I usually see these huge towering clouds rising over the hills behind my house after a storm and they are gorgeous. Cumulous congestus form in unstable areas of the atmosphere which are undergoing convection and that usually causes them to be taller than they are wide. They can reach an altitude of 15,000 to 20,000 feet.
It is amazing to watch them grow right before your eyes. Because they rise from behind the hills here at my house they sometimes look like giant mushroom clouds and the first time I saw one form I was absolutely amazed.
I'm looking forward to a weekend with a new moon/dark sky, Leonid meteor showers, a new place to explore with some incredible rock formations and a natural arch, and (hopefully this time please) low winds so I can do some night photography.