Mammatus are rounded, pouch-like protrusions that hang beneath the base of a cloud, most often on the underside of a thunderstorm anvil. They form where negatively buoyant, moisture-laden air sinks from the anvil into drier air below.
How they form
Mammatus develop where the boundary between a cloud layer and the drier air below becomes unstable. Small pockets of the cloud, laden with ice and condensate, are heavier than the environment underneath and sink. As they descend, they stay saturated at their tops but evaporate at their edges, and the visible outline of each pocket is the pouch shape observers see.
Several physical mechanisms contribute to that instability. Sublimation and evaporation at the cloud base cool the air and enhance negative buoyancy. Precipitation loading from ice crystals sinking out of the anvil adds mass. Turbulent mixing along the boundary can also seed the pattern. Multiple processes usually act together, which is why mammatus can look organized in one storm and chaotic in another.
How spotters interpret them
Mammatus are most commonly seen on the underside of a thunderstorm anvil, often after the parent updraft has weakened and the anvil is spreading and settling. They also appear beneath cirrocumulus, altocumulus, altostratus, and stratocumulus, so they are not exclusive to thunderstorms.
For a spotter, the most useful information from mammatus is atmospheric, not tactical. Their presence confirms sinking motion in a layer where moisture is available, which is interesting for understanding the storm's life cycle. It does not indicate anything about wind, hail, or tornado risk at the surface.
Common misreadings
A popular belief holds that mammatus mean a tornado is coming or that the storm above is severe. Neither is reliable. Mammatus can appear in ordinary weakening thunderstorms and can be absent in violent tornadic supercells. Judging storm risk from the underside of the anvil is unreliable.
Mammatus can also be confused with scud or fractus beneath a rain-free base. The distinguishing feature is that mammatus are smooth, coherent pouches embedded in an overlying cloud layer rather than ragged, detached fragments.