Convection is like a buoyant bubble, he said. In mammutus clouds, evaporation causes pockets of negative buoyancy as it cools the air inside the cloud. This makes the clouds puff downward instead of up like cumulus clouds, and they end up being like upside-down bubbles.
The reason they are smooth is the thermal structure right below them. The speed at which the temperature drops with increased height, known as the “lapse rate,” needs to be close to neutral, he said. In other words, if you put a warm little bubble of air in a particular spot, it won’t go up or down much at all — no heat goes in or out. This is typical of the thermal structure of thunderstorms. Without these circumstances, you’d get more common ragged-looking clouds or cloudy wisps coming out.