Thunder and Lightning


Why do we see lightning before we hear thunder and why does the thunder last so much longer than the lightning?


The first part of this question has an easy answer. The second part I am still wondering about.

Let's deal with the first part first. Lightning is an electrical discharge conducting electricity between cloud and ground. This electrical current travels at about 1/3 the speed of light, or 100,000,000 meters per second. The length of a lightning stroke may be about 1000 meters, for example, so the lightning makes it all the way to the ground is about ten millionth of a second. Way too fast for our eyes to follow, which is why the lightning stroke seems to appear all at once.

lightningThe passage of the electricity through the atmosphere heats the air along the path of the discharge to about 50,000 degrees F. This causes the air through which the lightning passed to glow white hot for a fraction of a second until it cools off. That is what we see as lightning. The light from the glowing air travels to our eyes at 300,000,000 meters per second so there is almost no delay in our seeing the lightning.

The other effect coming from the heating of the air by the electrical discharge is that the air has no time to expand during the temperature increase to over 50,000 degrees. It is contained by its own inertia. This causes a large pressure increase in the air along the lightning's path to perhaps 100 times normal. This channel of high pressure air expands rapidly into the surrounding air causing pressure fluctuations to spread out from the place where the lightning appeared. This is the clap of thunder which accompanies the lightning. Sound travels at about 1100 feet per second so if you are a mile or so from the strike, it will take the sound about five seconds to reach you. That is why the lightning is all over, from your point of view, before the thunder happens.

Now let's speculate a bit about the relative duration of the lightning and the thunder. The lightning disappears when the glowing air cools below a certain temperature. This happens very quickly, as anyone who has tried to photograph a lightning strike knows. The thunder may roll on for many seconds though. Why would the pulse of sound have any longer duration than the pulse of light? Two possibilities occur to me. One is that the expanding column of super heated air collapses to an over-pressure condition, setting up a reveberation within the air at the strike location. The other is that the initial pulse of sound bounces of the terrain so that we hear echoes long after the pulse has passed. I think I need more research to resolve this issue. If any of you have a good explanation, let me know.

By the way, there is an excellent source of weather related information on the web sponsored by USA Today. It is their "Ask Jack" page.

New thought: There is an obvious answer to the duration of thunder that escaped me previously. A lightning bolt may be miles long. If the distance from the bolt to the observer were to be constant the bolt would have to follow a spherical arc centered on the observer - not very likely. So the sound we hear, coming from the whole length of the lightning bolt, is stretched out in time due to the different travel times from different parts of the bolt.