Washington, August 23: A new research has suggested that heavy rains that often accompany thunderstorms may trigger off destructive tornadoes.

Till now, 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.

Now, according to a report in Discovery News, 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.

"The hook echo is usually thought to be a passive feature of tornadoes," he said. "I''m saying it''s not passive, it''s an active mechanism for tornado formation," he added.

Davies-Jones ran computer simulations of supercell storms to see if falling rain could provide the needed kick that turned diffuse updrafts rising off the warm plains into tight-spinning, lethal tornadoes.

As the rain falls out of a rotating supercell cloud, it is also twisting, and as it falls, he found it transfers the rotational energy into the updrafting air adjacent to it.

The rain also acts as a sort of wall, confining the swirling, rising air.

As it continues to head skyward, the air inside the rain curtain stretches out like a figure skater raising her arms. The spinning speeds up, and a tornado is born.

"The mechanism is a good one," David Lewellen of the University of West Virginia said. "But until these things are seen more conclusively out in the field, it''s not at all clear whether rain is involved in the formation of most tornadoes, a few, or none at all," he added. (ANI)