Mammatus clouds - what are they and how they form?

Mammatus clouds form on the underside of a thunderstorm’s anvil. Their name 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.

Sunset mammatus clouds over Texas Panhandle, USA on May 23rd, 2016. Photo: Marko Korošec

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.

Mammatus clouds over Istra, Croatia on Aug 3rd, 2014. Photo: Marko Korošec

Interesting fact: mammatus clouds have often been associated with approaching tornadoes. In fact, this is a common misconception. While mammatus clouds do form on most tornadic thunderstorms, they form on many thunderstorms in general (including non-tornadic) and there is no known direct correlation between tornadoes and mammatus clouds.

Mammatus clouds are often best seen after a thunderstorm has passed. Also, as these clouds form on thunderstorm anvils, which are much larger than the part of the thunderstorm where the worst weather happens, it may well happen that you see mammatus clouds without actually being impacted by the thunderstorm.

Mammatus clouds over northeast Montana, USA on June 19th, 2015. Photo: Marko Korošec

Fun fact: contrary to popular conspiracy theories, mammatus clouds are not a recent phenomenon and were first described in 1894 by William Clement Ley.

Recap: mammatus clouds are pouch-like structures protruding from the underside of a thunderstorm anvil. They are caused by sinking cool air and may persist for tens of minutes. While they can be associated with severe thunderstorms, they are not a direct sign of impending severe weather.

Got it? Great! Let us see some examples of these often spectacular cloud formations.