Mammatus clouds form on the underside of a thunderstorm’s anvil. The name mammatus comes from the Latin word mamma, meaning “udder” or “breast”. They appear as pouch-like structures protruding from underneath the anvil. Mammatus clouds are gentle downdrafts – sinking cool air – descending from the anvil, that form, evolve and dissipate over a time span of about ten minutes to half an hour. While they are generally well-understood as downdraft features, the exact formation mechanism or mechanisms are not well constrained and a subject of research.
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Mammatus clouds vary in size and definition: from small, barely recognizable features to large, extremely well defined pouches. Even rather small thunderstorms with not particularly well-defined anvils (Cumulonimbus capillatus) often form some mammatus clouds. Intense thunderstorms that form extensive anvils (Cumulonimbus capillatus incus) can produce enormous ‘fields’ of mammatus clouds.
There are some common misconceptions about mammatus clouds. We talk about these next.