Lightning is a large electrical discharge caused by a thundercloud. It can occur within a cloud as intracloud lightning, between clouds as intercloud lightning, or between the cloud and the earth as cloud-to-ground lightning. A lightning discharge consists of pulses of electric current carried by electrons. The current is driven by a high voltage between the cloud's charge centres or between them and the earth. During the development of a thundercloud, negative charge is accumulated in the hail-forming region at the central part of the cloud, and positive charge in the top region which consists of ice crystals.

When enough charge is accumulated, the air breaks down electrically and narrow, hot, highly ionised channels are formed where the movement of electrons neutralises the accumulated charges. Ionisation means that negative electrons are removed from air molecules which thus remain positive, and the insulating air becomes electrically conductive.

The main phases of cloud-to-ground lightning are leaders and return strokes. A leader generally comes down from the cloud, forming or re-charging the lightning channel, and a return stroke neutralises it, proceeding very quickly from the ground to the cloud. A leader-return stroke pair is called a stroke. A lightningflash consists of one or more strokes, which may follow the same channel of fork into two or more. The whole flash usually lasts less than a second.

In Finland, thunderstorms are divided into two general types: air-mass thunderstorms and frontal thunderstorms. In the former, the instability that makes thunderclouds to develop occurs in the same air mass, and is often caused by Sun's heating in the afternoon. In the latter, the main energy for the thunderstorms comes from the differences between the two air masses in the front. Cold-front thunderstorms are usually the most violent.

Thunder is the sharp or rumbling sound that accompanies lightning. It is caused by the intense heating and expansion of the air along the lightning channel. The rumble of thunder is caused by the noise arriving from parts of the channel at different distances. For this reason, thunder also lasts longer than the flash, and because sound speed is relatively low (see the next question).

Light travels 300 000 kilometres per second, so the flash can be seen immediately. Thunder starts at the same time, but its sound travels one million times more slowly, about 330 metres per second.

The distance can be estimated by counting the time interval between the lightning flash and the start of the thunder. If you count seconds and divide them by three, you get the distance in kilometres. Thunder is rarely heard at a distance of more than 20 km.

Thunder cannot hurt anybody (except by the sound-pressure shock if the lightning hits very close), and the risk of being struck by lightning is far less than that of being killed in a car crash. Most lightning occurs as cloud flashes, which are harmless on the ground. Most of the flashes that do strike the ground cause little or no damage or harm. Lightning often chooses a well-exposed target on the ground, usually a high well-conducting object standing alone.

Lightning strikes lone trees on high ground, so a wise thing would be to not seek a shelter under a tree. If you are in the open and find no shelter, make yourself low by crouching down, keeping the feet together. Indirect dangers from thunderstorms are lightning-ignited fires, overvoltages in electric and telecommunication lines, and gusts of strong wind.

Lightning strikes aircraft, but the people inside are safe because it runs around the outside. Lightning strikes tall buildings, but they have lightning conductors to carry the electricity harmlessly to the ground. The safest place might be in a car.