The first thing that you need for a lightning storm is a big cloud.
In particular, you need a big cloud that's moving. As the cloud moves
over the ground, it builds up a whole lot of electrical charge. This is
the same as when you drag your feet on a carpeted floor. You build up
charge that can shock you later. When one kind of charge builds up in
one place, its first priority is to spread out again. In order to
spread back out, the charge on the cloud will jump to the ground.
(Since the Earth is really big, the charge can spread out there.) This
is when we see lightning.
So how about thunder? Well, as the electricity of the lightning
passes through the air, it ionizes the air molecules. When this
happens, the air in that area gets very very hot. Hot air takes up more
space than cold air, so this suddenly hot air expands very rapidly.
When the air expands, it shakes the air molecules around it. Shaking
air creates soundwaves, which we hear as thunder. Thunder is literally
the sound of a giant explosion occuring all along the lightning strike.
But why do we hear a long rumble instead of just a loud bang? Well,
the top of the lightning bolt is a lot farther away from us than the
bottom (since it's a lot farther up). This means that the soundwaves
coming from the top have farther to go and will take longer to get to
us than the soundwaves coming from the bottom of the bolt. So we hear
the sound of thunder from the bottom of the lightning bolt first and
the sound from the top later.
(republished on 08/02/06)