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Mammatus occurs whenever there is a conditionally unstable cloud layer above clear air: when an air parcel from this cloud layer descends into the clear air below, the cloud particles such as snow crystals and water droplets evaporate, cooling down the air parcel and making it descend even faster. A blob of cloud material is dragged down, creating the pouch of cloud.

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Observing tips

Mammatus is one of the more bizarre cloud forms. Many people who see mammatus for the first time are awe-inspired. The mammatus with its bulges of smooth cloud, sometimes showing a high degree of ordering, is a remarkable sight.

Thunderstorms are ideal mammatus producers. The anvils of thunderstorms contain ice crystals, and the anvil is usually sharply bounded by clear air below it (since right next to a thunderstorm, convection is usually somewhat inhibited). Mammatus mostly occurs at the back (trailing) side of a storm, referring to its direction of movement: a trailing anvil is left over and quite frequenty shows the mammatus very well.

Mammatus is usually a small cloud, and subject to evaporation. This is why it is actually quite transparent, although it may not appear so as seen from below the cloud. When you are in an airplane and flying through mammatus, you will notice that it is actually hardly visible at close range.