To a layperson, any funnel-shaped column of air is a tornado, but meteorologists tend to be more discriminating. They classify twisters into two broad categories -- supercell and nonsupercell -- based on how they form and how much damage they inflict. For a storm to be called a supercell tornado, it must have three characteristics [source: Williams]:
- It must form within a massive thunderstorm and be pendant, or suspended, from that storm.
- It must come in contact with the surface of the Earth.
- It must have wind speeds in excess of 65 mph (105 kph).
Contrary to popular belief, a tornado doesn't have to take the shape of a funnel sidewinding its way across the sky. Those without a funnel will appear as a broad cylinder or cone, sometimes a mile wide, swirling near the ground. Either way, these types of large tornadoes can cause significant damage, dismantling homes and buildings, flinging vehicles and uprooting trees.
Any other tornado that fails to meet these standards would fall into the nonsupercell category. Waterspouts, for example, are nonsupercell tornadoes that form over a body of water. Although they look, at first glance, like their larger, land-based buddies, they form in different ways, arising from cumulus clouds that haven't become thunderstorms and may never mature into thunderstorms. As a result, they have shorter life cycles and often wind speeds in the range of 33 to 45 mph (53 to 72 kph); that is, they rage a lot less than their stronger tornado cousins.
The same holds true for landspouts and gustnadoes. Landspouts occur over solid land, but they look and act like waterspouts. As a result, they cause far less damage than supercell tornadoes. So do gustnadoes, which are weak, short-lived (but awesomely named) tornadoes that can arise along the boundary between descending cold air and warm air at the surface. Meteorologists refer to such a boundary as a gust front, and although the strong winds spawned by such a front signal an approaching thunderstorm and possible supercell activity, they're not themselves full-blown tornadoes.
Finally, weather scientists recognize a third class of tornado-like storms known as whirlwinds. These common types of atmospheric systems occur when the sun heats dry terrain and causes a column of warm air to rise rapidly. As it does, the column of air will whirl, or rotate around a vertical axis, much like water draining from a basin. They become visible when they pick up debris from the ground and are often named to reflect the nature of that debris: dust whirls (or dust devils), sand whirls, snow whirls, even hay whirls.
Fire tornadoes develop when a blaze, not the sun, heats air above the surface of the Earth. They're not really tornadoes at all, but a special type of whirlwind (makes sense why they're known as fire whirls or fire devils in many parts of the world). Although they occur far less regularly than dust devils, fire tornadoes can develop readily over large fires. And they obey many of the same principles that govern the formation of true tornadoes.