Mid-winter heatwaves

Following the recent short cold snap, everything has reverted to that familiar pattern of wind, rain, and above-average temperature which has been so typical of the winter season during the last 15 years. Maximum readings of 10-13C have been commonplace, contrasting with a January normal of 5-8C but still well short of the record at this time of the year.

Eight times in the last century the mercury climbed above 17C somewhere in the UK during January, most recently on the 10th in 1998 when 17.3C was reported from Prestatyn. Amazingly, all eight occasions occurred along the north Welsh coast, and on five of them the highest temperature was logged at the same site. The village of Aber which lies roughly half way between Bangor and Llandudno has twice recorded a maximum of 18.3C in January - on the 27th in 1958 and the 10th in 1971.

The reason for this concentration of warmth in north Wales is down to a mechanism known as the Föhn Effect, named after the foehn wind which is well-known in the Alpine region for its desiccating warmth. Britain's warmest winds in winter nearly always blow from between south and southwest, and they are usually very moist as well. They are forced upwards over the Welsh hills, depositing large amounts of rain there, then they descend the northern flank of the Cambrian mountains with much of moisture now rained out. Descending air is warmed by compression, and this warming is enhanced if the air is dry. Sometimes very dry air aloft is also brought down to sea-level.

The north coast of Wales is not unique in its geographical characteristics in the UK. Similar stretches of coastline in the lee of upland areas also enjoy rare days of exceptional winter warmth. These include that stretch of coastline from Ilfracombe to Minehead in the shelter of Exmoor, the Whitby district in the lee of the North York Moors, the Carlisle and Eden Valley area which is downwind of the Lake District, and the southern shore of the Moray Firth which lies to the north of the Grampian massif. The remote coastline of Sutherland and Wester Ross, the Edinburgh area, and the north coast of Northern Ireland also occasionally benefit from the foehn effect.

The most widespread January warmth happened around January 11 1971. Alongside Aber's 18.3C, Llandudno recorded 18.2C, Rhyl 17.7C, Prestatyn 17.6C, and Colwyn Bay 17.2C. Further afield, Lairg in Sutherland registered 16.7C, Poolewe in Wester Ross and Huddersfield both logged 16.6C, Corbridge in Northumberland 16.2C and Bromfield in Cumbria 15.9C. During the same spell, London failed to reach 13C.