Cumulonimbus clouds grow vertically and often quite rapidly. I heard thunder minutes after taking this photo late one August afternoon. It is common in the summer to get pop-up thunderstorms due to the warm, moist, uprising air. These clouds can grow up to 60,000 feet in height; the higher the cloud tops the more violent the storm will be. These clouds can produce downburst winds, torrential rain, and sometimes even hail or tornadoes.