A supercell thunderstorm is a powerful type of thunderstorm formed from a particular combination of winds, moisture, and temperatures, and often produces destructive weather effects such as tornadoes and hail. The one characteristic that makes a supercell unique is the rotation of the air mass inside the storm. When a part of the rotating column of air extends from the bottom of the storm toward the ground, it becomes a tornado. Even if a supercell does not produce a tornado (not all of them do), it is still an awesome display of nature's power, and an impressive sight to behold.
Although there is much about the formation of supercells that scientists still do not fully understand, there seem to be a number of key ingredients common to all supercell storms. The first is wind shear, which is a condition caused by winds at different levels in the atmosphere blowing at different speeds, in different directions, or both. At the boundary between these two moving masses of air, the offset motion causes rotation, and forms a horizontal, spinning column of air; the greater the difference in speed or direction of the air masses, the faster the rotation of the horizontal column. If the air near the ground is warm, it begins to rise, pushing the column upward into a vertical position. This is called a mesocyclone, and is the signature of a supercell.
Since warm air usually contains moisture due to evaporation from the Earth's surface, as the warm air rises the moisture is carried upward where it condenses in cooler air higher in the atmosphere and forms clouds. The supercell then develops its characteristic towering shape, and when the clouds reach a high enough altitude, usually around 50,000 feet, strong high-level winds spread the cloud tops out, so that the biggest storms take on the appearance of an anvil. Most of the rain and hail associated with a supercell falls on the side of the storm where the anvil forms, because the condensed moisture is no longer being held aloft by the rising, rotating column of air in the heart of the storm.
The majority of supercell storms do not produce tornadoes; field studies suggest that perhaps as few as 20 percent actually do, and the process is not completely understood. Different theories have been suggested to explain why some supercells spawn tornadoes and others do not. One theory is known as the Dynamic Pipe Effect, and seems to occur when the rotation that forms a supercell begins at higher altitudes. A partial vacuum is created inside the "pipe" of rotating air, drawing air into its center from near the ground. As more air enters the pipe it begins to rotate as well, gradually building downward until it emerges from the bottom of the supercell as a tornado. But this is just one of many possible causes that scientists are still studying.
Supercells can form anywhere two different air masses collide in just the right way, but they are most common in the central U.S., in the region known as "Tornado Alley." Here, especially during the warm summer months, cool, dry air moving from the northwest collides over a wide expanse of flat land with warm, moist air moving from the Gulf of Mexico in the south. Because the conditions are so ideal for the formation of supercells and tornadoes, weather researchers flock to the area each year to study these impressive storms. That, along with the romanticized notion of storm chasing popularized by movies such as "Twister" and various television programs, has started a dubious tourist industry, with vacationers spending weeks hoping to catch sight of a tornado. The tourist dollars are undoubtedly appreciated by the local businesses, and storm chasing may be exciting, but as the old saying goes, "it's always fun until someone gets hurt." Supercell storms, whether they produce tornadoes or not, are powerful and sometimes deadly, and are not something to be taken lightly.
National Severe Storms Laboratory information about supercells and tornadoes: