What Causes Tornadoes?
Answered by Justin Caudell
Tornadoes form in unusually violent thunderstorms when there is sufficient
instability and when there is wind shear present in the lower atmosphere. Instability
refers to warmer and more humid than usual conditions in the lower atmosphere,
and possibly cooler than usual conditions in the upper atmosphere. Wind shear in
this case refers to the wind direction changing, and the wind speed increasing, with
height. An example would be a southerly wind of 15 mph at the surface, changing
to a southwesterly or westerly wind of 50 mph at 5,000 feet altitude.
This kind of wind shear and instability usually exists only ahead of a cold front
and low pressure system. The intense spinning of a tornado is partly the result of
the updrafts and downdrafts in the thunderstorm caused by the unstable air and
then interacting with the wind shear, causing a tilting of the wind shear to form an
upright tornado vortex. Helping the process along, cyclonically flowing air around
the cyclone, already slowly spinning in a counter-clockwise direction (in the
Northern Hemisphere), converges inward toward the thunderstorm, causing it to
spin faster.
This is the same process that causes an ice skater to spin faster when he or
she pulls their arms in toward the body.
Other processes can enhance the chances for tornado formation. For
instance, dry air in the middle atmosphere can be rapidly cooled by rain in the
thunderstorm, strengthening the downdrafts that are needed for tornado
formation. Notice that, in virtually every picture you see of a tornado, the tornado
has formed on the boundary between dark clouds (the storm updraft region) and
bright clouds (the downdraft region), evidence which shows the importance of
updrafts and downdrafts to tornado formation.
Also, an isolated strong thunderstorm just ahead of a squall line that then
merges with the squall line often becomes tornadic. Isolated storms are more likely
to form tornadoes than squall lines, since an isolated storm can form a more
symmetric flow pattern around it and also have less competition for unstable air
than if it were part of a solid line of storms.
Because both instability and wind shear are necessary for tornado information,
sometimes weak tornadoes can occur when the wind shear conditions are strong,
but the atmosphere is not very unstable. This sometimes happens in southern
California in the winter, for instance, when a strong low pressure system comes
ashore. Similarly, weak tornadoes can occur when the air mass is very unstable,
but has little wind shear. For instance, Florida has many weaker tornadoes of this
variety. Of course, the most violent tornadoes occur when both strong instability
and strong wind shear are present.
In the United States, tornadoes are most commonly found in the mid west.
Overall, the U.S. experiences more tornadoes than anyplace else in the world.