Cumulonimbus mamma – Stormy, cloudy pouches

Mamma are pouch-like cloud features that can appear on the undersides of several of the ten main cloud genera. Their most dramatic displays occur beneath Cumulonimbus storm clouds, like this one spotted by SallyGrace Mindrebo over Sandia Crest, Albuquerque, New Mexico, US. Their distinctive appearance is the result of convection, but in reverse to the process that forms most clouds.

Most clouds form when warm, moist air rises through cooler air, causing water vapour to condense into droplets or freeze into ice crystals. In a towering Cumulonimbus, these rising currents can cause clouds to build all the way to the top of the troposphere. Mamma, however, form when pockets of air cool and sink, dragging the cloud layer down into rounded pouches.

In Cumulonimbus clouds, mamma usually develop on the underside of the broad anvil of ice crystals that spreads out near the cloud’s summit. The air there is already cold, but for reasons that are still debated it can become colder still, causing pockets of denser air to sink. One leading explanation is that some of the ice crystals begin to melt. Melting absorbs heat from the surrounding air, cooling it enough for it to descend and pull the cloud down with it.

Mamma tend to appear towards the rear of an advancing storm, often after the heaviest rain or hail has passed. Think of them as the farewell display of a departing thunderstorm.

Cumulonimbus incus mamma spotted over Sandia Crest, Albuquerque, New Mexico, US by SallyGrace Mindrebo. View this in the photo gallery.