How Does Lightning Form?

As explained here lightning is just an electrostatic discharge. Electrons themselves cannot be seen, but when they are moving extremely fast, the air around them glows, causing the lightning flash. The actual streak of lightning is the path the electrons follow when they move.

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 to other atoms.

If 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 to exist in the liquid state below 0°C as super-cooled water, but eventually they begin to turn to ice, usually at temperatures below -20°C. These ice particles often collide and the smaller particles can lose an electron to the larger, thereby gaining a positive charge.

The small particles are propelled towards the top of the cloud by updraughts while the larger and heavier 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.

We explain the different types of lightening in the next section.