The rare positive lightning bolt can strike from clear blue sky
Fewer than one lightning strike in twenty is positive, but those bolts can carry up to 400,000 amps, roughly double a typical strike's peak, and travel through clear air far from the storm. That is why they earned the name bolts from the blue: they arrive without warning.
Lightning is a sudden electrostatic discharge between two charged regions, at least one of them in the atmosphere. A single flash releases somewhere between 200 megajoules and 7 gigajoules and heats the surrounding air to about 30,000 degrees Celsius. That abrupt heating creates a pressure shock wave, which we hear as thunder. Thunderstorms are the usual source, but volcanic eruptions produce lightning too, and the science of it goes by the name fulminology.
There are three main forms. Intra-cloud lightning, inside a single storm cloud, is the most frequent. Cloud-to-cloud lightning jumps between separate clouds. Cloud-to-ground lightning is the least common, yet it is the best understood, because it ends on the ground where instruments can measure it, and it is the type that threatens people and property. It begins when a stepped leader works downward from the cloud and meets a streamer rising from the ground.
Ground strikes are classed by the direction of current. In a negative strike, electrons flow down the channel to the ground; an average one carries about 30,000 amps and roughly 15 coulombs of charge. Positive strikes send positive charge downward, often followed by long continuing currents that heat surfaces enough to start fires. They are more common in winter thundersnow, intense tornadoes and dying storms, and they help trigger sprites tens of kilometres overhead.
A popular belief holds that bolts launching from a storm's anvil top are positive because they seem to leave the cloud's positive region. Observations show these are actually negative flashes that start inside the cloud, exit through the positive zone, then cross clear air to hit the ground some distance away.
Source: Lightning
