"A whale's mouth forms when a downdraft of cold air descends from a thunderstorm. Upon reaching the ground, the pool of cold air spreads out, much like a puddle of water expanding. At its leading edge, it forces the warm, moist air surrounding the thunderstorm to rise and cool. This cooling causes moisture in the warm air to condense, producing the whale's mouth cloud.
The boundary between the warm and cold air is very turbulent, which is what gives the cloud its gnarled appearance."