A vocabulary of landscape — a landscape of words
1467 entries
mammatus
Mammatus are pouches hanging down from the underside of a cloud, usually the spreading anvil of a thunderstorm, in rows of smooth rounded lobes. They form where cold cloud-laden air sinks into clearer air below, which is the reverse of the way most cloud forms. They are commonest after the worst of a storm has passed, so they tend to appear in low evening light, lit orange from beneath.
Etymology
The word is Latin for breast-like and it is one of the more directly anatomical names in the cloud atlas. The World Meteorological Organization lists it as a supplementary feature rather than a cloud type in its own right, in the same category as arcus, which is the shelf on the front of a storm.
Notes
The formation mechanism is still argued over, and at least ten explanations have been proposed without one winning. What is agreed is that they mark sinking air, which is why they are associated with turbulence and why aircraft avoid flying under them. Their appearance does not indicate a storm is coming, which is a common misreading; it usually indicates one has just gone past.