May, 1996
Lightning severely damages of kills thousands of trees a year. Many of these trees line community streets and are around homes and schools. Lightning strikes ground somewhere on Earth 9 million times a day -- 6,200 times a minute -- 100 times per second. Georgia has between 50-70 thunderstorm days per year. Each storm can generate lightning which causes extensive damage to historic, rare, specimen, and sound trees along its path. On average, lightning strikes every square mile in Georgia about 16 times every year. Many of these strikes involve trees. Figure 1 provides a map showing thunderstorm days, a means of estimating lightning activity.
The electric charges that generate lightning are formed in large storm clouds that contain tiny ice crystals and larger wet ice particles. Collisions between tiny ice crystals and wet ice particles within huge storm updrafts knock negative charges off the small crystals. These positively charged ice crystals are blown to the top of the cloud. The negatively charged wet ice falls toward the bottom of the cloud. Inside the cloud, ice crystal and wet ice particles form best, and lightning can develop best, when the churning updrafts are around 8°F.
The negative fingers of charge, called "cloud leaders", stretch downward following the pathway of least electrical resistance. Precipitation, other lightning paths, and even cosmic rays help determine how the leaders move downward. The jagged nature of lightning, in part, comes from the stair-step pathway followed by the invisible leaders dangling down from a cloud.
The cloud leaders reach out for the ground at 450,000 miles per hour. The positive charges concentrated at the Earth's surface reach up toward the cloud in "ground streamers" flowing off of tall structures like trees. The charge difference between cloud and ground finally become too great and overwhelms air resistance. A conduit is now open for electrical discharge. The great exchange of negative and positive charges causes light to be generated along the pathway.
When the connection is completed, the visible return stroke moves upward at 1/3 the speed of light, spewing light and instantaneously heating the air. Usually several strokes occur in one lightning flash to reduce the charge build-up. Because of rapid changes in air resistance and ground streamer locations, the various return strokes may not follow the same pathway. Ground points exchanging charge in one lightning flash can be separated by a mile or more, but are usually closely grouped. At times, several trees in a row show damage from several different strokes following different paths in one lightning flash.
Occasionally storm clouds become tilted or pushed over so the top is not inline with bottom. In these cases a positive charged strike may occur between the cloud top and the ground. These strikes can be three times more powerful and last longer than the usual negative cloud-ground strike. Tree and tree clump damage can be especially severe under these conditions.
Lightning comes in a number of forms or varieties. There are internal cloud, cloud-ground, cloud-cloud, and cloud-air discharges. Most lightning, more than 60%, is inside one cloud's electrical system. The lightning that damages trees are cloud-ground discharges. Cloud-ground discharges begin 90% of the time as a cloud leader with a negative charge moving downward. Most of the rest of lightning (9%) begins as the stronger positively charged cloud leaders moving downward. Only one percent of lightning discharges are initiated upward from the ground to the cloud. These rare forms can be positively or negatively charged.
Trees and other tall structures help provide terminals for ground streamers. Once the charge channel has opened, lightning may jump from the side of these tall structures into houses or onto people. The path of a lightning flash is unpredictable because the strike itself changes the local air and material resistances to electrical movement. Each millisecond presents a new pathway for electricity flow which could be almost the same as the last pathway or could be completely different.
The voltage carried by a lightning stroke is highly variable. Average values for a stroke are 100 million volts and 500 amps. The temperature at the core of a stroke is 50,000°F. This heat source causes rapid air expansion generating shock waves heard as thunder. The flash of the return stroke and the sound of the thunder are generated by the same event. Because light travels so much faster than sound, we see the flash and wait on the thunder. The thunder sound wave is traveling nearly 770 miles per hour at 70°F.
If you count the number of seconds between the flash and the thunder, you can tell how far away the lightning flash occurred. Every second, thunder sound moves toward you at 1/5 of a mile (actually 0.214 mile). If you count 5 seconds (4.7 seconds) between the flash and the thunder, then the lightning was one mile away.
People are most susceptible to lightning strikes when they are the tallest object in a flat area. Boating, open field recreation (like softball), driving a farm tractor in a field, golfing, exploring a granite outcrop, or other activities leave humans prone to strikes. Also, taking shelter under or near tall objects that could be struck is dangerous. Animals have less electrical resistance than a tree, allowing lightning striking a tree to change pathways crossing to animals or people near the tree base.
About 500 people are injured each year from lightning in the United States -of these people, 100 (20%) are killed. Feed lot and pastured animal losses are significant. Direct property damage has been estimated to be $175 million annually in the South. Damage to utilities is immense. Both forest trees and trees along community streets and in yards are damaged heavily when struck.
Injuries to people from lightning can come directly from electricity, light, and sound shock. Just being close to a strike can have many effects. The most pronounced effect associated with lightning is cardiac arrest. CPR, general first aid, andprompt arrival of medical personnel can prevent many deaths. Other less threatening symptoms include dizziness, temporary paralysis, ruptured eardrums and hearing problems, temporary blindness, physical collapse and falling from step voltage, being thrown by the shock wave, and superficial burns.
Trees are damaged by several events during a lightning strike. A direct strike can electrically disrupt the most vigorous areas of a tree. The heat generated from the strike, and from resistance heating, can blow bark off the tree and shatter wood. The usual lightning injury is a thin, opening in the bark of a tree and is common in the woods.
Depending upon the state of the tree and time of year, much greater damage may occur. Large sections of the bark can be ripped apart by steam explosions from a lightning strike. Roots of the tree are not immune to massive damage in a strike, blowing bark pieces out of the ground. Lightning-caused root damage is the hardest type of mechanical disruption to diagnosis in trees. It is not unusual to have a clump of trees killed by lightning. Usually group death occurs because of massive root damage.
Bark damage from lightning allows massive water loss. The tree quickly reacts but has few tools to stop water loss near a long vertical break. Treatments for lightning struck trees include watering and careful observations for pest problems.
Historic, rare, and specimen trees, especially when they are the center of landscapes or they shade or frame recreational areas, are valuable and can be protected by a properly installed lightning protection system. Trees with special significance, or that people or animals might move under in a storm, should be protected. Trees closer than 25 feet from a building or structure should also be protected to minimize "side-flash." Parks, golf courses, and public buildings should have large or important trees protected to minimize liability risks.
Tree lightning protection is expensive in labor and materials. Lightning protection systems must be installed properly with correct materials to insure long term protection. For example, aluminum should not be used for any link in a system, nor should solid wire of any type be used. It is essential to consult with a trained arborist or urban forester, and a lightning protection system installer before designing a protection system for a tree.
Lightning protection systems in trees do not attract lightning. The purpose of a protection system is to dilute and slowly release electrical charge potential between the ground and cloud. Trees are not good conductors of electricity but can act as a better conduit than air. Protection systems dissipate the electrical charge before it can build to high levels.
Woven copper cables (minimum 32 strand -- 17 gauge) should be used for the main down-cable. Bends in the cable should be minimized. Smaller woven copper cables (minimum 14 strand -- 17 gauge) can be used for protecting major branches. Usually 3-8 branch cables are used depending upon tree size and shape. Be sure major branches are protected. Cables are attached to the tree, and away from the bark, by three inch long pylons spaced three feet apart.
The top-most ends of each main or branch cable should be tightly fastened to the tree and to a solid copper or copper-bronze conductor (air terminal or air point). New charge dissipators are now available that look like fuzzy balls. Technology continues to evolve for reducing lightning damage and reducing the charge differences between cloud and ground.
Figure 2 shows the lightning protection zones below an air terminal at various heights above the ground. Remember this diagram attempts to depict a three dimensional shape around and below a single air terminal. Figure 3 shows that "branch N" is outside the protection zone and would require a side branch cable for protection. "Branch Y" is within the protection zone afforded by the single air terminal at 75 ft. in height.
Large forked or multi-stemmed trees require separate cables for each stem. Trees larger than three feet in diameter require two or more down-cables on opposite sides of the tree and interconnection. Trees with lighting, wiring, cables, or other hardware should have all hardware interconnected with the lightning protection system.
All lightning protection cables and connectors in the tree must be firmly attached to the main down-cable. The main down-cable is run down the stem and into the ground at the base of the tree. The main down-cable is then put into a soil trench 1-2 feet deep out to a distance of at least two times the tree's crown radius away from the trunk.
Ground rods are at least 1/2 inch in diameter and 10 feet long. Ground rods are made of copper alloy. It is essential that ground rods be fastened securely to the down cable. Grounding methods differ in different soils. Normally, driving a rod at least 10 feet into the soil with good soil and soil-water contact is sufficient. Where soil space is limited or soils are shallow, fork the cable and bury several interconnected rods in separate trenches as deep as possible. Depth, spread, soil contact, and water/ moist soil contact are critical. Attach plates sunk into crevices or weave cable nets with multiple attachment point into grounding grids to maximize grounding potential in and around rocks.
For all grounding, a grounding resistance of less than 50 ohms is acceptable and less than 30 ohms is desirable. If you are unsure of grounding effectiveness install extra grounds and have the resistance of your system checked. The whole system is worthless if not adequately grounded.
All tree lightning protection systems should have an identification tag attached with the installer's name and address. Maintenance is required to check and reattach connections and prevent damage to the down cable where it enters the soil. All system components should be inspected every year. Figure 4 provided a graphical summary of important parts of a tree lightning protection system.
