The following is taken from the National Severe Storms Laboratory.

What is lightning?
Lightning, as best we understand, is a channel of negative charge, called a stepped leader that zigzags downward in roughly 50-yard segments in a forked pattern. This step leader is invisible to the human eye, and shoots to the ground in less time than it takes to blink. As it nears the ground, the negatively charged step leader is attracted to a channel of positive charge reaching up, a streamer, normally through something tall, such as a tree, house, or telephone pole. When the oppositely-charged leader and streamer connect, a powerful electrical current begins flowing. A return stroke of bright luminosity travels about 60,000 miles per second back towards the cloud. A flash consists of one or perhaps as many as 20 return strokes. We see flicker when the process rapidly repeats itself several times along the same path. The actual diameter of a lightning channel is one-to-two inches.

Does lightning strike from the sky down, or the ground up?
The answer is both. Cloud-to-ground lightning comes from the sky down, but the part you see comes from the ground up. A typical cloud-to-ground flash lowers a path of negative electricity (that we cannot see) towards the ground in a series of spurts. Objects on the ground generally have a positive charge. Since opposites attract, an upward streamer is sent out from the object about to be struck. When these two paths meet, a return stroke zips back up to the sky. It is the return stroke that produces the visible flash, but it all happens so fast – in about one-millionth of a second – so the human eye doesn’t see the actual formation of the stroke.

How many people are killed by lightning?
An average of 67 people are killed by lightning each year and about 300 are injured.

Can lightning strike the same place twice?
Lightning does hit the same spot (or almost the same spot) more than once, contrary to folk wisdom. It could be simply a statistical fluke (i.e., with all the lightning that occurs, eventually lightning will strike somewhere near a previous lightning strike within a short period of time). It could also be that something about the site makes it somewhat more likely to be struck. Typically, when lightning strikes something on the ground, the object that is struck sends a faint channel upward that joins the downward developing flash and creates the connection to the ground. Taller objects are more likely than shorter objects to produce the upward channel. But it is also possible that something that locally affects the ability of the ground to conduct electricity (such as the salt or moisture content of the ground at the time, the presence or absence of rock, standing water, pipes or other metal objects in the ground), the terrain shape, the shape of leaves or twigs, or something else might make a particular location more likely than another nearby location to be struck.

What is a stepped leader?
A stepped leader is a stream of weakly charged particles that flows from the cloud – it moves towards the ground, starting and stopping, and sometimes branching, trying to find the path of least resistance.

Does lightning always strike the tallest object?
Never say always! Lightning USUALLY strikes the tallest object. It makes sense that the tallest object is most attractive, because it is the easiest path for the lightning to take.

What type of electricity is lightning?
Lightning is an electrostatic discharge accompanied by the emission of visible light and other forms of electromagnetic radiation.

How many volts and watts are in lightning?
Lightning can have 100 million to 1 billion volts, and contains billions of watts.

How hot can lightning make the air?
Energy from lightning heats the air anywhere from 18,000 degrees Fahrenheit to up to 60,000 degrees Fahrenheit.

How long can a lightning bolt be?
Recent research from Vaisala-GAI’s LDAR and LDAR II lightning detection networks show that lightning can travel 60 miles or more. The longest bolts start at the front of a squall line and travel horizontally back into clouds trailing behind the squall line. The longest bolt they have seen to date was 118 miles long in the Dallas-Ft. Worth, TX area. Since 3-D lightning measurements are relatively new, scientists are learning more every day and these numbers may change.

How many flashes a year are there?
Over the continental 48 states, an average of 20,000,000 cloud-to-ground flashes have been detected every year since the lightning detection network covered all of the continental US in 1989. In addition, about half of all flashes have more than one ground strike point, so at least 30 million points on the ground are struck on the average each year in the US. Besides cloud-to-ground flashes, there are roughly 5 to 10 times as many cloud flashes as there are ground flashes.