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Extra - April 13, 2012
April showers
Sometimes wintry

By Philip Eden

Showers have featured prominently in this week's forecasts, but I suspect that this is not really the sort of rain we imagine when we think about April showers.

Many of this week's showers have been heavy and prolonged, perhaps lasting half an hour or more, with hail and thunder occurring widely too. These are rainstorms, cloudbursts, downpours, rather than mere showers. In my mind at least April showers are brief refreshing splashes of rain, five or ten minutes' worth, say, with strong spring sunshine fore and aft, laying the dust and relieving the drought resulting from those March winds.

April is associated with showers in our part of the world, not because they occur more often in this particular month compared with any other, but rather because the frequency of showery activity (inland, at least) increases sharply between March and April. Sunny mornings in winter usually presage fine days, but from April onwards sunny starts are often deceptive, usually being followed by a build-up of cloud and short sharp showers during the afternoon. Maybe it was this sort of weather that Shakespeare had in mind in Two Gentlemen of Verona:

O! How this Spring of love resembleth
The uncertain glory of an April day.

These springtime showers are a direct consequence of the growing power of the sun at this season. Strong morning sunshine warms the ground rapidly which in turn warms the lowest layers of air. Bubbles of warm air - thermals - rise, gradually cooling again as they do so. Beyond a certain level, moisture condenses to form clouds, and as the process continues these cumulus clouds grow big enough and deep enough to produce showers.

Most snow at this time of the year in lowland Britain comes in the form of showers, usually in a northerly or northwesterly airflow of Arctic origin. Such snowfalls occasionally cover the ground, but rarely last long. Very rarely, when such an Arctic air-mass has held sway for several days, a developing Atlantic depression makes inroads into the cold air, and heavy snow falls for several hours on the northern flank of the depression.

Such a circumstance befell with dramatic effect in late-April 1908 when a sequence of depressions tracked along the length of the English Channel and snow fell for long periods between the 23rd and 26th. Over the Downs of Hampshire and Berkshire, especially around Andover, Newbury and Abingdon, level snow lay 60cm (2 feet) deep at the end of the snowstorm. The Radcliffe Observatory in Oxford reported a depth of 43cm (17 in), probably the deepest snow of the entire century in the city.

'Easterly' snowstorms, so characteristic of the winter months, when continental arctic air has travelled directly to us from northeastern Europe, are almost unknown in April thanks to the rapid warming of the continent at this season. One did occur on April 14-15, 1966, however, with 15cm (6 in) of snow lying across counties south of the Thames.

By Philip Eden

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