A remarkable late heatwave – early-November 2003

The stunning weather of Thursday 6th and Friday 7th is now but a memory, but it is one that will stay with us a long time. Highest temperatures on Thursday afternoon were 18.8ºC at Northolt and Kensington, and 18.4ºC at Heathrow, while on Friday the warmest weather migrated westward and northward, giving 20.2ºC at Lochcarron and 19.5ºC at Aultbea, both in Wester Ross, 19.6ºC at Llanbedr in Merionethshire, 19.1ºC at Broadford on Skye, and 18.8ºC at Kinloss in Morayshire.

Exceptional warmth in November in the UK is almost always imported – delivered from sub-tropical latitudes by a strong southwesterly or southerly wind. In the lee of high ground the warmth is accentuated by the compression of air descending the mountain slopes, a mechanism known as the "foehn effect", after the warm and very dry foehn wind of the Swiss and Austrian Alps. November sunshine is feeble and low in the sky, so the warmth of the sun contributes relatively little to temperature levels on these rare warm days.

The highest November temperature ever recorded in the UK under standard conditions was 21.7ºC, measured at Prestatyn, Flintshire, on 4 November 1946. Nearby Hawarden Bridge reported 21.1ºC, and although maxima of 18-20ºC were observed widely elsewhere it is clear that on this occasion the foehn effect provided sufficient added impetus to give the record to the northeastern corner of Wales. Similarly, Edinburgh's 20.6ºC and Dublin's 20.0ºC on the same date are the highest November temperatures ever recorded in Scotland and the Irish Republic respectively courtesy of the Southern Uplands and the Wicklow Mountains respectively.

England's highest for the month happened on 5 November 1938 when 21.1C was recorded at several sites in East Anglia and the Southeast including London, Chelmsford and Cambridge. On this occasion a stiff southwesterly breeze was accompanied in eastern counties by several hours of bright sunshine which lifted temperatures by 2-3 degC when compared with cloudier regions further west and north.

The third example of extreme November warmth was a recent occurrence, in 1997, when new date-records were established on 15 to 18 November inclusive. Cloudy skies and a strong southeasterly wind meant that the foehn effect was very much in evidence, and this was emphasised by the fact that highest readings were obtained along the coast of north Wales. Indeed, the maximum on the 17th of 20.7ºC at Aber, four miles east of Bangor, occurred at 7pm – nearly three hours after sunset.

© Philip Eden

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