It is this flow of charge that is the visible lightning stroke.Īfter the first discharge, it is possible for another leader to form down the channel. Once connected, electrons from the cloud can flow to the ground and positive charges can flow from the ground to the cloud. Streamers are most often initiated from tall objects on the ground. As it nears the ground, a positively charged streamer fires upwards from the ground to connect with it. It is called a stepped leader because it travels in 50 to 100 m sections, with a slight pause in between, to the ground. Although invisible to the naked eye, this allows electrons to move from the cloud to the ground. At first a channel, known as a stepped leader, is formed. Once the voltage reaches a certain strength, the air between the base of the cloud and the ground develops an electrical conductivity. As a result of this, a potential difference or voltage is created across the cloud-to-ground gap. Lightning productionĪs the area of negative charge at the base of the thundercloud builds up, it induces a region of positive charge to develop on the ground below. The negative charges accumulate at the bottom part of the cloud and the positive charges towards the top. Within a thundercloud, the rapid upward and downward movement of water droplets and ice crystals can separate and concentrate these charges.
On colliding with air molecules, they produce a shower of lighter particles, some of which are charged. Cosmic rays are high-energy particles such as protons that originate from outside the solar system. The formation of electric charges in the atmosphere is due mainly to the ionisation of air molecules by cosmic rays. It is the formation and separation of positive and negative electric charges within the atmosphere that creates the highly intensive electric field needed to support this natural spark discharge that is lightning.
However, beyond 16 km, thunder is seldom heard. Distant thunder has a characteristic low-pitched rumbling sound. At about 1 km away, it is heard as a rumble with several loud claps. Thunder is effectively exploding air, and when heard close to the lightning channel, it consists of one large bang. At about 10 m out from the channel, it becomes an ordinary sound wave called thunder. The surrounding air is rapidly heated, causing it to expand violently at a rate faster than the speed of sound, similar to a sonic boom. Temperatures in the narrow lightning channel reach about 25,000☌. The combination of light from luminescence and incandescence gives the bolt of lightning its characteristic colour. At the same time, nitrogen gas (the dominant gas in the atmosphere) is stimulated to luminesce, producing bright blue-white. The extremely high temperatures generated heat the air molecules to a state of incandescence (white hot) such that they emit a vivid white light.
Lightning is visible as a flash of blue-white light.
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