Published on 06/17/96

Bolts from the Blue (Or Gray)

Summer brings flowers and green leaves. But it also brings lightning, which strikes thousands of trees a year in Georgia. Many of these are around homes and schools.

Georgia has 50-70 thunderstorm days per year. Each storm can produce lightning causing great damage to healthy, historic and rare trees. Lightning strikes every square mile in Georgia about 16 times every year.

The electric charges that make lightning are formed by icy collisions in large storm clouds. Tiny ice crystals and larger, wet ice particles run into each other. The crystals with positive charges get blown into the top of the cloud. Negatively charged wet ice falls toward the bottom.

The most common type of lightning begins when invisible fingers of negative charges move downward from the bottom of a cloud. Because positive and negative charges attract, a positive wave swells on the ground beneath the storm.

The negative-charge fingers reach downward along the path of least electrical resistance. Precipitation, other lightning paths and even cosmic rays help determine these jagged, stair-step pathways of lightning.

The negatively charged fingers reach out for the ground at 450,000 miles per hour. The positive charges bunched at the ground reach upward in streamers flowing off trees and other tall structures.

The electric fingers and streamers stay invisible until they connect. That happens a few hundred feet above the ground or a structure. When the connection is completed, the visible return stroke moves upward at one-third the speed of light, spewing light and instantly heating the air.

A single lightning flash is made of many high-voltage strokes, averaging four strokes per flash. The human eye can just see the individual strokes within each flash, which makes lightning flicker.

Every stroke is different. Average values for a lightning stroke's power are 100 million volts and 500 amps. A stroke's core is 50,000 degrees. This heat load causes air to expand fast, forming shock waves heard as thunder.

Because light travels so much faster than sound, we see the flash and wait on the thunder. If you count the seconds between them, you can tell how far away the lightning was.

Every second, thunder's sound moves toward you one-fifth of a mile. If you count five seconds between the flash and the thunder, the lightning was one mile away.

Lightning injures about 500 people each year in the United States. About 100 of these are killed. Feedlot and pastured animal losses are high.

Direct property damage is about $175 million each year in the South alone. Damage to utilities is immense. In forests, along community streets and in yards, trees are damaged badly.

In a dry forest, a line of lightning strikes can start many wildfires. If it doesn't start a fire, a lightning-struck tree is an open invitation to pests, like the Southern pine beetles in pines.

A direct strike can electrically disrupt a tree's most vigorous areas. Steam explosions from the heat can rip off large sections of bark and wood.

Roots aren't immune, either. A strike can blow root pieces out of the ground. It's not unusual for lightning to kill a clump of trees.

Tree-bark damage from lightning allows massive water loss. Treatments for lightning-struck trees include watering and carefully watching for pests.

You can protect historic, rare and specimen trees, especially in the center of landscapes or overshadowing recreational areas, with a properly installed lightning protection system.

Protect special trees and those that people or animals might move under in a storm. Protect trees closer than 25 feet to a building, too, to minimize "side-flash."

Large or important trees in parks and golf courses, or near public buildings, should be protected to minimize liability problems.

Kim Coder is a forester with the University of Georgia Warnell School of Forestry and Natural Resources.