Heavy Rain Triggers Destructive Tornadoes

Aug. 22, 2008 -- When the spring skies blacken over the Midwestern United States and the clouds begin to rotate ominously, residents know a tornado could be on its way.

But just how the deadly twisters form is a mystery that has puzzled scientists for decades. They know a range of atmospheric conditions must be in place, including strong, closely paired up- and downdrafts, windshear at high altitudes, and usually a foreboding supercell thunderstorm, spinning slowly in the sky.

Amid this stormy cocktail, new research suggests that the heavy rains that often accompany supercells may be key in triggering tornadoes to form. According to Robert Davies-Jones of the National Severe Storms Laboratory in Oklahoma, rain falling in heavy sheets from a supercell storm cloud provides a strong push to updrafting air, causing it to spin up into a fully fledged twister.

"You can have a rotating storm but until you have rain you don't get a tornado," Robert Davies-Jones of the National Severe Storm Laboratory said.

Scientists have known this simple fact since 1953, when radar first allowed meteorologists to peer into supercells. In tornadic storms, they noticed the rain swirled into a hook shape. The feature, called a 'hook echo' quickly became known as a tell-tale radar marker for a tornado.

But the swirling rain was regarded as a by-product of a tornado, merely the effect of its powerful corkscrewing winds. Davies-Jones believes the opposite may be true.