Mammatus clouds (also known as mammatocumulus, meaning "bumpy clouds") or simply "Mamma Clouds" is a meteorological term applied to a cellular pattern of pouches hanging underneath the base of a cloud. The name "mammatus" is derived from the Latin mamma (breast), due to the resemblance between the shape of these clouds and human female breasts.
When occurring in cumulonimbus, mammatus are often indicative of a particularly strong storm. These tend to form more often during warm months and they are much more common in U.S.A. than in Europe. Due to the intensely sheared environment in which mammatus form, aviators are strongly cautioned to avoid cumulonimbus with mammatus.
Mammatus may appear as smooth, ragged or lumpy lobes and may be opaque or semitransparent. Because mammatus occur as a grouping of lobes, the way they clump together can vary from an isolated cluster to a field of mamma that spread over hundreds of kilometers to being organized along a line, and may be composed of unequal- or similarly-sized lobes. The individual mammatus lobe average diameters of 1-3 km and lengths on average of 0.5 km. A lobe can last an average of 10 minutes, but a whole cluster of mamma can range from 15 minutes to a few hours. Their composition is usually mostly ice, but can be a mixture of ice and liquid water or almost entirely liquid water.
The first six photos of this post were taken by Jorn Olsen in Heartwell Park in Hastings, Nebraska in 2004 and are probably the best visual attestation of this curious phenomenon:
The following are other pictures of mammatus clouds taken everywhere in the world. The main source of them is the Cloud Appreciation Society, a fantastic site which is the "bible" for everything concerning clouds. Take a look to their photo galleries!
in Norfolk, UK ( © Tim Salter)
Taken in Colorado Springs, Colorado. © Tommy Pesavento.
in Milan, Italy, July 2005
San Francisco Bay
in Iraq, April 2006.
in Minnesota in 2005