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Lightning is a well known natural phenomenon that influences the lives of everyone. It can cause injury and death, thus fear is an associated phenomenon. It contributes both positively and negatively in ecosystem influence, for it provides natural areas with nitrogen in forms useable by plants, and it initiates wildfires with their many on- and off-site influences.
This brief section on lightning is a first-cut effort to collect knowledge that may allow us to determine, eventually:
Data for later analyses may be purchased from Statistical Climatology Branch, National Climatic Center, Federal Building, Ashville, NC.
One of the most powerful and uncontrollable natural phenomena is that of lightning. Each day approximately 1800 thunderstorms hurl thunderbolts to the ground causing millions of dollars of damage and personal tragedy. They contribute a natural source of acid rain. The lightning produced during these storms performs an important role in maintaining a dynamic balance within the ecosystem. Research has uncovered many of the chemical and physical properties which atmospheric electricity possesses. Much has been accomplished in the field of lightning research and its relationship to the biotic and abiotic components of the environment. One of the many areas of concern is the relationship of lightning to agricultural and natural-resource-related areas because of the tremendous effects it can have there.
Major global distributions of the occurrences of thunderstorms are prepared by the World Meteorological organization (Golde 1973:3). From these, it is evident that the number of thunderstorms which occur daily over a period of a year is higher over land masses and the equatorial belt than anywhere else on the globe (Sparrow and New 1971). These meteorological maps provide valuable detailed information about thunderstorm activity for a particular geographical region. Viemeister (1972:91-93) illustrated the average annual thunderstorm days (TD) per year and average annual thunderstorm days for major cities found in the continental United States using these maps.
Lightning storms develop by two major meteorological processes: 1) heat or convection, and 2) weather frontal convergences. Heat or convective storms occur mainly in mountainous and tropical areas. Their development is marked by hot air rising to high altitudes, and its gradual cooling, forming clouds (condensed moisture) and ice crystals (at higher altitudes). Frontal storms occur mostly in temperate areas where warm, moist belts of air meet cold fronts of air. As warm air rises and is replaced by colder air, the same process occurs as did in convective storms. The formation of clouds and ice crystals by these processes produces an electrically charged system. Lightning is the force which balances this system.
Workman (1967:540-557) stated that "the mechanism by which a cloud becomes electrically charged is not yet fully understood, but can be taken to be associated with the violent updraught of air in the center of a cell and the resulting impact of super-cooled water droplets on ice crystals." Byers and Braham (1949) found that "each cell has a diameter of several kilometers and undergoes a life cycle lasting some 30 minutes during which electrical charges are generated and lightning activity continues until the charging is exhausted." Ice crystals in the cloud are positively charged and water droplets negatively charged presenting the thunderstorm with a positive charged center in the higher altitudes and a negative charged center in the lower regions. The dipole formed is the initiating factor which creates the electrical conditions conductive to a surge of lightning.
Viemeister (1972:98-109) described the overall mechanism which causes lightning to strike the Earth. He stated that the primary cause is "the interchange of electricity between the Earth and the atmosphere, controlled primarily by the umbrella of the ionosphere that encloses the electrical processes in which the thunderstorm plays a part." The ionosphere is primarily an "uncharged" entity (with equal numbers of positive and negative ions) and the Earth has a 300,000 volt negative condition with respect to the ionosphere. It is this voltage difference that powers the flow of electricity from the earth to the atmosphere. There is a constant flow of electrons toward the ionosphere called the "air earth" or "ionic current" which has been calculated at 1,800 amperes. Evaporation and atmospheric ions carry these electrons up to the ionosphere through the ionic current daily, forming conditions conductive to electrical disturbances and thunderstorms.
The discharge of a bolt of lightning was first studied intensely by Sir Charles Vernon Boys (1926). More recent research by Schonland (1956) has described the discharge of lightning as beginning in the cloud and becoming visible as it penetrates its lower boundary. These phenomena have been studied by means of the rotating camera which reveals details about different characteristic parameters of lightning. A brief summary of these parameters and past research can be found in Golde (91973:10-12) and Viemeister (1972:111-123).
The polarity of 90 to 95% of the lightning charges that reach the Earth has been verified as being negative. (Golde 1973:12 and Viemeister 1972:111). Viemeister (1972:111) also listed the primary ways in which electrons are exchanged between the Earth and the atmosphere during a thunderstorm. They are: 1) negative charged center of the cloud to ground (cloud to ground type), 2) negative charged center to small positive charged pockets (cloud to cloud type), 3) negative charged center to main positive charge center in cloud, 4) Earth to small positive charge pocket (positive cloud to ground strokes), 5) Earth to main positive charge regions, and finally, 6) main positive charged center to upper air. It should be noted that a stroke from a cloud may dissipate into the atmosphere before reaching the ground or another cloud.
These 6 types of lightning flashes can be separated into 2 basic categories, 1) cloud-to-cloud strikes, and 2) cloud to ground strikes. Aircraft are the only objects which are susceptible to cloud-to-cloud strikes. Therefore, the following review of past literature and general scope of this section shall be confined to cloud-to-Earth flashes.
Viemeister (1972:110) explained briefly what happens during a lightning flash. A group of electrons from the cloud groups toward the earth in a succession of steps, and push forward each step creating a luminous tail called the initial or stepped leader. As this leader nears the ground, electrons are forced from the air near the surface creating the ionized streamer which rises from the earth to meet the advancing leader. Upon the joining of these two leaders, the air around their path is ionized and an enormous amount of electrons proceed down this path creating the main or return stroke. This is the "flash" we observe during a discharge of lightning. "Even today, many of the details of this sequence are still in the theoretical stage of understanding" (Viemeister 1972:112).
The magnitude of this surge of electricity to the Earth is phenomenal. On the average, 25 coulombs of electricity are brought to Earth in a lightning flash. The flash averages from 1,000 to 9,000 feet in length, although strokes of 100 miles have been reported. The average diameter of a main stroke is 6 inches. Although total power of a flash of lightning does vary with its duration, even the smallest strokes have more power than the combined maximum electrical power production of all the electrical power plants in the United States for a single day (Viemeister 1972; Golde 1973). Quantification of the lightning discharge has been performed by several authors using different instruments and techniques. Golde (1973:15-22) summarized some of the past work performed in this area. measurements relating to the frequency of Earth flashes can be divided into two groups, 1) visual observations along with electrical recordings, and 2) lightning strikes to transmission lines.
Analyses of the frequency of Earth flashes show great similarity considering the different parts of the world, different experimental techniques, and different scientists who performed the research. Approximately 2 Earth flashes per 10 thunderstorm days per square kilometer is a good representative estimate of Earth flash density (Golde 1972:21).
One of the positive effects of lightning is its fertilization potential to the soil. With lightning acting as the catalyst, nitrogen in the air combines with oxygen forming nitric oxide gas. This gas is leached from the atmosphere by rain and enters the earth as nitrate. Viemeister (1972:225) stated that "scientists have estimated that lightning plays a major role in bringing useful nitrogen to the soil and have estimated that hundreds of millions of tons of nitrates are produced by lightning each year." Spurr and Barnes 91973:181) stated that in the temperate zone of North America, about 5 kilograms of nitrogen are added annually to each hectare of soil by precipitation. Thus on the total Indian Head Naval Ordnance Station, 7500 total pounds are added. Between 3 and 5 kilograms is probably a good approximation of the amount of lightning-attributed nitrogen added per hectare (2.5 to 4.5 pounds per acre) of soil annually in this region of the United States.
Considering the negative effects which lightning can have on monetary and non-monetary entities, a high value of importance should be placed on the protection against these occurrences. Because of the vast acreage of land and high investment in machinery and equipment, the farmer and other natural resource managers must be particularly concerned with the near-disastrous effects of lightning or face the possibility of financial or personal tragedy.
Costs of lightning protection systems vary considerably with season and locality but can usually be estimated accurately. The costs of various materials used in constructing the system (copper, aluminum, galvanized steel) fluctuate annually, but raw materials are not always the major influence on cost. Various fittings raise the costs. Labor costs greatly exceed (usually) costs attributed to raw materials.
An hypothetical example of costs in relation to benefits received was described by Viemeister (1972:220) (Actual values have changed: the ratios probably are useful.) Assume installation costs $640. This averages out to $32 per year when spread over the life of the system. If buildings were insured for $40,000 and the base rate is $4 per thousand per year, the annual insurance bill is $160. The annual insurance bill would drop $16 if a 10% credit were granted for installing the lightning rod.
Much information is available on the occurrences of lightning to certain types of structures which aid in deciding when lightning protection is merited. (See the Lightning Protection Institute, P.O. Box 406, Harvard, IL 60333.) Several systems have also been developed utilizing this information. Examples of these information systems can be found in Golde (1972:46-47). The most comprehensive information concerning damage due to lightning has been collected by insurance agencies and government organizations. A statistical analysis of the information yielding number of buildings damaged as a percent of the total number of buildings in the region along with corresponding monetary losses was suggested by Golde (1973:44) as an aid for determining when to install lightning protections. Golde (1934:44) also cited Bruckmann (1961) and Popolansky (1969) as using these kinds of data to enable them to make significant decisions on lightning protection.
One should realize that the statistical and economic approaches described previously are "best" when applied to rural buildings or farm structures which constitute a high fire risk. Building codes requiring the use of steel and non-flammable materials greatly reduced the probability of lightning or fire damage in modern rural buildings.
Because of the location and proximity to fire stations, rural dwellers face greater lightning hazards than suburban or urban residents. Between 21 and 37 percent of all fires to rural buildings result from lightning (Viemeister 1967:219). Also, because of the high amount of combustible materials used in construction of farm buildings, the fires which result from rural lightning strikes can become very destructive in a short period of time. This is why the farmer and resource manager should have lightning protection as a major goal in his/her management plan.
There are 3 major components which are part of all good lightning protection systems. Lightning rods used to protect roofs and taller objects are part of the air termination network protection system. Its purpose is to intercept the lightning strike and deflect it from the fabric of the structure. Once lightning has been intercepted by a roof conductor, the current injected at that point must be transmitted to the ground by the shortest possible path. This is the function of down conductors and the current transmittal system. The Earth termination network of a lightning protection system comes into play after the strike has been intercepted and conducted to the ground. Its function is to discharge the current into the ground and secure its effective distribution over the mass of the Earth. All three of these systems must function properly for a roof (or any structure) to be protected adequately against lightning.
Trees, roofs, fences, silos, windmills, aerials are all potential hazards which present problems. Each object required special consideration with respect to lightning protection. A brief summary of the major things a farmer or manager should do to reduce risk, loss, and hazard to farm objects is presented below. For more specific information concerning the subject see Golde (1973:42-194) or Viemeister (1967:177-287).
On farms, trees are one of the most susceptible objects to lightning. Because of their height and location, they often provide the avenue of least resistance to a surge of lightning. However, the damage they receive from being struck is usually minimal (except to the tree itself) unless they are located adjacent to livestock, buildings or other property. In fact, when tall trees are properly protected with lightning conductors, they provide a safety "cone of protection" around them which shall be discussed in detail later under the topic of lightning rods.
Many factors contribute to the likelihood of a tree being struck. Moisture content of the wood, location (i.e., standing alone or in a group), height, soil, species, rooting type (shallow vs. deep) are but a few of these factors. Past research has attempted to analyze "lightning preference" for species type, location, and other of the above mentioned factors which contribute to a tree being struck. As indicated by Thompson (1946), an isolated tree occupying an exposed position in the landscape is much more likely to be struck than trees found in the midst of the forest. In his survey, of the trees struck, 96% were alone, and 84% represented the dominant trees of the area. He concluded that there was no significant evidence to justify greater or lesser susceptibility to lightning between species.
According to Golde (1973:160) protection of individual trees is costly and need only be considered where preservation of the tree is desirable for historical or similar reasons or where it is required for the protection of the adjacent building. A general rule to use when considering safe clearance distance between trees and other objects is: if the tree is taller than the structure which is in question and the main bole is within a radius one-third the height of that structure, then protection is merited. Protection is also necessary if the taller branches of the tree come within to 2.5 meters of the structure in question.
When a situation arises where a tree requires protection, the top of the tree (at its highest point) and taller main branches should be fitted with down conductors. Allowance for winds should be adjusted for by using bare-stranded conductors. Injury to the tree (by insects, fungi, etc.) can be avoided if care is taken when fitting the conductors. The British code of lightning protection (1965) required that at least one rod electrode (2 for larger trees) be driven into the ground near the main bole of the tree and that they should be bonded by radial buried strips connected to a ring conductor which should encircle the tree at a distance not less than 8 meters. In cases where clearance distance (as outlined above) cannot be attained, roof or down conductors (in addition to those used on the tree itself) placed on the structure closest to the tree will alleviate the possibility of "side flash" by lightning.
Some "common sense" solutions a farmer may apply to nuisance trees in the pasture is to cut the tree down or fence off the area around the tree to keep livestock away. Bringing livestock in from the fields under threatening conditions provides another solution in reducing lightning hazards.
Roofs of farm buildings, sheds, silos, etc. are also prime targets of atmospheric electricity. Protection is usually employed by the same devices which are used to protect trees (rods, conductors, and lines). These instruments operate by the same principle which causes lightning to stroke tall objects. Taller objects "draw" strikes away from shorter objects because of the shortest route theory, i.e., lightning will take the shortest, easiest route available.
A radius equivalent to the height of the taller object encircles an area from which lightning is diverted. This creates a cone of protection and smaller objects falling within this cone are free from lightning strikes. A farmer can reduce risk and hazards from lightning by installing lightning protection systems which will create these "cones of protection" throughout his/her property.
Fences also present special problems to the farmer. Metal fences are exceptionally good conductors of electricity and are capable of carrying current for great distances. Viemeister (1977:222) stated that if a fence is continuous and uninsulated from the ground, electrical current may travel for several miles if hit directly. The farmer can do 2 major things to prevent electrical currents from traveling along his/her fences. They are: 1) ground the fence, or 2) interrupt the fence's electrical continuity. Metal fences should be grounded with metal posts or rods made of galvanized steel placed every 150 to 300 feet along the fence. These should be driven a depth of 5 feet in the soil. Strips of wood (2-inches x 2-inches x 24-inches) placed every 500 feet along the fence are recommended by the U.S. Bureau of Standards to prevent induced currents from traveling great distances along fences.
Other entities found on almost every farm which present lightning hazards are door tracks, eave troughs, ventilators, vent stacks, down pipes, hay fork tracks, water pipes, and electrical lines. Underwriter's laboratories recommend interconnection of lightning rods, conductors, and grounds to all of the above and suggest consideration of the same treatment to stanchions, railings, metal posts and guy wires. Complete lightning protection for a farming operation is as (Viemeister 91967:220) stated, "not a simple job for the average farm handyman."
Golde (1973:195) listed suggestions for lightning protection. Golde's treatment of lightning protection is very helpful and provides much information on the subject. Golde's major points concerning lightning protection of miscellaneous structures are:
1. If you consider the need for lightning protection, make local inquiries concerning the frequency of damage due to lightning and be guided by the results.
2. In many cases, it will be found cheaper to insure a building against lightning damage than to install lightning protection. In such cases, the risk of injury to occupants can be greatly reduced by having all metal services, e.g., electrical, gas and water installations, earthed.
3. The masts of domestic television or broadcasting aerials, whether installed on a roof of in a loft, should be earthed. This can be done at little expense and anyone living in an isolated position who neglects to do so is at extreme risk of a lightning strike.
4. The most serious consideration should be given to domestic and farm buildings built of flammable material, such as timber.
5. Protection should also be considered for dwellings or farm buildings in isolated situations.
6. Domestic houses or office buildings of ordinary height in built-up areas are in much less need of lightning protection than those in categories 4 and 5.
7. In high-rise buildings and other modern structure with a steel frame or reinforced concrete, metallic connection (bonding) between extended metal components both in, and on, the building is an effective method of protection.
8. The protection of structures involving a risk of explosion should invariably be discussed with an expert.
9. If expert advice is sought, this should be done in the design stage and not after the construction has been started.
10. Roof conductors should be so arranged as to protect particularly the edges of a roof. All structural parts which protrude above the roof, e.g., chimneys, should be included in the roof-conductor system. These conditions are best met by horizontally arranged conductors and full use should be made of structural metal components, such as gutters.
11. No separate down conductors are required on buildings with steel frame or of reinforced-concrete construction of where metal service pipes, such as water pipes, can be utilized to convey a lightning current to earth.
12. Earth electrodes should consist of rods in deep soil and be of horizontally-buried strip conductors in shallow soil. Isolated buildings in the country are best protected by buried ring conductors surrounding the structure and it is in the interest of personal safety that a low earthing resistance be secured. For buildings in built-up areas, the value of the earthing resistance is much less important and, if bonding is applied, it is immaterial.
13. A properly designed lightning conductor does not prevent the occurrence of a lightning strike, but it ensures that the energy released in a lightning flash is harmlessly discharged into the earth. No advantage is gained by giving a lightning conductor a fanciful shape, nor is there any reason to increase its cross-sectional area beyond 25mm2.
14. If a telephone is used during a thunderstorm, it is a good precaution to keep away from large metal objects and from electrical household equipment.
15. Remember that lightning sometimes does strike twice, or even more often.
16. Try not to panic when hearing a violent thunder clap; if you hear it, you have not been struck. The odds of getting hit are 1 in 600,000.
17. If you are keen on outside activities, familiarize yourself with the principles of resuscitation - you may be able to save a life. Give victims mouth-to-mouth resuscitation and heart massage. In a group, treat the "dead"- appearing first. Get others to a doctor as soon as possible.
18. The following (from the National Weather Service) are techniques for personal protection from lightning. (About 100 people are killed by it in the U.S. each year.) Lightning may strike 10 miles away from rainfall, so quick action before a storm is essential.
When thunderstorms approach:
If caught outdoors:
These suggestions should provide valuable assistance to the individual concerned with lightning protection.
The future outlook regarding lightning protection is very promising. On the "macro scale", prevention of lightning damage by predicting where and when storms of major electrical activity are going to occur is a technique being investigated (Fuquay 1980). Turnan (1979:321) stated that through the use of satellites, equipped with high-frequency radio receivers and optical detectors, major areas of lightning occurrence can be monitored on a world wide scale. Lightning distribution maps similar to those prepared by the World Meteorological Organization have been developed by PBE satellites. Distribution and seasonal trends of lightning occurrence have been monitored over time. This information will enable lightning occurrence for a particular geographical area to be estimated quickly and accurately. Applications using this system are many. Damage due to lightning may become an insignificant factor for those who work and use the outdoors.
Monitoring for the total area will include total annual lightning-caused 1) human injuries, 2) total deaths, 3) total structural damage events (not value since a national median value is available), and total lightning-caused wildfires. The effort over time is to reduce the sum of the weighted scores attached to 1-3 over time by means of the cost effective deployment of techniques described within this section.
Literature Cited
Boys, Sir C. V. 1926. Progress lightning. Nature 118:749-750.
British Standards Institute. 1965. Code of Practice, Cathodic.
Byers, H. R., and R. R. Braham. 1949. The thunderstorm. U.S. Department of Commerce, Washington, DC.
Fuquay, D. M. 1980. Forecasting lightning activity level and associated weather. USDA For. Service Res. Paper INT-244, Intermountain Forest and Range Exp. Station, Orgon, Utah. 30 pp.
Golde, R. H. 1973. Lightning protection. Chemical Publishing Co., new York, NY. 220 pp.
Schonland, Sir B. F. J. 1956. The lightning discharge. Encyclopedia of Physics 22:576-628. Springer Veilag, Berlin.
Sparrow, J. G., and E. P. Ney. 1971. Lightning observations of satellite. Nature 232:540-541.
Spurr, S. H., and B. V. Barnes. 1973. Forest ecology. Ronald press, new York, 2nd Edition 571 pp.
Thompson, A. R. 1946. Final report of lightning struck tree survey. 22nd National Shade Tree Conference.
Turman, B. N. 1979. Lightning protection from space. American Scientist, Vol. 67(3):321-329.
Viemeister, P. E. 1972. The lightning book. The MIT Press.
Workman, E. J. 1967. The production of thunderstorm electricity. J. Franklin Institute 283:540-557.
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