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FEBRUARY 1996 — VOLUME 3, NO. 2
ARTICLES
While most glider pilots seek the gentle updrafts to soar through the skies, the pilots who fly the sailplane operated by the National Center for Atmospheric Research (NCAR) seek the potentially hazardous updrafts of a towering cumulus cloud to study embryonic thunderstorms. The new IMAX film Stormchasers features NCAR's Schweizer SGS2-32 sailplane heading into a developing thunderstorm cloud.
Because a sailplane can ride atop ascending parcels of unstable air called thermals, it is ideally suited for studying how those air masses feed into thunderstorms and how the storms' electric fields are generated. The unique sampling ability of the sailplane provides continuous coverage of the cloud region where development of precipitation and electrification occurs. Many scientists believe that the way positive and negative electrical charges develop on particles within a cloud is essential to a thunderstorm's evolution.
On the scale of a single thunderstorm, lightning is a discharge, or a means of releasing the tremendous electrical energy built up by the storm. But on a global scale, thunderstorms actually separate charge. Lightning and other storm-related electrical features act to maintain a permanent potential of some 300 kilovolts between the earth's crust, which is negatively charged, and the ionosphere (well above 30 miles, or 50 kilometers), which is positively charged. In between, the slightly conductive lower atmosphere allows current to flow between the two regions. Were it not for the constant recharging from thunderstorms, the Earth-atmosphere potential would disappear in a mere five minutes.
Why does lightning enhance the global electric field instead of dispelling it? The answer lies in the structure of thunderstorms. For reasons unclear, but probably involving millions of collisions among ice crystals and large ice particles, storms evolve with positive charge near their top and negative charge from their middle to cloud base. In a typical cloud-to-ground strike, negative charge descends from cloud base to ground. In response, trees, poles, and other objects release positive charge upward, thus keeping the earth's overall charge negative.
Many of the details in this general picture remain fuzzy. Clouds vary greatly in their ability to become electrified and produce lightning and the process of charge separation still puzzles scientists. This active research topic at NCAR is investigated with the sailplane. Scientists have used the glider since the 1970s to study cloud physics and the early stages of thunderstorms in Colorado, Florida, Montana, New Mexico, and Wyoming. Instrumented to look at cloud droplets, cloud water content, precipitation particles, and electric fields, the aircraft has been used to study wave clouds, precipitation development, and most recently the electrification in cumulus clouds.
The sailplane is towed and released near 12,000 feet (3.6 kilometers), a height usually near cloud base on the High Plains, but well within growing storms over Florida. Rising in a circular pattern, the sailplane samples atmospheric and electrical conditions with its probes, electric field mills, and an induction ring that can measure the charge on a single cloud particle.
Among other things, NCAR's sailplane studies have found that:
In the summer of 1995, scientists used the sailplane in northeast Colorado to study water and ice droplets inside young storms. The goal of the summer's work was to expand previous findings and clarify the kinds of particle growth taking place during early electrification. When ice particles collide within a young storm, the results lead to, or at least enhance, the stormÕs basic charge separation pattern (positive charge up high, negative charge at mid-and low levels). How and why those charges separate is the focus of the research.
The sailplane collected overall electric field data and sampled precipitation particles to measure their size, shape, and individual charges. Those data were combined with readings from a special radar that can help scientists estimate the sizes of precipitation particles and whether rain or hail predominates in a given part of a cloud. As scientists analyze the results of their work, they hope to gain insight on how radar data can be used to diagnose the onset of a storm's lightning.
Scientists want to develop a set of thresholds that can be used to gauge the onset of electrification, and thus lightning potential, from radar data. Lightning data can be used for short-term forecasting and to look at climate change indicators. Establishing the physical basis for the development of lightning should aid forecasts and other interpretations of those data.
Effective forecasting could in turn reduce impacts on telecommunications, as well as the number of injuries and fatalities caused each year by lightning.
Sudden, awesome, dangerous -- these timeworn adjectives take on new life when applied to lightning. In less than a second, one lightning flash can ignite a forest fire, kill a person, or interrupt electric power for thousands.
The frequency of "bolts from the blue" belies the mystery at the heart of lightning formation. How does the charge accumulate within thunderstorms? What is the catalyst that allows an embryonic bolt to travel earthward? Scientists have nudged closer to answering these and other questions by bringing instruments into the middle of developing thunderstorms. New detection devices have improved our global picture of lightning frequency. Still, much remains to be learned about the compact, intense, and sporadic event that is a lightning flash.
To tally each occurrence of lightning worldwide would be no small feat. According to estimates, up to 10 million cloud-to-ground strikes occur each day. The most accepted global measure of lightning frequency is the thunderstorm dayÐa day on which thunder is heard at a reporting site. By this standard, the tropics are the earth's lightning capital. From 100 to 200 thunderstorm days are reported each year across the equatorial belt from South America to Africa, southeast Asia, and northern Australia.
In the middle latitudes, North America receives the most lightning due to its unique geography conducive to thunderstorms. Two U.S. regions are especially prone to strikes. Florida is overall leader, with its peninsular shape causing ocean-land heat contrast and air circulations that trigger storms. The High Plains and the foothills of the Rocky Mountains receive intense summer lightning due to elevated heating, moisture from the Gulf of Mexico, and their high altitude (even small clouds over the Rockies are cold enough to carry the ice crystals crucial to lightning formation).
Thunderstorm days are not the ideal index of lightning, since this measure does not distinguish between a single clap of thunder and a prolonged severe storm. Automatic devices to detect cloud-to-ground strikes were developed in the 1970s to sense the radio-frequency pulses that travel outward from a lightning bolt. Satellites can also observe lightning; though previously limited to night-time detection, a prototype optical sensor, launched in April, is the first in a series of sensors that will detect lighting round the clock by the late 1990s.
RESOURCES
So you didn't get to read the last issue of Science Now? Or, do you wish to review a past issue on a topic of interest? Now available for you online are current and back issues, complete with photos, on the SIRS Web site at If you are looking for additional materials and resources related to weather, contact SIRS Customer Service toll-free at 1-800-232-SIRS. Inquire about the SIRS Photo Essays unit on "Weather" and SIRS Researcher CD-ROM.
The NCAR Digital Media catalog is now available on the World Wide Web as well. More then 1,000 of NCAR'S best slides and prints of solar and weather phenomena and related topics can be previewed in full color at http://www.ucar.edu/DMC/DMCHome.html.
Teachers can also access a listing of videos and film footage, as well as slide sets with suggestions for classroom use. Contact NCARÕS Visual Communications program at 303-497-8606 or 8212; razo@ucar.edu.
Science
Now
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Corporation for Atmospheric Research
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(Social Issues Resources Series.) Science
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Editor:
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Scientific
Editor:
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Contributors:
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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 1996 UNIVERSITY CORPORATION FOR ATMOSPHERIC RESEARCH. ALL RIGHTS RESERVED.
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