Lightning

Lightning (also called hilaac) is the bright flash that results from an electrical discharge in the atmosphere. It occurs when the electrical potential difference between parts of a cloud, or between a cloud and the ground, becomes large enough for electricity—which normally does not conduct through air—to pass through it.

Lightning is usually accompanied by thunder. Thunder is the sound produced by the rapid expansion of air when it is heated to an extremely high temperature. Light reaches the eye immediately, whereas sound travels gradually; this is why lightning is seen before thunder is heard.

How it occurs

Cumulonimbus (thunderstorm) clouds contain rising air carrying water droplets, small ice, and ice crystals. The collision of these materials separates electrical charge: negative charge moves downward, positive charge accumulates upward. When the electrical potential difference reaches a threshold, a rapid discharge occurs and is neutralized.

Types

Some lightning occurs between one cloud and another, or within the same cloud, which is the most common form. The type that affects people and property is cloud-to-ground lightning. There also exist discharges related to the upper atmosphere that originate from cloud tops.

Hazards and protection

Lightning can cause injury and death, and can also start fires in trees and buildings. General safety advice includes staying indoors or in a closed vehicle, avoiding isolated trees, heights, and water. Large buildings are fitted with metal apparatus that conducts electricity safely to the ground.

Research and measurement

Weather monitoring systems use networks of ground-based sensors and handheld devices to determine the location and time of each discharge. This information is used for aviation warnings, power management, and outdoor operations.