I think most of us absorbed this in earth science class, but if you need a refresher on how different air layers create different types of precipitation, here it is. (Also, the animations are quite pretty).
Looking around the site it turns out this is just one in a very large series of weather related interactives. Check it out. Some very cool stuff, including my hometown favorite: Lake Effect Snow (Irondequoit, NY).
An addictive collection of beautiful charts, graphs, maps, and interactive data visualization toys -- on topics from around the world.