A supercell is a thunderstorm that is characterized by the presence of a mesocyclone, it is a deep persistently rotating updraft. For this reason, these storms are also known as rotating thunderstorms. It is a continuously rotating updraft deep within a severe thunderstorm (a mesocyclone) and looks downright scary. These are among the most dangerous thunderstorms that occur on the planet, and almost always produce severe weather of one kind or another. Ordinary thunderstorms can pass by with no more than a few rumbles of thunder and a heavy but brief, deluge of rain.
Supercells are often put into three classification types: Classic, Low-precipitation (LP) and High-precipitation (HP). LP supercells are usually found in climates that are more arid, such as the high plains of the United States, and HP supercells are most often found in moist climates. The true sign of a supercell is a cloud dome poking up through the flat top of the anvil at the southwest corner of the storm , the visible sign of the mesocyclone updraft that makes the dangerous weather characteristic of the supercells. If you see the thunderhead with such a cloud dome rising from the flat summit, then this is probably a supercell however, if the thunderstorm is moderately close before you can see it, then the cloud dome will probably be invisible, because you will only be able to see the underside of the anvil from the angle you are looking at the storm from.
Supercells are often carriers of the giant hailstones and although they can occur anywhere in the world they are most frequent in the Great Plains of the US. The first storm to be identified as the supercell type was the Wokingham storm over England, which was studied by Keith Browning and Frank Ludlam in 1962. Some supercells can produce lightning strikes at a rate of 15,000 per hour, which is far in excess of the usual rate in thunderstorms.
Reason : Supercells derive their rotation through tilting of horizontal vorticity (an invisible horizontal vortex) caused by wind shear. Strong updrafts lift the air turning about a horizontal axis and cause this air to turn about a vertical axis. This forms the deep rotating updraft, the mesocyclone.