By Philip Eden

"Cast not a clout/ Till May be out" is an old saying which still has some currency even though a substantial part of the population no longer knows what a 'clout' is. There is also some disagreement whether May refers to the month or to the may blossom - the flower of the hawthorn - which in the UK can bloom any time between late-February and late-June depending on your location and the character of the season. My choice of a capital 'M' reveals whose argument I find the most compelling.

Although the sun is as high in the heavens as it is in late-July or early-August, May is certainly not a summer month. The seas around our shores remain cold - as cold as they are in November - and winds from within the Arctic Circle can still bring snow showers and overnight frosts.

Temperature records for Central England compiled by Professor Gordon Manley show that May has changed little during the last three centuries or so save a modest warming during the last 30 years. Since the record started in 1659, decadal temperature averages for the month have been between 10.6C and 11.8C with only a few exceptions. The 1690s were particularly cold in all months, and the mean May temperature for that decade was just 9.8C. In the 20th century there was hardly a single warm May between 1972 and 1987 inclusive, but the mean temperature for that period was 10.8C. The warmest decade in the entire series was the 2000s with 12.1C. Thus, as with several other months during the year, we see a marked upward step-change in the late-1980s rather than a prolonged warming trend.

As far as rainfall over England and Wales is concerned, the figures reveal no long-term trend during the last 300 years. There was a tendency for wetter Mays during the late-18th and early 19th centuries and also in the second half of the 20th century whereas the period 1830 to 1940 was comparatively dry. The wettest decade was the 1810s with a mean May rainfall of 92mm, while the driest was the 1830s with 47mm. During the 20th century the wet 1960s (73mm) contrasted with the dry 1990s (47mm). So once again the 1990s were completely out of step with the preceding several decades.

Sunshine records do not cover as long a period as those for temperature and rainfall, extending back only to the late 1870s. Averaged over England and Wales, sunny Mays predominated until the mid-1920s, while there was often a shortage of sunshine thereafter. Sunniest decade was 1901-10 with an average sunshine total of 213 hours while the dullest was 1931-40 with a mean of just 162 hours. Once more we find a sharp contrast between the 1980s (an average of 157 hours between 1981 and 1988) and the 1990s (an average of 210 hours between 1989 and 1997.

© Philip Eden