For those who have never seen this kind of thing, this is what a good old-fashioned midwestern thunderstorm looks like from slightly above and twenty miles to the side. Notice the flat, anvil-shaped top.
This kind of storm is really spectacular at night. In addition to the usual amazing displays of lightning, thunderstorms have a few other tricks up their sleeves. Google "red sprites" and "blue jets" to learn about some other cool phenomena that pilots sometimes get to see. (I've seen a red sprite only once, and I'll never forget it...)
When I took this photo, we were flying at FL390 (39,000 feet). This particular storm wasn't particularly huge, as you can see by the fact that we're above its top. Many summer storms top 50,000 feet. We always stay well away from those... 
