July 18, 2000

Lightning's Shocking Secrets



The Associated Press, top; Kevin Moloney for The New York Times, bottom;

Researchers thought they knew a lot about lightning strikes like this one, in June 1999, near Billings, Montana. But new studies show there is still much to learn. Bottom, Dr. Ron Thomas of the New Mexico School of Mines and Technology monitors signals from 13 lightning detectors in Kansas and Colorado.

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The Associated Press/The Arizona Daily Star/Sergey Shayevich
Lightning bolts rain down on Tucson in July 1999.

Zeus were alive and chucking lightning bolts down from the sky, he would be perched right over Goodland, Kan. That is where many of the nation's most violent thunderstorms are spawned, the kind that drop baseball-size hail, tornadoes, torrents of rain and furious winds across the Great Plains states.

So when scientists from nearly a dozen universities and government laboratories recently decided to carry out an advanced study on what causes lightning and severe weather, they deployed their instruments on the cornfields around Goodland, hunkered down and waited for Zeus to rock and roll.

Their eight-week experiment, called the Severe Thunderstorm Electrification and Precipitation Study, or Steps, ended on Sunday. Scientists say that the experiment turned up some stunning surprises that may force them to revise their theories of how lightning is produced.

Among the discoveries were many instances of a rare kind of so-called reverse lightning, in which electrons shoot upward from the ground to the cloud, instead of downward as in normal lightning, and new clues about what causes strange lights called blue jets, red sprites, elves and trolls that appear in the upper atmosphere above thunderstorms. Sprites, which last long enough to be seen with the naked eye, happen only above reverse, or "positive cloud to ground" lightning, adding to the mystery of both phenomena.

Most important, researchers witnessed several events within severe storms that seemed to predict when tornadoes would form. For example, they observed updraft regions in clouds where all lightning suddenly ceased. Moments later, tornadoes formed in that area. If lightning-free zones usually precede tornadoes, they said, weather forecasters might be able to watch for these "electrical holes" in the clouds and make better short-term predictions for severe weather, including large hail, heavy rain and tornadoes.

The study was organized because the details of precipitation, hail and lightning formation within severe storms are still not completely understood, said Dr. Morris Weisman, a scientist with the National Center for Atmospheric Research in Boulder, Colo., who helped coordinate each day's observations.

"Given similar initial weather conditions, we don't know why some storms produce downpours while others contain just as much water vapor but make very little or no rain," Dr. Weisman said.

With financing from the National Science Foundation, the scientists converged on Goodland armed with radar, weather balloons, mini-weather stations on wheels, lightning detectors and an airplane that could fly into the heart of thunderstorms.

They set up this equipment along the Kansas-Colorado border, where moist air from the Gulf of Mexico meets hot, dry air from the Southwest to cook up storms so huge they can last for days as they move east.

Scientists have thought for years that they understood basically what happens inside storm clouds, said Dr. Donald MacGorman of the National Severe Storms Laboratory in Norman, Okla. For example, lightning is generated in cold upper layers of air when particles of partly frozen water and ice crystals collide inside large clouds. These collisions generate positive and negative electrical charges the same way that walking across a carpet on a dry day can build up static charges and result in a shock when a person touches a doorknob.

In general, negative charges sink to the middle or lower parts of the cloud, while positive charges rise to upper levels, he said. When air can no longer insulate these opposite charges, an avalanche of runaway electrons initiates a lighting bolt. Most electrical discharges move sideways within or between clouds, but electrical charges can also flow to the ground in familiar flashes of lightning.

As for storms, as hot air near the ground rises into cooler regions, any moisture in the air condenses into cloud droplets. Strong updrafts promote the formation of rain, which can fall and drag air masses back down to the surface. Storms like this are short-lived. But if winds are strong or if there are significant temperature differences and large amounts of moisture, very large clouds can evolve into thunderheads that acquire a vortex of circulating air. These super cells can produce tornadoes and other violent weather.

But, as everyone knows, weather is fickle, Dr. MacGorman said. Some supercells produce rain, lightning, hail and tornadoes; some produce no rain or lightning but pound crops with large hail; others produce floods and tornadoes but no lightning and hail. The reasons for such different outcomes will eventually be found in the minute physical properties of storm clouds and a better understanding of chaotic systems, Dr. MacGorman said.

For the last two months, the Steps investigators met every morning at the National Weather Service center in Goodland and pored over weather data, looking for the storms.

On good days, when the weather was bad, the scientists scrambled into action. Dr. Erik Rasmussen, a scientist with the Severe Storms Laboratory, sent out a crew to chase storms in six Oldsmobiles fitted with ski racks mounted with weather instruments. These volunteers collected data once a second on winds, temperature, pressure and humidity under and near the storm front. Conditions measured on the ground might help researchers learn what is happening in the clouds overhead.

For example, on June 29 the volunteers discovered that the air falling to the ground along the rear flank of a very large storm had suddenly turned warm. Minutes later, the storm spawned quarter-size hail and a tornado. Still later, measuring other downdrafts at the back of the same storm, they found that the falling air had suddenly turned cold. Minutes later it rained and hailed but no tornado formed.

While this was going on, Charlie Summers, a pilot employed by the South Dakota School of Mines and Technology, took to the skies in a single-engine airplane designed to fly through severe weather. Resembling a 1940's Studebaker with wings, the craft is armored with 700 pounds of aluminum coating, a metal grate to keep hailstones out of its carburetor and thick acrylic windshields.

Strapped into a four-way harness, Mr. Summers flew the plane into the heart of severe storms where he was buffeted by 50-mile-an-hour vertical winds, direct lightning strikes, hail and enough ice to temporarily stall the engine. Instruments measured the spectrum of water and ice particles in the clouds, air pressure, temperature and electric fields.

The airplane conked out about a week before the experiment ended, said Dr. Andrew Detwiler, an expert on storm electrification at the South Dakota School. But the aircraft gathered large amounts of data that will be compared with information collected on the ground, he said.

While Mr. Summers flew, other scientists operated special radar stations that could directly measure the size and shape of water particles in every cloud. Conventional radar cannot do this. The added information is expected to shed light on exactly how lightning is generated.

To study lightning, scientists from the New Mexico School of Mines and Technology in Socorro turned on 13 specially built lightning detectors that were laid out in cornfields under radar observation. Each detector is about the size of a small coffee table and contains a receiver that measures how long it takes for radio signals, which are always produced by lightning strikes, to arrive, allowing researchers to calculate the exact time and location of every lightning strike in every storm that occurred during the experiment.

By comparing this detailed picture of where lightning occurs with radar images and airplane data on precipitation, it may be possible to learn more about how lightning is generated, said Dr. Paul Krehbiel, who helped develop the system.

Preliminary data show that charge fields -- the layers of positively and negatively charged particles within clouds -- do not fall into the conventional pattern, Dr. Krehbiel said. They are often upside down, meaning negative on top and positive below.

This inverted polarity was also observed by scientists from the National Severe Storms Laboratory, who released up to four weather balloons into the belly of each storm. Each balloon measured temperatures, humidity, pressures, wind directions and electric fields.

A surprising number of storms produced the reversed, "positive cloud-to-ground" lightning that was thought to be restricted to very large supercell storms, said Dr. David Rust, chief of the Mesoscale Research and Applications Division at the Severe Storms Laboratory. "We're seeing it in ordinary smaller storms" along with electrically inverted charge fields, he said.

In a positive-to-ground lightning strike, positive charges first rush from the cloud to the ground, creating a lightning channel through which electrons flow from the ground back up to the cloud. Such lightning strikes tend to carry more charge, last tens of seconds longer and be less branched than the more common negative-to-ground lightning. But how this reversal of charge occurs remains something of a mystery, Dr. Rust said.

"Storms may reverse their polarity all the time, but we just never knew it," Dr. Krehbiel said. While scientists on the ground and in the air over Kansas and Colorado ran around collecting information on each storm, other Steps researchers sat quietly at an observation post called the Yucca Ridge Field Station in the foothills of the Rockies near Fort Collins, Colo.

There, with an unobstructed view of the Great Plains, researchers waited for blue jets, red sprites, elves and trolls -- strange lights that could be seen with the naked eye or detected with instruments over the tops of thunderstorms to the east.

Blue jets are extremely energetic fields of charged particles that rise up to 30 miles from the tops of clouds. After they occur, lightning stops for several seconds. Red sprites are striated glowing ribbons that rise high above the jets and reach the ionosphere, lasting for 3 to 10 milliseconds. They happen only over regions of reverse lightning. Elves are thin, expanding doughnuts of light found on the lower edge of the ionosphere. They tend to occur above sprites after a lightning pulse. Trolls are propagating waves of energy that appear to come back out of cloud tops and hook up with sprites, but no one really knows what they are.

"If we were biologists, it would be like we discovered some new body parts," said Dr. Walt Lyons, a scientist with FMA Research, a company that runs Yucca Ridge. Somehow, he explained, lightning discharges in the lower atmosphere are having effects in the upper atmosphere.

During the experiment, Dr. Lyons and his colleagues observed more than 1,000 red sprites over the dissipating regions of storms where clouds flattened out and produced a type of horizontal discharge called spider lightning or creepy-crawly lightning. These clouds also produced reverse lightning over which the sprites were seen.

"We're happy but exhausted," Dr. Lyons said. It will take time to analyze what happened inside those clouds, he said, but whatever is going on may help explain sprites.

Indeed, each researcher said it will take many years to analyze all the data collected on the Great Plains this spring and summer.

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