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 lightning flash 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 weather 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 lightning channel at different distances. For this reason, thunder also lasts longer than the flash, and because the speed of sound is relatively low.

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.

Thunderstorms can be dangerous and it is best to take shelter during a thunderstorm. Lightning often strikes on a well-exposed target on the ground, such as high, well-conducting objects standing alone.

It is a good idea to take into account the following protection guidelines:

• Try to find shelter indoors. Even indoors, avoid the use and proximity of electrical devices, landline telephones, water pipes and fireplaces as the electric current from a lightning flash can travel on these surfaces and result in overvoltages in electric and telecommunication lines. Close all windows and doors.

• When outdoors, a car or other metal-bodied and closed vehicle provides good protection. Avoid biking and motorcycling. Avoid open areas but also seeking shelter under a tree. If you are outdoors in the open and find no shelter, make yourself low by crouching down close to (but not directly under) a large tree, keeping the feet together and protecting your hearing with your hands. Do not use an umbrella.

• Avoid water and the coastline. Go ashore well in advance. A sailboat requires lightning protection.

Thunderstorms can also result in indirect impacts, such as lightning-ignited fires and damage due to fallen trees resulting from strong wind gusts. Lightning striking airplanes is relatively common, but the people inside are safe because the electric current travels around the plane’s surface outside (similarly than in a car).The risk of being struck by lightning is very low. 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.