4. LIGHTNING 



Basic Processes of Lightning 



About 2,000 thunderstorms are in 

 progress over the whole earth at any 

 given time. These storms produce a 

 total of about 1,000 cloud-to-ground 

 and 500 intracloud lightning dis- 

 charges each second. It follows that 

 there are over S million lightning 

 discharges each day to earth, and 

 about 5 times as many discharges 

 within the clouds. 



Lightning is essentially a long 

 electric spark. (See Figure V-14) The 

 total electrical power dissipated by 

 worldwide cloud-to-ground lightning 

 is roughly equal to the total annual 

 power consumption of the United 

 States, about 500 billion watts. On 

 the other hand, the energy from a 

 single lightning flash to ground is 



only sufficient to light a 60-watt bulb 

 for a few months. It is the high 

 worldwide rate of lightning flashing 

 that provides the high power levels. 



The electrical energy that generates 

 lightning is transformed to sound 

 energy (thunder), electromagnetic 

 energy (including light and radio 

 waves), and heat during the discharge 

 process. The radio waves emitted 

 by the hundreds of lightning dis- 

 charges per second provide a world- 

 wide noise background. The level at 

 which many communications systems 

 can operate is limited by this back- 

 ground noise level. The radio waves 

 emitted by a single close (say, closer 

 than one mile) lightning discharge 

 can also cause malfunction of sensi- 



tive electronic systems (particularly 

 solid-state systems) such as are used 

 in modern guided missiles. 



The heat generated by the lightning 

 channel sets forest fires, ignites flam- 

 mable materials, and can be a cause 

 of individual death. Of the over 8 

 million discharges that hit the earth 

 daily, very few cause damage. For 

 example, most lightning to wooded 

 areas does not cause forest fires. 

 Still, there are about 10,000 forest 

 fires a year in the United States at- 

 tributable to lightning; and about 

 2,000 rural structures, roughly half 

 of which are barns, are destroyed 

 by lightning-induced fires each year. 



Lightning strikes about 500 U.S. 

 commercial airliners per year. Most 



Figure V-14 — LIGHTNING 



(1) This photograph shows a normal cloud-to-ground lightning flash near Mount 

 San Salvatore. Lugano, Switzerland. Note how the streamers from the main lightning 

 strokes branch downward. (2) In this photograph, a tall tower on Mount San Salva- 

 tore has triggered a lightning flash. Note how the streamers branch upward, indicat- 

 ing a reverse situation from the normal lightning flash. 



157 



