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 This
newsletter is a joint project of SIRS Publishing,
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MAY 1995 — VOLUME 2, NO. 4
ARTICLES
Thunderstorms represent one of the most intriguing and often-dangerous forms of weather we may encounter. While they are crucial for bringing much-needed summer rainfall around the globe, in their extreme form they are responsible for devastating flash floods, damaging hail falls, and tornadoes. The ability to predict thunderstorms and their attendant weather is thus a major part of meteorological research at the National Center for Atmospheric Research (NCAR).
What makes thunderstorms different from other weather phenomena is the intense vertical motions that accompany them. Updrafts may reach speeds greater than 100 miles per hour (40 meters per second), creating the potential for hailstones as large as grapefruit. Downdrafts tend to be weaker, but as they reach the ground and spread out, horizontal winds greater than 120 miles per hour (50 meters per second) have been observed. And, of course, the airflow in these storms can be concentrated into intense rotating columns or tornadoes with winds exceeding 250 miles per hour (110 meters per second).
Whether it's a slender, graceful tube or a massive, roaring wall of cloud, the tornado is among the most fascinating and frightening of atmospheric phenomena. In a typical year more than 900 tornadoes are reported in the United States, by far the highest frequency reported for any nation in the world. However, U.S. death and injury tolls have dropped considerably in recent decades due to better warnings. Scientists at NCAR and elsewhere are trying to unravel the causes of these evanescent whirlwinds.
Virtually all tornadoes develop out of thunderstorms. Dust devils, the rotating columns of dust you might see on a hot, sunny day, are only distant cousins of tornadoes. However, it is rising air motions that lead to both dust devils and tornadoes. Another ingredient for strong storms is a boundary between air masses, such as a cold front, that pushes surface air upward. Moisture in the warm air adds to the air's potential buoyancy.
As springtime unfolds, the clashing of warm and cold air results in frequent tornado activity in the Gulf Coast states in March, with the zone moving northward and westward to the upper Midwest by June and July. Tornadoes are rare west of the Continental Divide, where moisture for strong storms is usually lacking. However, every state in the union, including Alaska and Hawaii, has reported tornadoes.
It takes more than rising air in a thunderstorm to create a tornado. Somehow, the air must be given a spin. Thunderstorms often develop weak rotation as strong winds aloft, sometimes racing at 100 miles (160 kilometers) per hour or more, impart a spin to the column of rising air. On one side of a severe storm, you can sometimes see clouds moving in a circular fashion. A "wall cloud" may hang from a larger rain-free cloud base. Large hail and heavy rain may occur near the wall cloud.
What finally produces a tornado? By some mechanism not fully understood, the circulation in the storm's lower levels tightens into a narrow cylinder, elongates, and speeds up, much as a figure skater spins faster by pulling in his arms. Computers can simulate the large-scale rotation of a thunderstorm, and a newer NCAR computer model examines the motions in an area as small as a quarter of a mile (half a kilometer) across, providing insight on the tornado formation process. Doppler radar, able to sense the winds in the vicinity of a tornado, is another valuable tool. One Doppler radar trained on tornadoes in west Texas has measured winds of over 200 miles (320 kilometers) per hour.
The outward appearance of tornadoes can vary tremendously, even for the same storm viewed at different times or from different angles. A complete funnel need not be visible from cloud to ground for a tornado to be capable of damage. Large tornadoes may resemble a cloud on the ground rather than a narrow funnel. Especially in the eastern United States, rain may wrap around a tornado's circulation and cut off visibility.
Researchers collect many kinds of information on tornadoes by going into the field. On a storm "chase," scientists take instruments and cameras into the countryside and attempt to document tornadoes as thoroughly as possible. Wall clouds and other visual precursors of tornadoes discovered by chasers are now used by law enforcement and civil defense agencies to issue warnings. The most damaging twisters are often spotted well before they strike populated areas, enabling residents to take cover. The homes of some 20,000 people lay in the path of the 1979 Wichita Falls, Texas tornado, but extensive warnings kept the death toll to a relatively low 42.
Although understanding the formation of tornadoes will not lead to their prevention, better information will save lives and property. Research data can be applied to building codes and zoning for areas prone to tornadic storms. Simple construction techniques, such as bolting roofs to walls, can reduce tornado damage.
The cardinal rule for tornado safety is: get as close to the ground as you can. If inside a building that lacks a basement, find a small, centrally located room such as a bathroom; avoid auditoriums and other places with large free-span ceilings that may collapse. Cover your head and get under a mattress or blanket to help protect yourself from flying debris. If you are in a car, drive at a right angle to the tornado's direction of approach; if time is short, abandon the car and find a ditch or other low-lying area. Cars and mobile homes are where most tornado fatalities occur. Keep in mind that tornadoes can travel at 70 miles (110 kilometers) per hour or faster and can change direction without warning. They usually, but not always, move from west to east or from southwest to northeast. Scientists are growing increasingly concerned about weather enthusiasts who seek out tornadoes. Their advice: think of safety first, stay tuned to weather warnings, and only chase with an experienced guide.
A major research project is under way this spring in the southern Plains to help researchers better understand what spawns tornadoes. VORTEX, the Verification of the Origins of Rotation in Tornadoes Experiment, is deploying as many as 20 vehicles at a time to trace the life cycles of tornadic thunderstorms. Approximately 100 participants will help with the field project. Never have so many storm chasers with so much equipment been trained on individual storms.
This year marks the second spring of field activity in the two-year program. VORTEX's target is supercells, long-lived severe thunderstorms with circulation patterns especially favorable for producing tornadoes. Although supercells produce the most violent tornadoes on earth, less than half of them produce any tornadoes at all.
Field operations for VORTEX are based at National Severe Storms Lab (NSSL) in Norman, Oklahoma. The study area ranges from southern Kansas to northern and western Texas. From April to June, forecasters are starting each day by evaluating the severe weather potential and issuing likelihood forecasts for boxes of about 500 square miles (1,300 square kilometers) that cover the research area.
Because the project's goal is to document potentially tornadic cells from start to finish, field crews typically leave Norman each day well before thunderstorms have formed. Unlike typical storm-chase field programs, in which a few vehicles fan out to document several storms, VORTEX is keeping all of its vehicles on any day concentrated on one cell, providing high-resolution observations.
All vehicles have weather stations mounted atop them that report wind direction and speed, temperature, pressure, and humidity every six seconds. The data go to laptop computers within the cars and to the mobile field coordination center. Four NSSL mobile labs, along with one mobile sounding unit and two stationary units from NCAR, launch weather balloons when needed. Three camera crews film the evolution of tornadoes from as close as a half mile (about one kilometer), providing images that can later be analyzed to compare wind speeds with those measured by Doppler radar. Three aircraft will probe the storms with airborne Doppler radar and two portable Doppler units are being operated by University of Oklahoma (OU) professor Howie Bluestein. In April 1991, Bluestein's portable Doppler measured one tornado's winds at close to 300 miles (480 kilometers) per hour -- the strongest near-ground winds ever documented.
Last year's field phase of VORTEX tracked three tornadic and eight nontornadic supercells. One of the biggest surprises for VORTEX scientists was a storm that produced a tornado near Archer City, Texas on May 29, 1994. With winds estimated at close to 200 miles per hour, the twister moved west, then northeast, than northwest, defying the usual eastward movement of tornadoes. Analysis of the airborne Doppler radar data showed that the tornado's parent circulation developed very rapidly in a manner quite different from that normally thought to produce major tornadoes. Such results, though puzzling, will help VORTEX researchers address their hypotheses on tornado development.
Along with NSSL and NCAR, participants and funding agencies for VORTEX include the National Science Foundation; the OU Center for the Analysis and Prediction of Storms; The National Oceanic and Atmospheric Administration, Texas A & M and Texas Tech Universities; the University of Illinois; the University of California, Los Angeles; and Canada's Atmosphere Environment Service.
RESOURCES
Did you know that the United States has more tornadoes than any other country? Questions about how they form, their unpredictablity and destructive capabilities are often unanswered. SIRS Photo Essays are a useful resource in educating students about the structure of these storms. The "Hurricanes and Tornadoes" unit consists of eight posters, each with a photograph and brief explanation. The posters can be used for bulletin board displays or to stimulate group discussion.
SIRS Photo Essays units are designed to help develop students' inquiry and analytical skills. A study guide is offered with worksheets, exercises and a selected bibliography. New Photo Essays coming soon include: Man on the Moon, The Space Shuttle, Hubble Telescope and The Solar System.
For more information and a catalog, contact SIRS Customer Service
at 1-800-232-SIRS or via e-mail:
custserve@sirs.com
The National Center for Atmospheric Research developed a resource
poster called Thunderstorm Detectives as part of a traveling exhibit. For a
copy of the poster, which features a spectacular photograph of a thunderstorm,
student activities and general storm information, please send a check for $4
to cover postage and handling to: NCAR Thunderstorm Detectives, P.O. Box 3000,
Boulder, CO, 80307 or send e-mail to:
lcarbone@ncar.ucar.edu
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Now
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Corporation for Atmospheric Research
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(Social Issues Resources Series.) Science
Now is published three times during the
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Editor:
Caroline Hanson
Scientific
Editor:
Pat Kennedy
Contributors:
Bob Henson, UCAR Communications;
Morris Weisman, Mesoscale and Microscale Meteorology Division
UCAR is a consortium of over 60 universities in the U.S. and Canada with doctoral programs in atmospheric and related sciences. UCAR manages and operates the National Center for Atmospheric Research under the sponsorship of the National Science Foundation. Any opinions, findings and conclusions or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. Anyone who undertakes any of the activities described herein shall do so at their own risk; UCAR and SIRS Publishing, Inc. assume no liability, whatsoever, for any injury or harm, which may result therefrom.
© COPYRIGHT 1995 UNIVERSITY CORPORATION FOR ATMOSPHERIC RESEARCH. ALL RIGHTS RESERVED.
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