The Hidden Life of Thunderstorms

To take a closer look at what's happening, Blakeslee and colleagues have used a novel way of monitoring thunderstorms. They flew a modified version of the Predator unmanned aerial vehicle (UAV) used in the recent Iraq War, "conscripting" a version of the aircraft into civilian duty to fly over thunderstorms near Key West, Florida.

Their high-altitude version of the UAV, called the Altus II, can measure the invisible electric and magnetic fields enveloping the storm. As the UAV flies over the storm clouds, it passes through these fields, sensing the strength and direction of the fields at points along its flight path.

Combined with optical measurements of lightning flashes, the result is a much more complete picture of the electrical infrastructure of thunderstorms.

"One of the things we can measure with this experiment is the current flowing above a thunderstorm," Blakeslee says. "It may be that there's very little current flowing out of the top of the storm, but if you take all the thunder clouds over the whole earth that are producing that small amount of current, it may very much account for this discrepancy in the atmosphere's electricity budget."

Where does this current come from? Blakeslee suggests one possibility.

Image courtesy NASA The Altus unmanned aerial vehicle.

"When the storm's electric fields get strong enough, the earth's surface under the storm becomes charged. Grass, trees, people and everything start giving up a charge that flows up into the atmosphere. Sometimes that's manifested in something that can be seen like St. Elmo's Fire. Generally you don't see it, but it's occurring all the time under storms," he says.

The hunt to quantify this missing current continues. Blakeslee's team is still processing the data they gathered, and they hope to publish some of their results within the next few months. One thing their research has already shown, though, is the usefulness of these unmanned aircraft.

"These unmanned craft are an important new tool for studying storms," he says. "They allow us to have much longer and more continuous observations compared to manned airplanes, so they give us a fuller picture of what's going on."

The UAV flies much slower than normal planes (150 mph vs. ~500 mph) and can turn around much quicker. So while a manned plane would quickly zoom over a storm and then spend a long time turning around for another pass, UAV's can simply do slow "figure 8's" over the storm for hours, gathering data continuously.