Published: 1 August, 2007

Charge separation - its cause not fully understood - causes lightning

What causes lightning?

There are several phases to the formation of lightning. The first phase is to create a separation of charges within a cloud. There are many theories on how this is achieved. The two most popular theories deal with wind, water, ice, and collisions, within tall clouds called cumulus . However, neither explains the process fully, mainly because they fail to generate the observed frequency of the lightning.

Water droplets are carried from the bottom of the cloud to the top by the strong updrafts. Eventually they pass through the freezing level and the droplets will begin to freeze forming an outer thin shell. The positive charges within the water droplet migrate to this thin ice shell. Turbulence is generated within the cloud and this causes the partially frozen droplets to collide, shattering the outer ice shells. Since ice is less dense than liquid water, the ice particles will be carried higher in the cloud by the updraft, while the heavier liquid droplet, which may or may not freeze, will fall. This creates a charge separation within the cloud, with positive charges at the top and negative charges at the bottom. This thermoelectric effect is still disputed to some degree.

Another process, called induction, may contribute to charge separation. After a thunderstorm develops a positive charge at the top and negative near the bottom, an induced charge will develop on the ice crystals within the cloud. The negative charges will migrate toward the end of the crystal facing the top part of the cloud and positive charges toward the bottom part. As updrafts bring small droplets and other ice crystals upward, they will collide with the larger ice crystals. The particles will "bleed" off the positive charges and carrying them to the top part of the cloud.

Once the charge separation is established, the second phase occurs in a two-step process. The first part consists of a stepped leader. This is a negatively ionized particle channel about 10 cm (4 in) in diameter, which moves towards the surface, and develops many branches extending from a main channel. Each section travels about 50 m (165 ft) in a microsecond. The sections continue until they approach the ground where there is a strong buildup of positive charges at the surface in that region. The positive charges will move upward through various objects such as trees or people in an effort to reach the stepped leader. Once the distance between the stepped leader and the ground is small enough, the attraction force between the positive and negative charges will break down the resistance of the air allowing the positive charges to "jump" across the air to meet the descending stepped leader. Air in the channel heats to about 30,000 K (54,0000oF), which causes the air to emit visible light and expand rapidly. The visible stroke travels upward from the ground to the cloud and the rapidly expanding air creates the thunder. Usually more than one stroke is needed to neutralize all negative ions so another leader, or dart leader, is initiated and a return stroke follows. The process is repeated about four or five times on average to neutralize all ions resulting in a flickering of the lightning flash. Since the positive charges fill the entire channel, some of the branches of the stepped leader which have not come in contact with the ground will be filled-in, thus giving the appearance that lightning is traveling downward. Both phases are constantly renewed during the life cycle of the thunderstorm.