Title: Tornadoes
1Tornadoes
2A tornado is a violent windstorm characterized by
a twisting, funnel-shaped cloud. Most tornadoes
spin cyclonically but a few spin
anti-cyclonically.
3The United States experiences more tornadoes than
any other country (over 1200/year)
4Although they form in storms all around the
world, they occur most commonly in a broad area
of the Midwest and South known as Tornado Alley.
Note that tornado alley corresponds to the same
area where intense thunderstorms form in the
Spring.
5A tornado begins in a severe thunderstorm called
a supercell. Supercell a severe thunderstorm
with an intense, long-lived, rotating updraft,
known as a mesocyclone. Tornadoes generally
form toward the trailing end of the thunderstorm.
6April 28, 2002 Aerial view of a supercell
thunderstorm over Southern Maryland. This
thunderstorm was producing twin tornadoes at the
time of this photograph. Photo Credit Steven
Maciejewski
7Within the supercell, wind from two directions
causes a shear that can create a roll of
horizontal currents. These currents are dragged
into a vertical rotation by an updraft in the
thunderstorm. This cell sags to below the cloud
base to form a wall cloud.
8Wall cloud an isolated lowering of a cloud that
is attached to the rain-free base of a
thunderstorm, generally to the rear of the
visible precipitation area. Wall clouds
indicate the updraft of or the inflow to a
thunderstorm.
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16Mammatus clouds a cloud with groups of rounded
pouches sagging from the cloud. These clouds are
indicative of severe thunderstorms and potential
tornadoes.
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18As a tornado evolves, it becomes wider and more
intense. At the waning stages, it can tighten
and begin to spin faster.
19Cordell, OK 22 May 1981
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23Dimmitt, TX 2 June 2005
24Dimmitt, TX 2 June 2005
25Tornado Damage
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30The Fujita scale (F-Scale) rates a tornado's
intensity by the damage it inflicts on
human-built structures and sometimes on
vegetation. The Enhanced F Scale accounts for
different degrees of damage that occurs with
different types of structures, as well as damage
to things other than structures.
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38Lightning is generally very active during the
formation stages of a wall cloud and mesocyclone.
Rapidly rotating wall cloud, with a tail cloud
streaming in from the east.
39Tornado Watch indicates that conditions are right
for a tornado to develop and that the sky should
be watched. Be alert to changing weather
conditions. Tornado Warning indicates a
tornado has been sighted or that radar indicates
one has developed or could develop within
minutes. Warnings will give the location of the
tornado and the area immediately affected by the
warning. When a warning is issued, move quickly
to shelter.
40During a Tornado If at home, go to a basement or
storm cellar, away from windows. If neither a
basement or storm cellar is available, find
shelter under a piece of sturdy furniture such as
a work bench or heavy table and hold on to it.
Use arms to protect head and neck. If at work
or school, go to the basement or inside hallway
at the lowest level. Avoid rooms such as
auditoriums, gymnasiums, cafeterias or large
hallways. If outdoors, get inside a building,
if possible. If unable to get indoors, lie in a
ditch or low-lying area Use arms to protect head
and neck and stay low to the ground. Remember If
you are in a ditch or low-lying area, be alert
for flash floods that often accompany
tornadoes. If in a vehicle, never try to
outrun a tornado. Get out of the vehicle
immediately and take shelter in a nearby
building. If there is no time to go indoors, get
out of the vehicle and lie in a ditch or
low-lying area between the vehicle and the
tornado. Do not take shelter in a ditch downwind
of the vehicle. Use arms to protect head and
neck. If in a mobile home, get out of the
mobile home quickly, take shelter in a building
with a strong foundation. If shelter is not
available, lie in a ditch or low-lying area
between the tornado and mobile home. Do not take
shelter in a ditch downwind of the mobile home.
If a tornado hits it, debris could fall on top of
you. Use arms to protect head and neck.
41Cones
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44Wedges
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48Stovepipes
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56Elephant Trunks
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