What causes a tornado?

Everybody loves a good suspense story, and movies that depict weather catastrophes rank high up in the movie-goer ratings.  The public eye likes to see people face, battle, and triumph against the natural phenomenon.  However, the adrenaline rush we get when watching a film and the adrenaline spike that occurs in real life are vastly different.  Tornadoes in the movies don’t touch us.  Actual tornadoes are fantastic and fearsome events that deserve high respect and wonder.  Why?  Let’s explore the causes of these tremendous weather wonders.


Tornadoes are rotating wind tunnels that are most visible when accompanied by moisture droplets, dust, and debris in the rotation.  A tornado begins in the clouds – most often a cumulonimbus cloud, or more rarely, a cumulous cloud.  If sufficient wind shear and instability in the atmosphere exist, the conditions are ripe for tornado formation.  Of all the atmospheric types of storms, tornadoes are the most violent.  According to the National Severe Storms Laboratory, the ultimate cause of tornadoes is unknown.  What we do know is that the most deadly tornadoes form from rotating supercell storms.  For a tornado to form, the “ingredients for regular thunderstorms must be in place.  


A regular thunderstorm occurs when there is warm, moist air near the surface of the ground and cooler, drier air above.  The warm air rises forming an updraft.  For a tornado to form, both the right wind shear and instability must be present together.  Fast-moving winds “roll” air beneath them forming a horizontal spiral, or vortex.  Wind shear refers to a combination of the wind speed changing the height and increasing and the wind direction changes.   Instability typically means that the lower atmosphere is humid and warm while the upper atmosphere is unusually cool.  So when the fast-moving horizontal vortex hits the updraft caused by the rising warm air, the vortex begins to change direction from horizontal to vertical.  Thus, you have the classic twister.  The combination of wind shear and instability leads up to tornadic activity, which precedes a cold front with low pressure.  Weaker tornadoes can form when one or the other is stronger, but they must still both be present.  A rain that quickly cools the atmosphere makes storm downdrafts stronger.  These downdrafts play an essential role in tornado formation.


A clearly defined radar circulation is called a mesocyclone.  Meteorologists theorize that once one of the mesocyclones are well underway, the temperature differences around the downdraft of the storm may result in tornado development.  However, this is just a theory.  Some models have shown that the temperature variances matter little, as in the case of the Oklahoma tornado on May 3, 1999.  Regardless, when the supercell winds rotate and blow in the upper atmosphere of the mesocyclone, and they combine with the updraft of the warm air and the cooler downdraft, often provided by rainfall, tornadoes are likely to form.  


Don’t take tornadoes lightly.  Weather and circumstances may change in the blink of an eye, so be “weather smart!”  Take precautions and take cover!