Visible discharge of electricity when part of the atmosphere acquires enough electrical charge to overcome the resistance of the air. During a thunderstorm, lightning flashes can occur within clouds, between clouds, between clouds and air, or from clouds to the ground. Lightning is usually associated with cumulonimbus clouds (thunderclouds) but also occurs in nimbostratus clouds, in snowstorms and dust storms, and sometimes in the dust and gases emitted by a volcano. A typical lightning flash involves a potential difference between cloud and ground of several hundred million volts. Temperatures in the lightning channel are on the order of 30,000 K (50,000 °F). A cloud-to-ground flash comprises at least two strokes: a pale leader stroke that strikes the ground and a highly luminous return stroke. The leader stroke reaches the ground in about 20 milliseconds; the return stroke reaches the cloud in about 70 microseconds. The thunder associated with lightning is caused by rapid heating of air along the length of the lightning channel. The heated air expands at supersonic speeds. The shock wave decays within a metre or two into a sound wave, which, modified by the intervening air and topography, produces a series of rumbles and claps. See alsothunderstorm.