Lightning is formed in cumulo-nimbus clouds, which are very tall clouds reaching up several miles, to where the air is very cold. Scientists are not quite sure how clouds generate electricity, but one theory is that ice particles and water drops become charged when they come into contact (just like rubbing fur on plastic), with the result that rain brings down charges of one type, leaving an increasing negative charge on the cloud. Lightning bolts then bring the the negative charges from the cloud to combine with the positive charges on the ground. The majority of lightning bolts go from cloud to cloud; only about a quarter of them go to the ground.

The clouds are charged to 10,000,000 volts; a single lightning bolt lasts about a thousandth of a second and delivers about ten thousand amperes (the same total charge would be delivered by a 1 ampere current flowing for several seconds, but that wouldn't be as exciting) over a distance of several miles. The total energy released is approximately 100 Kilowatt-hours ($10 worth of electricity). The energy released heats the air in a channel less than an inch wide to 50,000 degrees, producing the flash and the boom. The heated path becomes electrically conducting, and there are frequently several lighting flashes along the same path within a few hundredths of a second.

Sound travels much more slowly than light (1/5 mile per second, versus 186,000 miles per second). The thunder is a long rumble because we hear different parts of the lightning bolt at different times. If you can hear thunder at all, the source is within a few miles -- which is also how far lightning bolts travel: time to seek shelter now!

Despite the large numbers, lightning bolts hit tall buildings regularly, without significant damage. The current is carried by the steel frame of the building.

Why study sources of electrical energy?
The section on sources of electrical energy