Thunderstorms

An atom consists of three basic parts, a proton (which has a positive charge), a neutron (which has no charge) and an electron (which has a negative charge). Electrons cling to the positively charged centre of the atom because they have a negative electrical charge. During a thunderstorm, some of the atoms in the cloud lose electrons while others gain them.

When a cloud is composed entirely of water droplets, there is very little transfer of electrons. As a storm cloud grows in height, the temperature of the water droplets higher up falls. They continue in the liquid state below 0 °C as super-cooled water, but eventually they begin to turn to ice, usually at a temperature below -20 °C. These ice particles often collide and the smaller particles lose an electron to the larger, thereby gaining a positive charge.

The small particles are propelled towards the top of the cloud by strong internal winds while the larger particles start to fall. This causes the top of the cloud to develop a strong positive charge.

The larger, negatively charged, ice particles begin to ‘capture’ super-cooled water droplets, turning them instantly to ice and thereby growing, some reaching a sufficient size to start falling.

This leads to the base of the cloud becoming negatively charged which, in turn, induces a positive charge on the ground below. In time, the potential gradient between cloud and ground, or between adjacent clouds, becomes large enough to overcome the resistance of the air and there is a massive, very rapid transfer of electrons, which appears as a lightning flash.

There are several types of lightning, all of which are made up of different parts and none of which are alike. Lightning that shoots from the cloud to the ground is made up of four main parts: a stepped leader, upward streamers, return strokes and dart leaders.

As negative charges collect at the base of the cloud, they repel the electrons near the ground’s surface. This leaves the ground and the objects on it with a positive charge. As the attraction between the cloud and the ground grows stronger, electrons shoot down from the cloud. The electrons move in a path that spreads in different directions – like a river delta. Each step is approximately 50 metres long and the branching path is called a stepped leader. Further electrons follow, making new branches. The average speed at which the stepped leader cuts through the air is about 270,000 miles per hour.

As the stepped leader approaches the ground, positive electrical sparks rise from tall objects such as trees and buildings. These sparks are known as upward streamers. When the stepped leader meets the upward streamer, the lightning channel is completed. When the lightning channel is complete, the electrons in the channel rush towards the ground. This is the return stroke which lights up the channel. The first electrons to reach the ground light up the bottom of the channel. The upper part of the channel glows as the electrons move rapidly down it. Therefore, the light from the flash starts at the ground and moves upwards. The branches of the stepped leader are also lit up, but not as brightly as the main channel as there are less electrons present. The lightning flash ends when there are no electrons left in the channel.

If lightning flickers, it is probably because there has been more than one return stroke. Following a lightning flash, the lightning channel is momentarily empty and it is then possible for electrons from another part of the cloud to enter it. The movement of the electrons into the channel is called a dart leader. It causes another return stroke to occur. The repeated return strokes and dart leaders make the lightning appear to flicker because of the great speed at which they occur.