What causes tornados —

how a great air mass begins its lethal swirling pattern

What causes tornados?  Lanney, Sandia Park, New Mexico

From late winter to early summer, tornados sweep across parts of the country, terrifying those in its path.  In its 10-minute average lifetime, the dark, slanted vortex can wreak devastation.  On the 18th of March in 1925, the Tri-state tornado raced across 220-miles of Missouri, Illinois and Indiana, killing nearly 700 people, and leveling houses to their foundations.  What causes this most violent of all storms?

How tornados form. Image courtesy of the National Severe Storms Laboratory.  Larger image.

Though fearsome, the funnel shrinks to mote size compared to the black mother cloud (hovering above and towering up to 50,000 feet) — which spawns a tornado.  The birth sequence for dangerous tornados, says NOAA's National Severe Storms Laboratory, follows a pattern.

The spawning supercell thunderstorm (called a mesocyclone) starts building about an hour or so before a tornado forms.   Various factors can bring a thunderstorm into being, but a rotating updraft is critical for development that spawns tornados. Shear winds can start the updraft column swirling. 

For example, a ground wind, slowed by friction against the Earth, pokes along at about 5 mph, coming from the southwest.  Whereas, winds about a mile up, race at 25 mph.  As the two winds, blowing from different directions, shove against a horizontal tube of air, the air moves.  The southwest wind, blowing harder, starts the air tube rotating in a counterclockwise direction, somewhat like a pinwheel (whose stick is held horizontally) starts to spin. 

The rising column of air tilts the horizontal spinning tube of air up, so now the rotating column extends up through much of the supercell thunderstorm.  See the red spiral line in the figure.  Given that start, the spiraling air must be fed by warm, moist air from below, at ground level.  Then a tornado can form.

We are still puzzling out details.  We think tornado development might be related to temperature differences across the edge of the downdraft air (blue arrow) wrapping around the spiraling upward air.  But this idea fails sometimes. 

"In fact, very little temperature variation was observed near some of the most destructive tornadoes in history on 3 May 1999," writes lead forecaster Roger Edwards of NOAA's Storm Prediction Center.  Moreover, only about 20 percent of supercell thunderstorms actually produce a tornado. 

We continue experimenting and learning more of a tornado's mechanism.  Difficulties abound, not the least being tornados destroy instruments.

Further Reading

The online  tornado FAQ, by Roger Edwards, NOAA

A severe weather primer: Questions and Answers about tornadoes, National Severe Storms Laboratory

(Answered May 11, 2009)