New Ways to Fight Lightning

In a stormy summer day in 1752, Benjamin Franklin stood in a field outside Philadelphia waiting to conduct his famous key-on-a-kite-string experiment. His research led to the development of the lightning rod, which is still in use today as a lightning protection device. Following Franklin's lead, University of Arizona scientists are waiting for stormy days at the Kennedy Space Center in Florida, where rockets and copper wire replace kite and string in a space-ageversion of Franklin's experiment - rocket triggered lightning. This is the eighth year that an international collaboration of scientists, including those from the UofA, will gather information in the NASA-sponsored program.

The rocket research has led to the development of a system that can pinpoint the location of lightning strikes, a valuable tool in helping combat forest fires. Utility companies, weather services, and others also find the system helpful. For example, some research projects now under way suggest that lightning detection may help identify the onset of the wind shear conditions that are so threatening to aircraft.

To trigger rocket lightning, a three-foot solid-fueled rocket dangling a copper wire to the ground is launched near a thunderstorm. The rocket climbs to an altitude of one kilometer (nearly 3,300 feet). Most strikes occur before the 200 meters of wire (about 650 feet) unravel from the launcher's spool.

UofA researchers wait out the storms in grounded structures nearby. If the wire, when struck by lightning, touched someone or an ungrounded structure on the ground, the charge could inflict serious damage or perhaps fatal injury.

"When the rocket reaches about 180 meters (approximately 600 feet), it produces an 'upward stepped-leader' that searches out charges in clouds," says Bill Jafferis, NASA triggered-lightning program manager. "Rocket and wire create an avenue for lightning to travel."

Rocket launchings make it possible to study the physics of lightning discharge and the interaction of lightning with structures in a partially controlled environment, according to Dr. E. Philip Krider, head of UofA's Department of Atmospheric Sciences and director of the university's Institute of Atmospheric Physics.

"The wire guides the lightning to a point where various sensors measure the physical properties of the discharge and its damaging effects," says Krider.

Although Florida experiences more lightning strikes than any other state, Arizona's thunderstorms are more spectacular. The peak season in Arizona is July and August, when moisture-laden winds from the gulfs of California and Mexico interact with the hot, dry desert. As the warm air rises, it cools. Thunderstorms form when the cooling air can no longer hold the moisture (see "Monsoon Season," Arizona Highways, August 1986).

Scientists have studied lightning for more than 200 years. In the early 1900s, it was proven that thunderclouds have two electrically charged regions a negative charge at lower levels and a positive at higher altitudes. When the positive andnegative charges become large enough, they create a large spark. When this occurs within a cloud, it's sometimes called sheet lightning.

In cloud-to-ground lightning, an almost invisible highly branched negative discharge zigzags intermittently toward the Earth. This discharge is called the stepped-leader. When the tip of any branch of the stepped-leader gets close to the ground, it induces a positive charge on the Earth until one or more upward positive discharges form at the surface and rise to meet the stepped-leader.

When contact occurs between the positive upward discharge and the negative stepped-leader, the first return stroke begins. "The return stroke moves upward at about one-third the speed of light," says Krider. "Most cloud-to-ground flashes contain two to four return strokes, and each effectively neutralizes a different portion of the cloud charge. Lightning appears to flicker because the human eye can just resolve the time intervals between different return strokes.

"Although most lightning occurs within clouds, approximately 20 million cloud-to-ground flashes strike the continental U.S. each year," he adds. "Loss of life and property damage combine to make it one of our worst weather hazards."

Lightning is dangerous because of its powerful electric current. National Climatic Data Center statistics show that from 1959 through 1988, lightning killed 2,869 persons and injured 7,666 in the 48 contiguous states.

Property loss due to lightning is estimated in the hundreds of millions of dollars annually. As an example of property loss, a lightning strike to a KVOA-TV tower in Tucson on August 7, 1981, severely damaged a control panel in the television station. UofA researchers documented the strike in photographs that reveal the unpredictability of lightning - the lightning's path changed abruptly from a vertical to a horizontal direction approximately 80 meters (about 260 feet) above the ground and 70 meters (about 230 feet) from the tower structure.

If lightning threatens people and structures on the ground, it is also hazardous to space vehicles. On November 14, 1969, 36.5 seconds after lift-off, the Saturn V rocket carrying Apollo 12 to the moon was hit by lightning. Fifty-two seconds after liftoff, another bolt struck. The strikes knocked out a power supply, the guidance system tumbled, and several noncritical sensors were destroyed. Fortunately, the astronauts regained control of their spacecraft and completed their mission.

Apollo 12's lightning strikes alerted

A time exposure of a summer storm over the Grand Canyon (PRECEDING PANEL, PAGES 10 AND 11) radiates multiple lightning strikes. GARY LADD (BELOW) Lightning bolts rain down from stormy skies over Tucson. Using an extremely long exposure and colored lens filters to distinguish a number of strikes over time, the photographer captured the fury of an Arizona electrical storm. JOHN WALDEN Continued from page 6 NASA to the possibility of triggered lightning, a previously unsuspected hazard. In creating the rocket-triggered lightning program, the space agency hopes to find ways to improve lightning forecasting and protection systems not only for space vehicles but for ground facilities as well. Because lightning research has a broad range of applications, NASA has invited certain nations, other federal agencies, and several universities to participate in its rocket-triggered lightning program. The UofA is one of those universities. Our research addresses problems of concern to many organizations the Office of Naval Research, the U.S. Air Force, NASA, the National Science Foundation, and the Department of Energy," says Krider. Krider's research began in 1972 as a joint project with Professor Martin Uman of