Lightning originates from the towering mass of cumulonimbus cloud, the bringer of thunderstorms. It can strike from cloud to cloud, cloud to ground or inside one cloud (intracloud).

Water droplets freeze to form ice crystals inside the higher part of the mass at a certain altitude, which varies with ground conditions.

The separation between the ice and water droplets in the lower part of the clouds and friction between the cloud and the air causes a positive electrical charge to build in the top of the cloud and a negative charge to grow at the bottom. When the difference between the two charges becomes strong enough to break the resistance of the air to create a conducting path (often hundreds of millions of volts), lightning is caused.

Air in the path of the lightning heats rapidly to thousands of degrees, expanding and then quickly shrinking back. It is the air pushed outwards around the discharge of electricity that makes the sound of thunder.

Because light travels far faster than sound (300,000km per second as against 20km per second), lightning is seen before the thunder is heard. The further away the cloud is from the viewer, the greater the delay between the thunder and the lightning that precedes it.

Lightning strikes can kill or destroy as the force passes to ground, with anything high or made of metal at particular risk as the closest and best conductors.

The US is one of the most active areas for lightning and, according to the World Meteorological Organisation, the phenomenon kills around 100 people there each year.