Lightning aloft: a new clue to tornado prediction?
In April, 1995, NASA launched a new satellite with this euphonious (defined) moniker: "Optical transient detector," AKA OTD. Using new optics and electronics, OTD can see quick lightning flashes, even during the day.

Courtesy of Marshall Space Flight Center (NASA). Note: the tornado occurred near the most intense bunch of flashes. Cloud-to-cloud flashes get most intense just before a twister touches down. At least, that's what the OTD found on April 17, 1995, when it passed above a storm in Oklahoma and "saw" about 200 lightning flashes. During the same period, only nine flashes were seen on Earth. One minute after the satellite detected a peak in the cloud-to-cloud lightning, a twister touched down. Researchers at Marshall and elsewhere are following up on the results now.

Surprisingly, the detector found that much more lightning flies between clouds than between clouds and Earth. Says Hugh Christian, principal investigator for the OTD at Marshall Space Flight Center in Alabama, "Using the instrument we have determined that, in some cases, there are up to 20 times more lightning flashes within clouds than is observed by the ground-based network" of lightning detectors.

"The intensity of the lightning (flash rate) seems to be related to storm intensification," Christian says. "The formation of a downdraft causes a decrease in flash rate and may help lead to tornado formation."

Unfortunately, the OTD sees a small part of the Earth at once, since its orbit is low. Furthermore, the technique may not become a basis for tornado warnings, since it seems to detect actual tornadoes, not their formation. "It's a now-casting tool," says Christian. "It's not going to tell you what happens in one hour. It tells you what is happening now."

Finally, notes Robert Davies-Jones, it's not likely that all tornadoes show this distinctive pattern of lightning.

And while Doppler radar (radar that detects wind speed and direction) is the "tool of choice for tornado detection now," Christian notes that it takes five minutes to do a full scan with Doppler radar. The OTD technology, he says, could crank out a real-time warning that a tornado had begun in less than a minute.

Nevertheless, OTD's mission was not tornado detection, but studying the global climate by getting a firmer grip on where, when and how often lighting flashes take place. Lightning signals convective storms, primarily thunderstorms, Christian says. The detector has measured the global rate of lightning flashes at 40 to 50 per second. That's about half the previous estimate, 100 per second, which dated to 1925 (not bad for an observer who had no computers, no airplanes, not to mention satellites!). Changes in the frequency of storms could give clues to global climate change.

Researchers at Marshall have applied to send another OTD aloft, Christian says, in an orbit with a broader field of view that would allow nationwide now-casting of tornadoes. If everything goes smoothly, the launch could occur in 2000.

So tornadoes are in the wind. How can I protect myself?