Transcript Tornadoes: What and why they occur

Tornadoes: What and why
they occur
Kevin Scott
&
Kevin Fitzgerald
What is a Tornado?
 A local storm of short
duration.
 Violent wind storms
that take the form of a
rotating column of air
extending down from a
cumulonimbus cloud.
 Produces severe
thunderstorms, heavy
winds, rainfall, & often
damaging hail.
Occurrence & Development
 Meteorologists are not sure what triggers
tornado formation.
 Can form in any situation that produces
severe weather.
 Important precondition linked to tornado
development in severe thunderstorms is the
development of a mesocyclone.
– Mesocyclone is a vertical cylinder of rotating air,
about 2 to 6 miles across, that develops in the
updraft of a severe thunderstorm.
Atmospheric Conditions.
 Where cold dry
continental polar air
meets the warm humid
tropical air.
 The greater the
contrast when these
air masses meet, the
more intense storm.
 Tornadoes occur
slightly ahead of the
cold front, in the zone
of southwest winds.
Where Tornadoes Occur
 Most tornadoes occur
in the central part of
the United States
known as “Tornado
Alley.”
– Climatologists can only
conclude that the
weather conditions in
this area are particularly
suited for tornado
development
Watches, Warnings, & Doppler
 Tornado watches
– For areas already identified in severe weather
outlooks. Watch covers about 25,000 square
miles for 4-6 hours.
 Tornado warnings
– When a tornado has actually been spotted.
Last for about 30-60 minutes.
 Doppler radar
– Can detect the initial formation and
development of a mesocyclone.
Fujita Tornado Damage Scale.
 F0 -Light Damage (<73 mph); Some damage to
chimneys; branches broken off trees; shallowrooted trees pushed over; sign boards damaged.
 F1 - Moderate Damage (73-112 mph); Peels
surface off roofs; mobile homes pushed off
foundations or overturned; moving autos blown off
road.
 F2 - Considerable Damage (113-157 mph); Roofs
torn off frame houses; mobile homes demolished;
boxcars overturned; large trees snapped or
uprooted; light-object missiles generated; cars
lifted off ground.
Fujita Scale Continued…
 F3 - Severe Damage (158- 206 mph); Roofs and
some walls torn off well-constructed houses, trains
overturned; most trees in forest uprooted; heavy
cars lifted off ground and thrown.
 F4 - Devastating Damage (207- 260 mph); Wellconstructed houses leveled; structure with weak
foundations blown off some distance; cars thrown
and large missiles generated.
 F5 - Incredible Damage (261- 318 mph); Strong
frame houses lifted off foundations and swept
away; automobile sized missiles fly through the air
in excess of 100 meters (109 yards); trees
debarked; incredible phenomena will occur.
What the Fujita Scale Really Means
Tornadoes
 Average of 770 tornadoes reported annually in the
United States.
 Occur most April through June.
 Lowest occurrences December and January.
Where Tornadoes Occur
•This graph, taken from
Lubbock, TX, (located
in the heart of “Tornado
Alley”) shows a definite
annual tornado cycle.
•This graph shows that in
Hattiesburg, Mississippi
has no discernable annual
tornado cycle.
Profiling Tornadoes
 Average tornado has a
diameter of 500 – 2000
feet.
 Travels at about 30 miles
per hour.
 On average cuts a path
about 6 miles long.
 Maximum winds beyond
310 miles per hour.
 Typically tornadoes move
towards the northeast.
Continued…
 63% of tornadoes are
weak (F0-F1)
 2% of tornadoes are
classified as violent
(F4-F5)
– Account for 70% of
tornado related
deaths.
 Tornadoes most
difficult natural
phenomena to
forecast precisely.
Two Days of Devastation
 April 3-4, 1974, super tornado outbreak. It was the worst
tornado outbreak in U.S. history with 148 twisters touching
down in 13 states. Before it was over 16 hours later, 330
people were dead and 5,484 were injured in a damage
path covering more than 2,500 miles.
A Final Image
An F-4 in Kansas