Boodler [1] is an open-source system for generating soundscapes. It comes with a hour long thunderstorm soundscape [2] that grows and then fades away.

The author of Boodler also has an iOS app [3] specifically for the thunderstorm.

[1]: http://boodler.org/ [2]: http://boodler.org/package/com.eblong.ow.storm/ [3]: http://zarfhome.com/pocketstorm/