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General Lightning Information



Introduction

While large scale natural disasters such as hurricanes, earthquakes and tornadoes grab most of the headlines, the most dangerous and also one of the most common is lightning. In the United States alone there were more than 13 million lightning strikes in 1989, resulting in 67 deaths, 322 injuries and causing over 100 million dollars in property damage.

A Brief History

As our understanding has increased, lightning has become less an object of superstition and more one of scientific interest. The Greeks and Romans thought it was a weapon of the Gods, and feared that thunderstorms occurred when the Gods were angry. Aztecs felt the same, resulting in the propitiation of virgins. By the Middle Ages some scientists were beginning to experiment with static electricity, and by the middle of the seventeenth century, electrical phenomena made for great entertainment such as making hair stand on end and shocking the really curious.

By the mid-1700, several scientists were starting to talk about the similarity between electrical sparks and lightning bolts. One of these was Benjamin ("Some people are weatherwise but most are otherwise") Franklin. Franklin is credited with the invention of the lightning rod, although he was not exactly correct in how it worked. He thought that it would draw electricity out of a passing cloud and prevent lightning from occurring. In fact, he devised an experiment, to test to see if clouds were indeed electrified. In 1750 he wrote

"On top of some high tower or steeple place a kind of sentry box ...big enough to contain a man and an electrical stand [insulator]. From the middle of the stand let an iron rod rise and pass bending out of the door, and then upright twenty or thirty feet, pointed very sharp at the end. If the electrical stand be kept clean and dry, a man standing on it when such clouds are passing low might be electrified and afford sparks, the rod drawing fire to him from the cloud. If any danger to the man should be apprehended (though I think there would be none), let him stand on the floor of his box and now and then bring near to the rod the loop of wire that has fastened to the leads, he holding it by a wax handle; so the sparks, if the rod is electrified, will strike from the rod to the wire and not affect him."

Luckily for Franklin, there were construction delays in erecting such a structure in Philadelphia, and he was unable to perform his experiment himself. Unluckily for a Swedish physicist, G. W. Richmann, who had read about Franklin's suggestion, there were no construction delays in erecting his structure, and he was killed when his sentry box was subject to a direct lightning strike. Meanwhile, Franklin had devised another method of gaining access to the electricity in the clouds: fly a kite. He drew sparks from a key attached to the bottom of his kite string, proving that clouds indeed were electrified.

Franklin, it turns out was weatherwise, otherwise and lucky, and there have been a number of people who have since died trying to imitate this experiment. So don't try this at home!

The Lightning Bolt

There are several types of lightning strikes; cloud-to-cloud, cloud-to-air, and cloud-to-ground. By far the most dangerous is the cloud-to-ground (this is assuming that you are not in an airplane. Cloud-to-plane-to-ground lightning is common, because of the charge differentials caused by planes. These bolts scare the be..sus out of the paying customers, but only cause a moderate sweat for the crew, unless the navigation is knocked out).

Lightning is created when a separation of the positive and negative charges within a cloud becomes so great that the insulating power of air is exceeded and great amounts of electricity are then released.

Exactly how such a charge separation is created in a cloud is not entirely understood, but several necessary ingredients to the process have been identified. The main two are ice pellets and rising air. Both of these are found in cumulonimbus clouds, most common in the late-spring and summer. The ice pellets pick up net charges through collisions, and are sorted out by the updrafts, leaving an abundance of the larger negatively charged pellets at the bottom of the clouds and the smaller, positively charged ones near the cloud tops. As the negative charge grows, it begins to draw a positive charge on the surface of the earth under the cloud, since opposite charges attract. When the negative charge is great enough it leaves the cloud and heads towards the surface as the first stage of a lightning bolt.

This first surge of electrons is called a stepped leader. The leader moves towards the ground in a succession of 50m steps, creating a pathway of electrical flow. As the stepped leader nears the ground, a strong positive charge is induced beneath it. Eventually, the electrical attraction is so strong, that the positive charge rises up out of the ground or out from objects protruding from the ground (houses, people, especially people on roofs of houses fixing television antennae), in an attempt to meet the stepped leader.

This rising positive charge is called a streamer. When the streamer and the stepped leader meet, the electrical circuit between the cloud and the ground is complete. At that instant, the negative charges in the leader explosively flow to the ground. The negative charges closest to the surface flow out first, followed by the charges progressively high up the leader.

This is called the return stroke. The violent surge of electrons within the return stroke heats the air to temperature in excess of 50,000 F (that's about 5 times the temperature of the surface of the sun!) and causes it to brightly glow. This is the brilliant flash that we recognize as lightning. It actually propagates upward! And so do the souls of T.V. Antenna repairmen or State Climatologists connected to the roof if they've been good and virtuous! Generally, however, this entire process happens so fast that the human eye cannot separate its components, and we see only a complete flash.

After the initial return stoke, the channel through the air remains very conductive, and thus provides an easy pathway from the cloud to the ground. Oftentimes, several subsequent strokes will follow this same channel, with each stroke draining the negative charge from higher up in the cloud. These strokes follow within milliseconds of each other and produce the flickering effect that is sometimes observed.

Thunder is produced when the air heated by the return stroke rapidly expands into the air around it. This rapid expansion produces a sound wave, which propagates away from the strike at about 1100 feet per second ( ~ a fifth of a mile per second). Since the return stroke begins at the ground, thunder begins at the ground. The rumbling of thunder is caused as the sound from the different sections of the lightning bolt reach the observer at different times. The distance to the lighting strike can be estimated by counting the number of seconds it takes from the time you see a flash until the time you hear the thunder. Dividing this number of seconds by five will give a good estimate of the number of miles between you and the strike. You know things are starting to get hairy when you don't get past one or two!

Lightning Protection

Since the power contained in a flash of lightning is tremendous (several hundred millions of volts, and several tens of thousands of amps), it is best to take precautions which lessen the chance of you and your property being struck.

The proper installation of lightning rods on buildings and trees can save immeasurable amounts of monetary as well as sentimental value by rechannelling the energy of a direct stroke. The idea behind a lightning rod is that it serves as the highest object around and so it intercepts the stepped leader on its way to the ground. A sharp tip on the rod facilitates the rise of a streamer from the rod and increases its effectiveness. Lightning rods will generally protect an area within a 45 cone beneath the rod. For large structures, several rods may be needed to insure complete protection, or perhaps an overhead ground wire. For further and more complete information on protecting your home from lightning strikes check with an expert in lightning protection (often found in your local telephone directory). (For more information on protecting your trees from lightning, click here)

There are also precautions that should be taken to protect yourself from bodily harm. A list of these from the National Weather Service is provided here. The main thing to remember is to make sure you are not in the vicinity of the highest thing around (get off the roof, opeday), and also not in contact with, or near electrical appliances such as the telephone. The electricity of a strike to a phone line, power line or even a tree, can travel long distances and still provide a jolt strong enough to kill. (Click Here for Lightning Saftey Precautions)

Lightning Research

Since cloud-to-ground lightning strikes cause so much destruction they have been the object of much study. Recently, technology has been introduced which allows very precise determination of lightning strike location and energy. A national network of 114 lightning ground stroke detectors has recently been put in place by the Electric Power Research Institute (EPRI), a private organization, that serves the needs of power companies and other subscribers interested in lightning across the country. These detectors sense the characteristic electromagnetic impulses of cloud-to-ground lightning strikes that occur up to several hundred kilometers away. Then, by using triangulation techniques, the network is able to describe the location of every ground stroke that it detects in the continental U.S.

As such it provides invaluable information for scientific studies, power and telephone company storm response, and even corporate and individual insurance claims. The illustration below uses data from this network to show the lightning strike density in the state of Virginia for the year 1989. The maximum is located over southeastern Virginia where they average 5 to 6 ground strikes per year per square kilometer (~250 acres). Here, summertime temperatures are the highest in the state and moisture is abundant from precipitation and the Atlantic Ocean, which are necessary ingredients for the formation of lightning producing storms.

The strike density gradually lessens as one moves north reaching a minimum of about 1 to 2 strikes per square kilometer in northern Virginia. Here, normal daily temperatures and precipitation are lower, and the chance of thunderstorms, reduced. Its is important to realize that the contours on the map are very general, and that much greater detail of strike density is available and would provide much more information for localized areas. Also, due to the use of very new technology, accurate, long term records of lightning strikes do not exist, so that the illustration below represents only those strikes which occurred during 1989, and therefore may not be representative of long-term patterns. ( Lightning Strike Density Map for Virginia)

Other Forms of Lightning

There are other forms of lightning that are still not very well understood. One of these forms is ball lightning. Ball lighting is a luminous globe ranging in size from a baseball to a basketball. Sightings of ball lightning are difficult to verify scientifically, and as such carry with them an aura of mystery. Many descriptions are very strange indeed. Here are two favorites:
"During a thunderstorm I saw a large, red hot ball come from the sky. It struck our house, cut the telephone wire, burnt the window frame, and then buried itself in a tub of water which was underneath. The water boiled for some minutes afterwards, but when it was cool enough for me to search I could find nothing in it."
"The whole family was sitting at the supper table when a ball about 4 inches in diameter came and hovered about the center of the table. It made a buzzing noise and was about 8 to 10 inches above the dishes. Its color was a mixture of blue and orange and may have had some red. It fluttered about 5 to 6 seconds. I could easily have grabbed it had I dared to do so. Then it exploded with a loud bang like a large fire cracker and gave off a smell somewhat like city cooking gas which lasted several minutes. After it exploded the table was left as before, no dishes broken or moved about."

This article originally appeared in the Virginia Climate Advisory, Volume 16, Number 2, Summer 1992
 
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