Lightning

Lightning is a massive electrostatic discharge caused by unbalanced electric charges in the atmosphere, and resulting in a strike, from a cloud to itself, a cloud to a cloud or a cloud to ground, and accompanied by the loud sound of thunder.

A typical cloud to ground lightning strike is over 5 km (3 mi) long. A typical thunderstorm has three or more strikes per minute at its peak. Lightning is usually produced by cumulonimbus clouds up to 15 km high (9 mi) high, based 5–6 km (3-4 mi) above the ground. Lightning is caused by the circulation of warm moisture-filled air through electric fields. Ice or water particles then accumulate charge as in a Van de Graaff generator. Lightning may occur during snow storms (thundersnow), volcanic eruptions, dust storms, forest fires or tornadoes. Hurricanes typically generate some lightning, mainly in the rainbands as much as 160 km (100 mi) from the center.

When the local electric field exceeds the dielectric strength of damp air (about 3 million volts per meter), electrical discharge results in a strike, often followed by commensurate discharges branching from the same path.

Mechanisms that cause lightning are still a matter of scientific investigation. The science of lightning is called fulminology. The fear of lightning is called astraphobia.

Lightning strikes 40–50 times a second worldwide, for a total of nearly 1.4 billion flashes per year.

Cloud-to-ground (CG) lightning accounts for 25% of lightning globally. The base of the negative region in a cloud is typically at the elevation where freezing occurs. The closer this region is to the ground, the more likely cloud-to-ground strikes are. In the tropics, where the freeze zone is higher, 10% of lightning is CG. At the latitude of Norway (60° lat.) where the freezing elevation is lower, 50% of lightning is CG.

Lightning is not distributed evenly around the planet. About 70% of lightning occurs on land in the tropics, where most thunderstorms occur. The north and south poles and the areas over the oceans have the fewest lightning strikes. The place where lightning occurs most often is near the small village of Kifuka in the mountains of eastern Democratic Republic of the Congo, where the elevation is around 975 metres (3,200 ft). On average this region receives 158 lightning strikes per 1 square kilometer (0.39 sq mi) a year. Other hotspots include Catatumbo lightning in Venezuela, Singapore, Teresina in northern Brazil and "Lightning Alley" in Central Florida.

According to this cloud particle charging hypothesis, charges are separated when ice crystals rebound off graupel. Charge separation appears to require strong updrafts which carry water droplets upward, supercooling them to between -10 and -40 °C (14 and -40 °F). These water droplets collide with ice crystals to form a soft ice-water mixture called graupel. Collisions between ice crystals and graupel pellets usually result in positive charge being transferred to the ice crystals, and negative charge to the graupel.

Updrafts drive the less heavy ice crystals upwards, causing the cloud top to accumulate increasing positive charge. Gravity causes the heavier negatively charged graupel to fall toward the middle and lower portions of the cloud, building up an increasing negative charge. Charge separation and accumulation continue until the electrical potential becomes sufficient to initiate a lightning discharge, which occurs when the distribution of positive and negative charges forms a sufficiently strong/high electric field.