How does a storm cloud grow a tornado, and why is it rated by its damage?

The strongest tornadoes usually grow when a rotating storm, a supercell, lowers its spin to the ground, and because conventional instruments cannot survive the winds, we rate tornadoes by the damage they leave. A tornado is a rapidly rotating column of air from the ground to the base of a cumulonimbus or cumulus cloud. Most have winds under 180 km/h and are about 80 metres across, but the most extreme can exceed 480 km/h and be over 3 kilometres wide.

The usual recipe starts in a supercell, a storm with a mesocyclone, an area of organized rotation a few kilometres up. Most intense tornadoes, EF3 to EF5, develop from supercells. The life cycle begins when increasing rainfall drags down a pocket of quickly descending air called the rear flank downdraft. It accelerates toward the ground and drags the rotating mesocyclone down with it. Meanwhile the updraft creates low pressure at the surface, which pulls the mesocyclone down as a visible condensation funnel.

The end comes when the downdraft wraps around the tornado and chokes off its air supply. The vortex weakens and turns thin and rope-like. Even so, researchers admit that many aspects of tornado formation, such as why some storms form tornadoes while others do not, are still poorly understood.

Now, why the damage rating? Direct measurement of the winds of the most violent tornadoes is nearly impossible, because conventional anemometers cannot withstand the intense winds and flying debris. So the Enhanced Fujita scale rates tornado intensity from the severity of the damage and gives only an estimate of actual wind speeds.

One last lesson in safety. Many people think a highway overpass is a good shelter. It is not: tornadic winds accelerate in the confined space, and in the 3 May 1999 Oklahoma outbreak three overpasses were directly struck, with a fatality at each.