A large discontinuous discharge is produced through the air, generally under turbulent conditions of the atmosphere associated with thunderstorms. The electricity is generated in cumulonimbus clouds by separation of the electric charge associated with the upward movement of air and the freezing of water droplets. The main discharge of lightning runs from the earth upward (return stroke) along a channel prepared by a leader discharge. The peak value of the lightning current exceeds 35 kiloamps (35 kA, or 35,000 amps) in 50% of cases, with one percent exceeding 200 kA. The temperature in the lightning channel may reach 20,000 to 30,000 deg C and the rapid heating of air produces an explosion which is heard as thunder. When lightning strikes it may cause loss of life, destroy structures or cause fires. The high voltage or current produced in electrical and electronic equipment by lightning causes severe damage to them. Microelectronic devices are especially sensitive to the secondary effects of lightning (such as induced voltage) if they are not protected.
Lightning strikes the earth an average of 100 times a second totaling over 3 billion strikes a year.
Electronic counters produce data which provide an estimate of about about two lightning strikes per sq km in temperate zones and between 10 and 15 strikes per sq km in tropical zones each year. Lightning discharges between clouds occur about twice as frequently. In forest areas in the USA it has been estimated that one sizeable fire results per annum from lightning for each 40 sq km of forest. Estimates for lightning-caused deaths are: USA, 160 per annum; West Germany, 41 per annum (92 people struck each year). Despite assumptions to the contrary, lightning may cause fatal damage to aircraft. It has been estimated that over Europe one aircraft is hit by lightning for every 2400 flying hours, and once every 6,000 flying hours over oceans. The cost of damage to electrical and electronic devices due to secondary effects of lightning may be ten times higher than that produced directly by a stroke of lightning.