Montana Earth Science Picture of the Week

This photograph was taken in Broadview, Montana (30 miles NW of Billings). It shows a cloud type known as "mammatus" (or "mamma"). They were given this name because they resemble mammary glands, especially the udders of cows.

Blacksmith's Anvil . . .
Mammatus clouds often form on the underside of cumulonimbus clouds (thunderclouds), but are sometimes seen underneath other clouds as well. When updrafts carry precipitation enriched air up to the cloud top, upward momentum is lost and the air begins to spread out horizontally. Sometimes this flattening out doesn't happen until the rising air reaches the boundary between troposphere (lower layer) and stratosphere (above). There the air stops rising and begins to spread out horizontally, giving the cloud a distinct anvil shape.

Not like other clouds . . .
Although the vast majority of clouds form as a result of rising air, the shape of the pouch-like clouds in the pictures on this page develop as a result of a sinking motion. Because of it's higher concentration of precipitation particles (ice crystals and water droplets), this saturated air is heavier than the surrounding air and therefore sinks back towards the earth. Rising warmer, clear air pushes against the descending cool air, forming the pouches.

Although they can appear threatening, the sinking air required to make these clouds actually indicates a weakening of the storm associated with them. While mammatus clouds may be observed with severe thunderstorms, they do not produce severe weather, nor should they be assumed as a definitive indicator of severe weather. When they are associated with a severe thunderstorm, they will probably be seen after the worst of a storm has past.

Watch the "Recipe for a Cloud" video.

Below: Mammatus clouds move over Crown Butte near Simms (20 miles west of Great Falls). To take a "virtual field trip" to Crown Butte, click on the Hot Link below.

Sources: University of Illinois at Urbana-Champaign - Department of Atmospheric Sciences, American Meteorological Society, The Weather Book

Term: troposphere