Rotation and Tornado Formation
How does a tornado form? Below is one example of how a typical tornado may form.
There is still much we don't know about tornadoes, but this gives you a good idea of how
they might form under these conditions. Warm moist air shoots upward meeting colder, dryer
air. Warm moist air is lighter than the cold dry air making for a strong updraft
within the thunderstorm. As the warm moist air rises, it may meet varying wind directions
at different altitudes.
Read all about
wind shear here (Pictures included)
If these varying winds are staggered in just the right manner with sufficient speed,
they will act on the upward rising air, spinning it like a top. This would be
similar to spinning a pencil held between your hands with your
palms facing each
other, moving your hands in opposite directions. The storm will begin to show visible
rotation, often forming a wall cloud. Inside
the storm these spinning winds can begin the formation of a tornado. On the outside of the
thunderstorm rotation might be visible as in this photo and here.
Only about 30 percent of mesocyclones actually produce a tornado, but about 95 percent
produce severe weather. Most of our information about the inside of a
tornado-generating thunderstorm comes from data gathered by Doppler radar. The
illustration (1a) would be an
example of how a tornado would typically form in this type of thunderstorm.
What makes a tornadic thunderstorm more severe than an ordinary
Much of the difference is located inside the storm, and what the
winds affecting the storm are like. In your everyday thunderstorm you have the warm moist
air rising causing updrafts within the storm, and colder air descending causing
downdrafts. Typically these thunderstorm downdrafts will be separate from the
downdrafts caused by the rain falling. Rain causes a downdraft because it is cooler
than the warm rising air. As an ordinary thunderstorm matures the falling rain cause
more down drafts, and finally begins to negatively affect the updraft that builds the
storm. This would cause a typical thunderstorm to weaken, less updraft, less power.
But in a severe thunderstorm the downdrafts may be so strong that they
pull the moisture or falling rain into them further strengthening the down drafts. As
these down drafts intensify, they can actually act like a wedge increasing the intensity
of the updraft. These updrafts can get so strong they even intrude into the stable
stratosphere. When a storm updraft breaks into the stratosphere this condition may be
referred to as overshooting. When updrafts are this strong it becomes possible to
suspend hail for long periods of time further adding moisture to the hail, sometimes
building it to incredible sizes. Grapefruit
sized hail is possible within severe storms. (cont.
Tornado frequency and Location
Where and when tornadoes are most likely
Chasing Dryline Storms
Upper air - Jet streams and chasing
|(cont) In a severe thunderstorm the
down drafts and updrafts actually feed each other giving the storm a life of its
own. Whereas an ordinary
thunderstorm might only last 30 minutes to an hour a severe
thunderstorm self generates its life so that it may last several hours or more. Severe
storms produce downdrafts that can get very strong causing a microburst, as in this photo.
These can be as damaging as an f1 tornado, toppling mobile homes,
knocking down trees, damaging buildings, and even throwing
items across town.
(Video of Microburst)
Printable Version 3.8 mb
Many times the damage caused from a microburst is mistakenly thought to have been by a tornado. This is
because the damage they leave can be so severe it looks like a
tornado hit. But the
damage path from a microburst is unidirectional, leaving a straight-line path of
destruction, whereas a tornado spreads debris all directions and leaves a circular path of
destruction. This is why the National Weather Service conducts inspections of the damage
from severe storms, to determine the cause of the destruction. People in mobile homes
should understand this because it may save their life. Even if there arent any
tornado watches or tornado warnings issued, if a microburst hits your mobile home it can
pickup your mobile home and topple it. You could be seriously injured or even killed by a
microburst if you dont have adequate shelter. So when the National Weather
Service issues a severe thunderstorm watch or warning, be on the lookout for down drafts,
they can be dangerous.
Besides dangerous downdrafts and powerful updrafts, a severe thunderstorm usually is acted
on by strong wind sheer, winds changing speed and direction with height. These winds act
on the updrafts and down drafts causing the entire storm to develop strong rotation. This
rotation is visible by Doppler radar. Putting all these ingredients together makes for a
What is Wind Shear
A thunderstorm is classified as a mesocyclone when cyclonically
rotating air is detected within it as seen in this
The National Weather Service uses Doppler radar to detect this rotation
so it can keep the public apprised of the severity of the storm. Doppler radar is also
used to see into the rotation of the storm looking for a tornado signature. A tornado
signature or tornado vortex signature shows up as an area within the storm with rapidly
changing wind directions. This is used to issue a tornado warning for people in the
path of the storm often well before the tornado is even on the
ground saving many lives every year.