Originally posted here by Striek
But because oxygen (O^2) is lighter than air it rises. At a specific altitude (I'm not sure which), it stops rising as the air around it becomes lighter too.
Most of the post is pretty much right, but this is a little off. Air is composed Of a mixture of Nitrogen, Oxygen and a few other gasses in small amounts. They are all mixed equally and don't seperate out, and least not in the earth's atmosphere (Otherwise, you might have some toruble breathing) The ozone is simply created out of whatever oxygen happens to be up at that altitude.