Lightings are a natural phenomenon through which clouds unload an enormous quantity of energy over the Earth. Even if we know from a long time ago the necessary conditions to produce lightning, it isn’t possible to predict when and where lightning will take place.
Generation of the electric field
Storms and very turbulent environments. Strong hot air flows (that go up) and cold air flows (which go down) move through the storm at high speeds. Hot air contains small water droplets. However, in the cold air there are small ice crystals. In this opposite movement of airflows, the crystals and water droplets collide. This constant rubbing of particles in the clouds produces static electricity.
Just like a battery, these clouds have a positive end and a negative end. Positive charges are in the top part of the cloud, and the negative charges are at the bottom part.
However, the atmosphere is a very powerful insulator, and an enormous quantity of energy has to accumulate before producing lightning. Once the energy threshold is reached, the force of the electricity is able to overcome the insulating properties of the atmosphere, and lightning is produced.
The electric field of the cloud isn’t the only one that is produced. Under the base of the clouds forms an area of positive charge on the Earth’s surface. These positive charges follow the storm wherever it goes.
How is lightning produced?
The positive zone that forms on the ground is the one that enables the lightning to hit the surface because negative charges are attracted by the positive ones. However, the electric field in the clouds is much more powerful that the one between the base of the storm and the Earth’s surface. For this reason, most lightings (around 80%) take place in the clouds themselves.
For lightning to hit the ground, negative charges of the cloud form a “stream” through the air that is directed towards the positive charges of the surface, until they connect the ones with the others.
The resulting electrical transfer when the two charges meet is what we see as lightning. Even if we know how these phenomena are produced, no one knows why lightings follow a zigzag trajectory as they move.
When lightning is produced, the latter emits a powerful flash of light, which heats up a lot the surrounding air (the temperature of lightning varies between 15,000 and 50,000 Celsius degrees). In comparison, the surface of the Sun is at 10,000ºC. Being so hot, the air rapidly expands, which provokes the strong sound that we hear as thunder.