Three bolts from the blue

Fundamental questions about atmospheric electricity

One of a series of stories covering the quadrennial International Conference on Atmospheric Electricity, June 7-11, 1999, in Guntersville, Ala.

June 8, 1999: Does lightning affect the ozone layer? What causes "sprites?" And why does "messy" lightning follow a simple lightning model?

Martin Uman of the University of Florida asked these questions yesterday at the opening of the International Conference for Atmospheric Electricity. While most of the scientists will share what they have learned in their specific areas of study, Uman instead decided to ask a few general questions about atmospheric electricity. Uman hopes to motivate discussion among all the conference scientists so they will work together to solve these atmospheric mysteries.

"I'm asking these questions because nobody ever worries about putting it all together," Uman said.

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Uman's first question was: How much nitric oxide (NO) is produced by atmospheric electric discharges? It is important to study nitric oxide levels because of its impact on the ozone layer in the troposphere and stratosphere. This ozone layer protects us from harmful ultraviolet (UV) radiation, but nitric oxide can destroy ozone.

Left: The ozone hole at the Earth's South Pole. The TOMS (Total Ozone Mapping Spectrometer) false-color image shows low levels of ozone in blue, high levels in red. This time-elapsed image shows the ozone hole as it develops each year between late August and early October.

Increasing concentrations of nitric oxide and other chemicals in the Earth's atmosphere contribute to the ozone hole over Antarctica. Although most of this nitric oxide is produced by human activity, lightning also produces a small but significant amount. Still, estimates vary about exactly how much nitric oxide is produced by lightning.

"The literature about NO production is confusing," said Uman. "Everyone cites different production levels, so it's still an unsolved mystery."

Right: Moonrise over the Earth's atmosphere. This fragile-looking shell protects us from powerful radiation coming from space.

Also, scientists don't really know what sort of electrical discharges produce nitric oxide. Although it is well established that lightning produces some nitric oxide, other electrical discharges such as sprites could also be a source.

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Uman then asked about sprites - huge colored emissions coming from the tops of thunderclouds. Once thought to be extremely rare, high-altitude cameras have shown us that sprites are often produced by thunderstorms. While the upper portions of sprites are red, they also have wispy blue tendrils that extend downward.

According to current models, only the most powerful lightning strikes generate enough energy to produce sprites. Uman questioned whether these models are accurate representations of the energy needed to generate sprites.

"The models don't agree with the all the measurements, so there's a big debate over which is wrong," said Uman. "Are the models wrong, or do we need to get better measurements?"

Left: A red sprite photographed by a team at the University of Alaska, Fairbanks. Sprites are emitted near the tops of thunderclouds and reach up into the ionosphere.

"The transmission line model is a case of a model that works, but probably shouldn't," Uman said.

The transmission line model for lightning shows a smooth upward curve of current. Lightning, however, is not a smooth and steady phenomena.

"Lightning is a mess," said Uman. "The transmission line model is the oldest and simplest model about lightning, so it shouldn't work. Lightning is much more complicated than that."

Lightning does not follow a simple path across the sky. For instance, downward lightning can meet upward lighting in the middle of a cloud. Despite the 'messiness' of lightning, it somehow still obeys the simple current curve of the transmission line model.

Right: Lightning often exhibits complex patterns in the sky. Credit: Australian Severe Weather/ Michael Bath.

Uman asked these questions because he believes most theorists are too intent on developing their own models. By posing these questions at the International Conference on Atmospheric Electricity, Uman hopes to stimulate further thought about the physics of lightning. When these questions are answered, they could fundamentally alter our understanding of how electricity interacts with the atmosphere.

External links

45th Weather Squadron at Patrick AFB, lightning reference page.
Australian Severe Weather has a large collection of storm pictures, most from Down Under.
Atmospheric Structure - a chart at the University of Wisconsin at Stevens Point describing the Earth's different atmospheric layers
National Severe Storms Laboratory, Norman, OK
National Severe Storms Laboratory Photo Library, where we got a lot of the neat pictures for these lightning conference stories.

More Space Science Headlines - NASA research on the web

NASA's Earth Science Enterprise Information on Earth Science missions, etc.

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For more information, please contact: Dr. John M. Horack , Director of Science Communications

Author: Leslie Mullen Curator: Bryan Walls NASA Official: John M. Horack