When developing into a cumulonimbus, or thunderstorm, cloud, a cumulus becomes exceptionally dense and vertically developed, with a distinct base from which rain may begin to fall. Toward the cold top of the cloud are ice crystals. Swift winds at these higher altitudes, together with the outflowing air from the storm's updraft, can sculpt the cloud into the shape of a huge flattened anvil. Shown here are the upper parts of a cumulonimbus, with an anvil beginning to take shape (upper right).
