November heatwaves

Warm winds from the south
by Philip Eden

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 dry foehn wind so well-known in the Swiss the Austrian Alps. The warmth of the sun contributes little to temperature levels on these rare warm days. November sunshine is pretty feeble: in southern England the sun is above the horizon for barely 9 hours and is only 18 degrees above the horizon even at midday, at Aberdeen the equivalent figures are 8 hours and 13 degrees, and at Lerwick in Shetland the sun climbs no higher than 9 degrees. This in turn means that the sun is less able to burn off the fog which so often forms at night in quiet weather; the same synoptic pattern with a gentle southerly breeze which brought record temperatures a month ago would now produce a day of widespread fog.

The highest November temperature ever recorded in the UK under standard conditions was 21.7C, measured at Prestatyn in Flintshire on November 4 1946. Nearby Hawarden Bridge reported 21.1C, and although maxima of 18-20C were observed 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 20.6C and Dublin's 20.0C on the same day are the highest November temperatures ever recorded in Scotland and the Irish Republic.

England's November highest for the month happened on the 5th in 1938 when 21.1C was recorded at several sites in East Anglia and the Southeast, including London and Cambridge. On this occasion a stiff southsouthwesterly breeze was accompanied in eastern counties by 4-6 hours of sunshine, lifting temperatures by 2-3°C 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 November 15-18 inclusive. Cloudy skies and a strong south to 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 coasts of north Wales and northwest Scotland. Indeed, the maximum on the 17th of 20.7C at Aber, four miles east of Bangor, occurred at 7pm - nearly three hours after sunset. For the record, the twentieth century produced no authentic example of a temperature above 20C between November 18 and March 1 inclusive.