Have you ever seen tall pillars rising up from the clouds when you look out of the window of an airplane? Those spectacular formations, which can rise 50,000 to 60,000 feet high, are supercells, the most violent of thunderstorms. They look peacefully majestic from the air, but people on the ground are probably experiencing thunder and lightning, large hail and damaging downbursts of wind. Those cauliflower-like contours made of water droplets and ice crystals hide the activity of winds as high as 170 mph. About 30 per cent of supercells turn into tornadoes.
A supercell is characterized by vertical wind shear (a change in wind direction and/or speed) and a steady, strongly rotating updraft called a mesocyclone, which continues to suck up warm air, allowing the storm to sustain itself for hours. Ordinary thunderstorms quickly wear themselves out when the falling rain cools the air and keeps it from rising. The rotating, rising column of the mesocyclone preserves the shape of the storm because it keeps precipitation from falling. Sometimes, if it is not hidden by surrounding clouds, it is possible to see a twisted corkscrew cloud in the heart of the storm.
A supercell starts just like any other thunderstorm, with warm, humid air in the lower atmosphere and much colder air aloft. If there is surrounding, colder air, the warm air rises because it is lighter than the cold air. The greater the temperature differential, the faster the warm air shoots upwards. The water vapor in the air condenses into cloud droplets or sublimates into ice crystals, releasing more heat, which adds to the storm's power.
The winds at various heights vary in speed and direction. With just the right combination of winds pushing against the rising column of air, a twisting movement can be initiated. A complex pattern of rising and falling air keeps rain and hail from dropping back and cooling the warm humid air which powers the storm. Once the mesocyclone has started twisting, it continues to suck more and more warm air into the heart of the storm, giving it more power. As long as falling rain and hail doesn't cut off the supply of warm air, the supercell continues, sometimes moving hundreds of miles.
If the air is rising quickly enough, the water droplets do not turn into ice crystals, even when the temperature is well below freezing. Water droplets reflect a lot of light, making the cloud look solid. In parts of the clouds where the air is rising more slowly or not at all, ice crystals are formed instead. These are larger than the water droplets, and reflect less light, giving the cloud a softer look.
Exceptionally fast rising air overshoots the top of the air column, creating a dome. If a dome lasts longer than about ten minutes and has a hard look, the storm is probably a supercell. Take cover!
Sources and rescources:
photos of supercells
explanation of supercells