Creation and threat of lightning
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Strokes of lightning kill more people in Europe and North America each year than floods or tornados, causing billions of dollars in damage. The annual number of lightning-induced forest fires throughout the world easily exceed 10,000.
Since the experiments performed by B. Franklin, Romas and other lightning researchers, we know that lightning is a physical phenomenon. It is created in thunderstorm cells. The cold storm front which penetrates a hot area forces the warm and humid air to rise. Temperature decreases with altitude and the water vapor condenses to small water droplets. This process is accompanied by the creation of heat which accelerates the air current. Reaching altitudes with subzero temperature, the water drops freeze to ice crystals. Again, heat is produced simultaneously. The air speed increases once more – reaching a velocity of several hundred km/h – and propels the small ice particles to higher altitudes of up to 12 km.
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The growing ice crystals convert to hail stones which fall down due to their weight or remain in certain balanced positions. This causes electrons being stripped from the ice crystals. As a result of this process, charges are separated across a wide surface area. With field strengths of several 100 kV/m, discharges may be triggered in the form of cloud-to-cloud or cloud-to-earth lightning strokes, and in rare cases even as earth-to-cloud lightning.
The electrical charge of a lightning stroke may exceed 100 As. It is discharged to the earth within 10 to 100 ms. The temperatures created in the lightning channel are higher than those on the sun's surface. The air is heated so quickly that it expands with the force of an explosion. The resulting sound waves can be heard as «thunder» as far away as 20 km. Lightning flashes may be as long as 50 km, but are only a few millimeters thick.
Thunderstorms occur most frequently in the tropical and subtropical belts surrounding the earth, where the temperatures and the air humidity are very high. At any given time, almost 2000 thunderstorms are in progress on earth, and every 1/100 second or 6000 times a minute a bolt of lightning strikes the earth.
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For many reasons, the world is mapped concerning thunderstorm days – or the ground flash density (GFD maps) – and number of hits per area (square miles, square km, etc.). Also satellite flash event maps are available.
In the USA alone, lightning strikes 40 million times each year. Its occurrence in the USA is greatest within a 100-kilometer-wide strip crossing the state of Florida, called «lightning alley». In this area, thunderstorms can be observed on 90 days every year.
Such maps are an important tool to determine the hit risk for a certain location. But for a final conclusion a lot more factors have to be considered, and the calculation models consist of complicated formulas. Considerations are altitude, the height of the building, the surrounding profile, buildings in the neighborhood, the distance to water, earth material and even if a lightning protection system is installed, to name only a few of them. In many cases – especially in the areas of lower latitude, the more northern and southern regions of the world – the theoretically calculated hit risk might look negligible. But the interference of nearby hits is very dangerous as well. This is exceptionally critical within a distance of 100 m.
The lightning hazard to electric and electronic equipment consists in the interferences of direct lightning current injections and high surge voltages induced by the electromagnetic field of nearby lightning channels or down conductors. The damage caused depends on the energy involved and on the sensitivity of the electronic systems. The electric surge pulse generated by lightning is called LEMP (Lightning Electromagnetic Pulse).
Lightning research has produced a large number of suitable protective measures that are reflected in international and national safety standards. These instructions and recommendations for the installation of lightning protection systems, together with the application of HUBER+SUHNER lightning protectors, provide a high degree of safety for electronic equipment.
The installation of a lightning protector costs only a fraction of today's transceiver equipment. In the case of damage by EM interference in general - natural, but also man-made - the repair of the equipment as well as the loss of revenue and good reputation due to downtime have to be considered. All in all, there is not left much choice to an operator of mobile communications or other wireless services than to establish the best protection available.
