There has always been an impression that certain types of weather recur at certain times of the year � indeed, the idea is enshrined in ancient country lore which contains myriad sayings and proverbs which tie climatic characteristics to the calendar.
But most thinking people would regard as preposterous the notion that the atmosphere can remember how it behaved on a particular date in previous years so that it can do the same this year. Put as starkly as that, the idea is certainly irrational and unscientific. But a gargantuan heap of statistical work over the last century and half has identified some significant tendencies to unusual weather at particular times of the year. These seasonal tendencies are called �singularities� � a word coined by the German climatologist, A.Schmauss, in 1938.
Work on singularities can be said to date back to Alexander Buchan's analysis of Edinburgh temperatures in 1869 in which he identified times of the year which were regularly warmer or colder than would be expected from the smoothed annual curve of mean temperature. These famously became known as -Buchan's cold and warm periodsö and were widely quoted out of context during the last century. More detailed and extensive work was conducted on the continent � notably in Germany, the Netherlands, Austria and Sweden � during the early-1900s, and the subject was taken up in the UK Meteorological Office by C.E.P.Brooks, J.E.Belasco, and H.H. Lamb from the 1930s onwards.
Rigorous statistical techniques were applied to daily sea-level pressure patterns over Europe and the north Atlantic over a period of 60 years. The end result was that several key periods were identified throughout the year when these synoptic patterns deviated markedly from the normal seasonal progression. The events certainly did not happen every year, nor were any exact dates set in stone, but more than 20 singularities which occurred in more than half the years of the analysis were detected in the British climate:
|Event||Avg. Start||Avg. End||Peak||Frequency %|
|Early-Jan storms||5 Jan||17 Jan||8 Jan|
|Mid-Jan settled||18 Jan||24 Jan||21 Jan|
|Late-Jan storms||24 Jan||1 Feb||31 Jan|
|Early-Feb settled||8 Feb||16 Feb||13 Feb|
|Early-Mar storms||26 Feb||9 Mar||1 Mar|
|Mid-Mar settled||12 Mar||19 Mar||14 Mar|
|Late-Mar storms||24 Mar||31 Mar||28 Mar|
|Mid-Apr storms||10 Apr||15 Apr||14 Apr|
|Late-Apr rains||23 Apr||26 Apr||25 Apr|
|June monsoon||1 Jun||21 Jun||16 Jun|
|July heatwave||10 Jul||24 Jul||16 Jul|
|Late-Aug winds||20 Aug||30 Aug||28 Aug|
|Early-Sep warmth||1 Sep||17 Sep||10 Sep|
|Mid-Sep storms||17 Sep||24 Sep||20 Sep|
|Old Wives� Summer||24 Sep||4 Oct||29 Sep|
|Early-Oct storms||5 Oct||12 Oct||9 Oct|
|St Luke�s Summer||16 Oct||20 Oct||19 Oct|
|Mid-autumn storms||24 Oct||13 Nov||29 Oct|
|St Martin�s Summer||15 Nov||21 Nov||18 Nov|
|Early-Dec storms||24 Nov||14 Dec||9 Dec|
|Mid-Dec settled||18 Dec||24 Dec||21 Dec|
|Christmas storm||25 Dec||1 Jan||28 Dec|
After a further sixty years these singularities are still identifiable on many occasions. Even in this era of high-tech weather forecasting, the list can still come in useful from time time: for example, when medium-range ensemble forecasts point in two contrasting directions the real atmosphere is most likely to follow the route closest to any relevant singularity.