What do mammatus clouds look like

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These choices will be signaled globally to our partners and will not affect browsing data. And what do they signal is coming? Mammatus clouds often form on the base of storm clouds but can be associated with other types of clouds as well. NOAA describes them as "in essence an upside-down cloud.

They appear rounded and pouch-like or sack-like and are found on the bottom of a storm's anvil. But they don't mean that a tornado is about to touch down -- actually the opposite. Mammatus clouds most often signal that a storm is on a weakening trend. These clouds are formed in part by sinking air.

One theory has that ice crystals that form high up in the storm's anvil become heavy enough to fall. The falling ice melts and turns into vapor on the way down, which cools the nearby air and causes it to sink, producing the pouch-like indentations in the cloud base.

Sometimes mammatus may form on other cloud types which produce no rain, though this is far less common. Mammatus comes from the Latin mamma which translates to "udder" or "breast".

Their striking appearance is most visible when the sun is low in the sky and their pouches are framed by the sunlight. This supplementary feature is a firm favourite with many meteorologists as well as cloud and photography enthusiasts. Mammatus usually form on the base of a cumulonimbus anvil, but they have also been sighted to form on other cloud types, such as stratocumulus, altostratus and altocumulus.

Mammatus have also been observed to form on the underside of volcanic ash clouds.