How do they form?
We need three basic ingredients to make a thunderstorm. The basic fuel
is moisture (water vapor) in the lowest levels of the atmosphere. The
air above the lowest levels has to cool off rapidly with height, so that
2-3 miles above the ground, it is very cold. Finally, we need something
in the atmosphere to push that moist air from near the ground up to
where the air around it is cold. This "something" could be a cold front
or the boundary between where the cold air from one thunderstorm meets
the air outside of the storm (called an outflow boundary) or anything
else that forces the air at the ground together. When that happens the
moist air is pushed up. What happens to a blob of moist air as it
rises? It cools off and after a while, some of the water vapor turns
into liquid drops (that we see as clouds). That warms up the rest of
the air in the blob so that it doesn't cool off as fast as it would if
the air was dry. When that blob of air gets to the part of the
atmosphere where it is very cold, it will be warmer and less dense than
the air around it. Since it is less dense, it will start to rise faster
without being pushed, just like a balloon filled with helium does. Then
more water vapor turns into liquid in the blob and the blob warms up
more and rises even faster until all of water vapor is gone and the blob
eventually reaches a part of the atmosphere where it isn't warmer than
the environment (typically 5-10 miles).
How are they detected?
We can see thunderstorms with a variety of tools. Radars let us see
where rain and hail are located in the storm. Doppler radars also let
us see how the wind is blowing within and near the storm. Some features
of thunderstorms, such as the anvil that spreads out at the top of the
storm, can be seen from satellites.
What type of damage can they cause?
Many hazardous weather events are associated with thunderstorms.
Fortunately, the area affected by any one of them is fairly small and,
most of the time, the damage is fairly light. Lightning is responsible
for many fires around the world each year, as well as causing deaths
when people are struck. Under the right conditions, rainfall from
thunderstorms causes flash flooding, which can change small creeks into
raging torrents in a matter of minutes, washing away large boulders and
most man-made structures. Hail up to the size of softballs damages cars
and windows, and kills wildlife caught out in the open. Strong (up to
more than 120 mph) straight-line winds associated with thunderstorms
knock down trees and power lines. In one storm in Canada in 1991, an
area of forest approximately 10 miles wide and 50 miles long was blown
down. Tornados (with winds up to about 300 mph) can destroy all but the
best-built man-made structures.
How does atmospheric pressure change in and around thunderstorms?
Conditions in the atmosphere change a lot over a small distance in the
vicinity of thunderstorms. Where the rain is falling, the pressure goes
up by a few millibars (about 0.1 inches of mercury). This is because as
the rain falls, some of it evaporates, which makes the air cooler and
heavier. Another process is going on, however, that makes the picture
complicated. As the air goes up in the thunderstorm's updraft, it
creates an area of low pressure under the updraft that acts to pull air
in from around the thunderstorm. This low pressure region is also
typically a few millibars lower than the environment of the storm. At
the top of the storm the pressure is high compared to places far away
from the storm and air is blown out.
Why would the sky appear orange after a
thunderstorm?
Most thunderstorms occur in the late afternoon.
By this time of day, the sun is setting. The orange hue is
caused by the same reason sunsets vary from yellow to orange to red: shorter
wavelengths of light (blue) are scattered quickly, leaving only the yellow-orange-red
end of the spectrum. For further explanation on the optics of sunsets,
including pictures and diagrams, see Weather World 2010 (you can use the
"up" arrow at the bottom of the page to go backward and learn more
about the mechanisms of light and optics in the atmosphere): http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/opt/air/sun.rxml
Is there such a thing as software that helps track thunderstorm development?
There is no computer program available that;
By the way, there are computer algorithms that run on our display of the WSR-88D
NEXRAD radar that track storms and supercells...but they are not used much. Why?
Because they are still not as good as a trained radar operator at looking at the
structure of the storms.
Thunderstorms may be 10 to 15 miles in diameter, and have average lifetimes of
20 to 30 minutes. Add to this the fact that even PhD researchers can't agree on what
exactly a supercell is...and maybe you can see the problems.
I have heard Meteorologists mention numbers like -6 to -10 when talking about thunderstorms. What does this mean?
When Meteorologists talk about -6 to -10, or numbers like that, they are
probably referring to the Lifted Index (LI). This is a measure of how unstable
the atmosphere is at any given moment when the measurement is taken, usually by
a weather balloon. The larger the negative number, the more unstable the air is
and the greater the chance of forming thunderstorms with violent updrafts, hail,
strong winds, and perhaps even tornadoes.
Can I get data from NSSL about certain thunderstorms?
NSSL doesn't generally keep records about thunderstorms and tornadoes, but
your local NWS office might
be able to help. Try contacting them!
What is a derecho?
A derecho is a fast-moving windstorm that is made up of thunderstorms that
repeatedly develop along the leading edge. These lines of storms can move
very quickly and produce widespread straight-line winds over long periods
of time. Derechos can move anywhere from 35-70 mph and last 8 hours or more.
Most derechos that produce severe weather move at speeds greater than 50
mph. Warm season events probably move a little slower than cold season events.
For additional information and links to other sites check the Thunderstorm section of the NSSL Web Guide.
Last updated July 28, 2004