Wall clouds can be warnings that a tornado is about to form in a bank of cumulonimbus clouds. As a mesocyclone expands and gains strength below the cumulonimbus clouds, it begins rotating as warm, humid air moves upward and condenses. Convergence causes the gathering wall cloud to rotate cyclonically, though slowly compared to the tornadoes it could form. Storm chasers who observe such wall clouds know that they can form full-fledged tornadoes within an hour’s time.