Lightning is a sudden, violent flash of electricity between a cloud and the ground, or from cloud to cloud. A lightning flash, or bolt, can be several miles long. It is so hot, with an average temperature of 34,000o Centigrade, that the air around it suddenly expands with a loud blast. This is the thunder we hear.
Lightning occurs in hot, wet storms. Moist air is driven up to a great height. It forms a type of cloud called cumulo-nimbus. When the cloud rises high enough the moisture freezes and ice crystals and snowflakes are formed. These begin to fall, turning to rain on the way down. This rain meets more moist air rising, and it is the friction between them which produces static electricity, it discharges it as a lightning flash.