How do Thunderstorms Work?

The Science of Thunder and Lightning

Scientists began to suspect that lightning must be a form of electricity as early as the 1700s because it looked similar to the sparks you could produce by rubbing certain materials together. Scottish scientist Robert Symmer had this off to a fine art and earned the dubious title of "the barefoot philosopher" because he was always removing his silk socks and rubbing them on things to produce sparks.
But it was a daring experiment by Benjamin Franklin in 1752, and one which he was lucky to survive, that proved it once and for all. Franklin flew a kite into a thundercloud and was rewarded with a stream of sparks flowing from the bottom of the kite string.

How is lightning generated?

Franklin's experiment worked because lightning is a multi-million volt electrical discharge between one cloud and another, or between a cloud and the Earth. It's produced when friction between tiny water and ice particles in clouds, called "hydrometeors", generates static electricity. For reasons that scientists don't fully understand, the smaller particles pick up a positive charge, and the larger particles pick up a negative charge. As these hydrometeors jostle about, updrafts push the smaller positively-charged particles towards the top of the cloud, leaving the negative charges concentrated at the bottom. It's possible that the solar wind, a million mile an hour maelstrom of cosmic radiation streaming out of the sun, may help in this sorting process.
Before long the cloud accumulates a massive potential difference measured in millions of volts. This electrical potential creates a powerful electric field, a bit like the contour lines on a map, which stretches from the bottom of the cloud to the ground (Earth). As a result the ground becomes positively charged as electrons are repelled away by the negative charge in the clouds. Tall and sharp objects, like buildings, trees, lightning conductors, and even golfing umbrellas, deform the contour lines of the field and push them close together, concentrating the electric field around the top of the object and making it a target for a strike. This happens when the field becomes sufficiently strong to overcome the insulting properties of the air, and the cloud discharges to Earth, producing a lightning bolt.
Each lightning flash is about 3 miles long but only about a centimetre wide. It discharges about 1-10 billion joules of energy and produces a current of some 30,000 - 50,000 amps, which heats the surrounding air to over 20,000 degrees Celsius, three times hotter than the surface of the sun (6000 degrees Celsius). In fact a single lightning bolt unleashes as much energy as blowing up a ton of TNT. And although it might look like a single flash, a strike is actually made up of between three and twelve individual lightning 'strokes', each lasting only a few thousandths of a second. This is what makes lightning appear to flicker.

And what about thunder ?

The intense heat of the lightning discharge superheats the surrounding air causing it to expand explosively. This creates a compression or 'shock' wave - the thunder - which spreads out through the air in all directions, travelling at about a fifth of a mile per second.
The flash and the thunder clap are produced simultaneously - as anyone unlucky enough to have ever got very close to a lightning strike can tell you - but the light from the flash travels much more rapidly (186,000 miles per second) than sound (0.2 miles per second approximately). The light therefore reaches you first, then a short while later (depending upon how far away the storm is), the thunder rolls in.