The northern hemisphere springtime marks the pinnacle of severe weather in the US as violent thunderstorms develop over the midwest and Great Plains. Most of this severe weather is spawned from a specific type of thunderstorm: the supercell.
One of the first studies on supercells was conducted by the British meteorology professor Keith Browning in 1962, which described in detail a violent thunderstorm that struck Wokingham, England, in 1959. Unlike most kinds of thunderstorms, a supercell is required to have a mesocyclone at its heart. A mesocyclone is a deep, persistently rotating updraft that, in essence, suggests the rising motion within a thunderstorm is spiralling its way upwards into the atmosphere.
These rotating updrafts allow supercell thunderstorms to hold their structure for long periods, generating copious amounts of lightning, large hailstones and violent downpours of rain. The spinning motion of the storm can, in some circumstances, become focused to ground-level with the formation of tornadoes.
So far this spring, several severe weather outbreaks in the US Great Plains have spawned supercell thunderstorms, some of which have produced deadly tornadoes. Such storms are much rarer in the UK but they do happen. The most recent supercell outbreak was on 28 June 2012, when a series of storms across the Midlands generated large hailstones and even a tornado.
