Not all lightning is created equal
BOSTON — To most people, lightning is a mysterious, awe-inspiring spectacle to admire from a distance. To meteorologists and atmospheric scientists, these bursts of electricity are something to look at closely.
New research presented last month at the International Conference on Atmospheric Electricity in Guntersville, Ala., shows lightning provides an important part to understanding, and possibly predicting, severe weather, such as hail storms and tornadoes.
"Lightning is just another piece of the severe-weather puzzle," says Richard Blakeslee, one of the lightning scientists at the Global Hydrology and Climate Center in Guntersville. "It can provide a lot of information on the type of precipitation that is coming from a thunderstorm."
The GHCC team found that sudden increases in lightning flash rates may be precursors to tornadoes. In spring 1995, the Optical Transient Detector (OTD), a satellite lightning sensor orbiting Earth, captured images of lightning intensifying minutes before a tornado dropped out of a storm over Oklahoma.
The theory is that strong upward convection, the transfer of heat from one part of a cloud to another, results in high lightning rates. When convection plummets, lightning flashes sharply decrease and downward-moving air currents take over, setting the stage for tornadoes.
Much of lightning research has focused on cloud-to-ground lightning, which makes up only 25 percent of total lightning. Most lightning actually occurs inside thunderstorm clouds, making it difficult to study.
"For forecasting and nowcasting, we need to know what goes on inside of clouds," Dr. Blakeslee says. "Nowcasting" is a term used by meteorologists to calculate weather conditions in real time. For example, following what's happening in a storm, second-by-second.
In the last 10 years, sophisticated technology has led to a shift in the way scientists study and measure lightning. The most recent development has been the placement of lightning sensors in space to observe the atmosphere. The OTD and Lighting Imaging Sensor, both in orbit, observe lightning from above the clouds.
Here on Earth, William Rison and colleagues at the New Mexico Institute of Mining and Technology in Socorro, have developed a way of getting real-time three-dimensional images of lightning as storms evolve. The system consists of 10 ground stations with radio receivers that pick up lightning signals. Lightning, being an electric discharge, emits a distinct noise - that crackle you hear on AM radio.
With precision timing and applied math, researchers then plot the signal's location of origin on a three-dimensional grid, giving them a view of a lightning bolt's passage through a storm. What Dr. Rison and colleagues found was that "all lightning is not created equal."
"Our standard concept of lightning is that it looks like a very discrete event in time and space. We see one lightning flash, and then a few moments later we see another flash." But this notion doesn't really fit in the type of storms in the Midwest, he explains. The lightning is actually continuous. If flash rates can't be used as an indication of storm intensity, Rison questions what scientists can use to characterize a storm.
What scientists can do, he says, is compare and combine data from ground-based observations with those made from space to get a clearer picture of what goes on inside storm clouds.
A team of NASA scientists studying gamma-ray emissions from the depths of space, serendipitously detected gamma rays bursting from the tops of thunderstorms.
"No one had ever seen this type of thing before, we weren't aware that thunderstorm clouds could produce such high-energy radiation," says Robert Malozzi, a gamma-ray astrophysicist who was part of the team. "Something like this is really a nice clue for people to actually figure out what's going on inside a cloud."
(c) Copyright 1999. The Christian Science Publishing Society