I have been reading that the chair of the UK鈥檚 Environment Agency, Lord Smith of Finsbury, has been warning of 鈥渁 new kind of rain鈥�. Apparently, 鈥渃onvective rain鈥� does not sweep across the country but dumps its deluge in just one place, putting great strain on that particular area. Is this a real and new phenomenon caused by climate change? And if it is, what causes it?
鈥� is by no means a new phenomenon. Indeed, every time the phrase 鈥渟unshine and showers鈥� is used in weather forecasts, convective rainfall is implied. It is caused by a sharp temperature gradient in the lower atmosphere.
鈥淓very time sunshine and showers are mentioned in weather forecasts, convective rain is implied鈥�
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Warm air at the surface of the Earth rises through cold air higher in the atmosphere in columns known as thermals. As a thermal rises it cools, and the moisture contained within it condenses to form towering cumulonimbus clouds. These can then deposit heavy but short-lived bursts of rain or snow.
In summer, the sun can heat the air just above the ground so much that huge thermals rise to give us thunderstorms.
In winter, during a cold snap, warmer air over the sea can gather lots of water and rise into a bitterly cold air mass, causing intense falls of snow. This is sometimes called 鈥渓ake effect鈥� snow, so named after a similar phenomenon over the Great Lakes of North America which gives severe blizzards in winter.
Convective precipitation is very different from frontal precipitation, which is caused by the sideways collision of two air masses of different temperatures forcing warm air to rise and resulting in thick cloud.
Slow-moving convective precipitation can be caused by 鈥渃onvergence zones鈥�, where two warm air masses meet and are forced upwards. This was responsible for the severe flooding in Boscastle in Cornwall, UK, in August 2004.
Although this is not a new phenomenon, warmer air in summer caused by climate change perhaps makes these events more likely.
William Torgerson, York, UK