NSF PR 96-23 - May 22, 1996
Media contact: |
Cheryl Dybas |
(703) 306-1070 |
Program contact: |
Jewel Prendeville |
(703) 306-1521 |
This material is available primarily for archival purposes. Telephone
numbers or other contact information may be out of date; please see current
contact information at media
contacts.
Tornado Season Strikes
Spring is tornado season, with about 50 percent of
all reported tornadoes occurring between April and
June. Most tornadoes strike between noon and sunset,
with the eastern two thirds of the United States home
to the greatest concentration of tornadoes on earth.
The National Science Foundation (NSF) supports research
in several fields of atmospheric sciences related
to the study of tornadoes.
NSF researchers have found that the strongest tornadoes
develop in "supercells," long-lived thunderstorms
with large scale cyclonic circulations that can persist
for hours and extend well beyond the storm itself.
These strong circulations are often associated with
the most violent twisters, yet only a small number
of supercells produce tornadoes, says Jewel Prendeville
of NSF.s lower atmospheric facilities section. Recent
data indicate that gust fronts and other wind boundaries
near the ground are important factors in a tornado's
eventual formation.
As storm-spotter and Doppler-radar networks improve
and public awareness increases, the number of reported
tornadoes is rising. From 1953 to 1991, an average
of 768 tornadoes reported per year, but since 1990,
more than 1,000 tornadoes have been reported annually.
The year 1992 produced both the annual record of 1,293
and the monthly record of 399, reported in June. The
1995 total was 1,233.
Tornadoes have killed more than 3,700 people since
1953. Though fatalities have been dropping in recent
years, tornadoes killed 69 people in 1994 -- the most
deaths since 1984 -- and 29 people in 1995.
Tornadoes have been reported in every U.S. state,
but they are most concentrated in .Tornado Alley,
which runs north from central Texas through Oklahoma
and Kansas into eastern Nebraska, South Dakota, and
Iowa. Other areas of high frequency include the Corn
Belt of Illinois and Indiana and the Deep South states
of Mississippi, Alabama, and Georgia.
The typical track length is about one mile for all
tornadoes and 23 miles for the strongest ones. The
longest track length recorded for a single tornado
is the 219-mile track of the Great Tri-State Tornado
of March 18, 1925. The typical track width is only
48 yards, but it can range to more than a mile.
Thunderstorms and their attendant tornadoes typically
move northeasterly or easterly, but tornado tracks
can be erratic. Tornadoes move forward at speeds ranging
from nearly stationary to 60 mph or more, and averaging
20 to 40 mph. Measurements from Doppler radar give
top speeds in the range of 250-300 mph. Most tornadic
winds are below 150 mph.
EXPERT SOURCES ON NSF-FUNDED TORNADO AND SEVERESTORM
RESEARCH:
Dan Breed (303-497-8933)
National Center for Atmospheric Research, Boulder,
Colorado Atmospheric electricity, including cloud
electrification and lightning; aircraft studies; precipitation
development
Howard Bluestein (303-497-8924 until August 15, 1996)
(405 325-3006)
School of Meteorology, University of Oklahoma Observational
studies of tornadoes and severe storms. Note: Bluestein
is on sabbatical at the National Center for Atmospheric
Research, Boulder, Colorado this spring and summer.
Morris Weisman (303-497-8901)
National Center for Atmospheric Research, Boulder,
Colorado Thunderstorm modeling and observation; mesoscale
convective systems
James Wilson (303-497-8818)
National Center for Atmospheric Research, Boulder,
Colorado Short-period forecasting (nowcasting) of
storms, microbursts, and related phenomena, using
Doppler radar
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