm from the 1710s to the 1750s, but since then until 1997 mean decadal temperature had been almost static - between 13.0C and 14.0C throughout. The warmest Septembers of all were in 1729, 1865, 1949, and 2006, the latter being the warmest of all.


During three centuries of systematic recording, September rainfall averaged 83mm over England and Wales. There was one very wet episode in the latter part of the 18th century, and decadal means exceeded 100mm in both the 1770s and 1790s. Dry Septembers were most frequent between 1880 and 1930, and the lowest 10-year mean was 48mm between 1906 and 1915. A trend towards wetter months during the mid-20th century ended in the 1970s. The wettest September (averaged across England and Wales) was in 1918 with 189mm, while negligible rain fell during the Septembers of 1743, 1754, 1865 and 1959.


Sunshine records which began in 1876 show a preponderance of sunny months until 1930, but Septembers since then have delivered roughly ten per cent less sunshine until about 1970. A modest recovery made its presence felt from the 1970s to the 2010s.


By Philip Eden


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