The bolt snakes out of the cloud, traces a jagged path left or right across the sky, and touches the ground. Or does it?
For most of us observing lightning - from a safe distance or inside a building - a lightning bolt is a flat, two-dimensional creature painted against a distant backdrop. But as researchers have shown over the last three decades, lightning has a complex shape that may let scientist pry a few secrets from a storm - including where a tornado might form.
A number of techniques have been developed over the years to look at the 3-D structure," said William Rison of the New Mexico Institute of Mining and Technology in Socorro, N.M. Rison works with Paul Krehbiel who talks today about "3-D Lightning Mapping and Observations" at the International Atmospheric Electricity Conference in Guntersville.
"The primary method is time of arrival of the radio signal," Rison continued. Besides a blinding flash of visible light, a lightning bolt also emits a distinct pulse of radio noise. The background crackle you hear on AM radio is the sound of distant lightning.
If several radio receivers are set up to record the radio pulse with precision timing, then the location of the pulse origin can be backtracked with a little math. It's similar to the principle used in navigating by satellite where several satellite broadcast the same timing signal and are heard, at different times (depending on distance) by a receiver.
The time of arrival method was pioneered in the 1960s by David Proctor of South Africa using VHF radio receivers. His work was laborious, with the data being collated and analyzed by hand.
In the 1970s and '80s, the technique was expanded by Carl Lennon and Launa Maier at NASA's Kennedy Space Center who developed a real-time system linked by microwave relays. This lets meteorologists tell launch directors if an electrical storm is too close for a safe launch. It's also optimized for Florida thunderstorms that are different in structure from what is seen in the central United States, and the microwave relays are bulky and difficult to move for studies in different areas.
By adapting a number of new technologies, Krehbiel and his colleagues have developed a Lightning Mapping System that can provide real-time 3-D images of lightning as storms develop.
Rison said the system comprises 10 automated ground stations, each with a radio receiver, a precision clock developed for satellite navigation receivers, a signal processor to extract and time-tag just the lightning pulse, and tape recorder. In two field campaigns, the network was set up northwest of Oklahoma City in June 1998 and outside Socorro in August 1998. It is scheduled for another campaign in New Mexico in the summer of 1999 and the Nebraska-Kansas-Colorado area the summer of 2000.
After a storm, the data tapes are collected and sent to Krehbiel's team. They then produce 3-D plots showing a lightning bolt's trip through a storm.
"This gives you, with a few qualifications, the full flash," Rison said. The principal qualification is that the system is most sensitive to negative streamers, discharges in which a stream of electrons burrows its way through the air to a positively charged area.
Time of arrival techniques locate impulsive radio emissions. This happens most often with negative leaders propagating into negative charge regions. It does not locate positive leaders into negative charge regions because the positive leaders do not radiate strongly. It often does not locate negative stripped leaders of cloud-to-ground lightning. These radiate strongly but are continuous, not impulsive.
As the lightning streaks across the sky, it ionizes new points in the air. In effect, the transmitter is moving through the sky, sending a new signal from each point. Each is recorded and, when the storm is reconstructed in a computer, becomes an individual point in a three-dimensional grid. The points are color coded to help the human eye follow a bolt's path across the sky. The positional accuracy is best when the storm is close to the array of receivers, but is still good out to ranges of 250 km (155 mi).
Lightning research at NASA/Marshall and the Global Hydrology and Climate Center.
The data plots also can be used to show where lightning occurs most often within a storm, matching lightning activity to storm structure, including convection.
The results are impressive and highly promising, Rison said.
"The initial discharge goes up," he said, describing one bolt recorded during a July 11, 1998, storm. The negative charge was at about 6 km altitude, and the positive charge was at 9 to 10 km. When discharge occurred when electric potential between the two regions became great enough to ionize a channel through air and become as conductive as a metal wire.
"Once that channel is established, then the discharge continued in the horizontal direction," he said. The upper charge center expanded east-west, and the lower center expanded north-south. Like snowflakes, each bolt is unique. "It depends on the structure of the cloud."
Storms also are unique. While some spit lightning at a seemingly leisurely pace, others are like hailstones hitting the roof. One supercell storm outside Oklahoma City fired almost nonstop.
"There was no time without lightning," Rison said. "The data are very amorphous. It just fills the plot and appears to be continuous. There's no gap in between. It's a real challenge to sort out. You can see tendrils, but not the start of one discharge and the start of another. That hasn't been seen before."
On the other hand, the Lightning Mapping System has recorded storms with apparent dead zones, where almost no lightning appears. But these are not hurricane-like eyes of the storm.
"That's a region of a very strong updraft moving at about 100 to 160 km/h (60-100 mph), vertically," Rison said. Storm watchers observed "a very high turret penetrating to the stratosphere." A little lightning does appear at extreme altitudes.
"We see clear evidence of an updraft," he continued. "We see continued discharging that has to be explained somehow." In this case, no tornado appeared. But Rison and his team have seen lightning dead spots where tornadoes did appear.
"The lightning wraps itself around the area where the tornado formed," he said. "This is where a hook echo forms on radar."
Radar is the tool that is needed now to complement studies with the Lightning Mapping System. Observations with two or more Doppler radar units would provide three-dimensional data on wind speed and direction.
New Mexico Tech is talking with the Global Hydrology and Climate Center in Huntsville about establishing a system in Huntsville, and with the Federal Aviation Administration, National Severe Storms Laboratory, and Global Atmospheric Inc., about setting up a prototype unit at the Dallas-Forth Worth Airport.