2. What is a
tornado?
• Tornadoes are relatively
small, localized low-
pressure areas associated
with powerful
thunderstorms under
cumulonimbus clouds.
• The “typical” tornado is
820 feet in diameter, with
whirling winds of about
150 miles per hour. The
twisting funnel cloud
typically travels at about
40 miles per hour over the
surface.
• Most tornadoes (but not
all!) rotate cyclonically,
which is counterclockwise
in the northern
hemisphere and clockwise
south of the equator.
7. Tornadoes are most consistently associated with fast-moving cold
fronts that sweep across the midsection of the United States, drawing
warm, moist, tropical air from the Gulf of Mexico.
The cold front is usually associated with a strong low-pressure
storm system that rotates counterclockwise as it swirls across land
in the prevailing westerly wind pattern.
8. Tornadoes can appear from any direction. Most move from southwest
to northeast, or west to east. Some tornadoes have changed direction
amid path, or even backtracked.
Hail, any particular pattern of rain, lightning or calmness, are not reliable
predictors of a tornado threat.
Tornadoes can last from several seconds to more than an hour, however
most tornadoes last less than 10 minutes.
11. Some of the damage caused by tornadoes results from the rapid
passage of low pressure. Most houses are built to withstand downward
pressure from much water, snow, or wind against the structure,
especially weight on the roof. When a tornado passes over a house,
however, the low pressure above, countered with high pressure inside
that cannot leak out quickly enough plus wind pressure under the
eaves, causes the house to appear to “explode” from within.
13. The most widely used method worldwide, for over three
decades, was the F-scale developed by Dr. T. Theodore
Fujita. In the U. S., and probably elsewhere within a few
years, the new Enhanced F-scale is becoming the standard
for assessing tornado damage.
14. The National Weather Service usually can predict severe weather
regions several hours in advance, but the exact location of a tornado
must wait for a visual sighting or the occurrence of a tornado signature
on Doppler radar.
15. Balloons carry instrument packages aloft twice a
day from about 90 of the 250 weather stations of
the continental United States.
16. When dangerous storms begin to develop, Doppler radar is available in
most parts of the nation. Doppler radar detects the wind component
parallel to the radar beam, then examines the pattern of the wind field
to find locations of potential tornadoes.
17. Meteorologists continue to improve their ability to forecast,
locate, and track tornadoes. Space satellites, a worldwide
network of manned weather stations, and sophisticated
computer systems enable meteorologists to “see” weather as it
develops.
