October 1, 2000
Q: How do tornadoes form?
A: The classic answer—"warm moist Gulf air meets cold Canadian air and dry
air from the Rockies"—is a gross oversimplification.
Many thunderstorms form under those conditions (near warm fronts, cold fronts
and drylines respectively), which never even come close to producing
tornadoes. Even when the large-scale environment is extremely favorable
for tornadic thunderstorms not every thunderstorm spawns a tornado. The
truth is that we don't fully understand. The most destructive and deadly
tornadoes occur from supercells—which
are rotating thunderstorms with a
well-defined radar circulation called a mesocyclone. [Supercells can also
produce damaging hail,
severe non-tornadic winds, unusually frequent
lightning,
and flash floods.]
Tornado formation is believed to be dictated mainly by things which happen
on the storm scale, in and around the mesocyclone. Recent theories and
results from the VORTEX program suggest that once a mesocyclone is
underway, tornado development is related to the temperature differences
across the edge of downdraft air wrapping around the mesocyclone
(the occlusion downdraft). Mathematical
modeling studies of tornado
formation also indicate that it can happen without such temperature
patterns; and in fact, very little temperature variation was observed
near some of the most destructive tornadoes in
history on May 3, 1999.
