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Mammatus (also called mamma or mammatocumulus, meaning "mammary cloud") is a cellular pattern of pouches hanging underneath the base of a cloud, typically a cumulonimbus raincloud, although they may be attached to other classes of parent clouds.

The name mammatus is derived from the Latin mamma (meaning "udder" or "breast"). According to the WMO International Cloud Atlas, mamma is a cloud supplementary feature rather than a genus, species or variety of cloud. The distinct "lumpy" undersides form as cold air sinks, creating pockets that contrast with the rising puffs of clouds caused by the convection of warm air.

Mammatus are most often associated with anvil clouds and also severe thunderstorms. Due to the intensely sheared environment in which mammatus form, aviators are strongly cautioned to avoid cumulonimbus with mammatus as they indicate convectively induced turbulence.

The existence of many different types of mammatus clouds, each with distinct properties and occurring in distinct environments, has given rise to multiple hypotheses on their formation, which are also relevant to other cloud forms. One environmental trend is shared by all of the formation mechanisms hypothesized for mammatus clouds: sharp gradients in temperature, moisture and momentum (wind shear) across the anvil cloud/sub-cloud air boundary, which strongly influence interactions therein. The anvil of a cumulonimbus cloud gradually subsides as it spreads out from its source cloud. Because of the differential warming, the cloud/sub-cloud layer destabilizes and convective overturning can occur, creating a lumpy cloud-base. The problems with this theory are that there are observations of mammatus lobes that do not support the presence of strong subsidence in the lobes, and that it is difficult to separate the processes of hydrometeor fallout and cloud-base subsidence, thus rendering it unclear as to whether either process is occurring. One problem with this theory is that observations show that cloud-base evaporation does not always produce mammatus. There may also be destabilization at cloud base due to melting. If the cloud base exists near the freezing line, then the cooling in the immediate air caused by melting can lead to convective overturning, just as in the processes above. The cloudy layer destabilizes due to evaporative cooling and mammatus are formed. One is that, because clouds radiatively cool (Stefan–Boltzmann law) very efficiently at their tops, entire pockets of cool, negatively buoyant cloud can penetrate downward through the entire layer and emerge as mammatus at cloud-base. Gravity waves are proposed to be the formation mechanism of linearly organized mammatus clouds. Indeed, wave patterns have been observed in the mammatus environment, but this is mostly due to gravity wave creation as a response to a convective updraft impinging upon the tropopause and spreading out in wave form over the entirety of the anvil. Therefore, this method does not explain the prevalence of mammatus clouds in one part of the anvil versus another. Kelvin–Helmholtz (K–H) instability is prevalent along cloud boundaries and results in the formation of wave-like protrusions (called Kelvin-Helmholtz billows) from a cloud boundary. Mammatus are not in the form of K-H billows, thus, it is proposed that the instability can trigger the formation of the protrusions, but that another process must form the protrusions into lobes. This mixing would take the form of mammatus clouds. The last proposed formation mechanism is that mammatus arise from Rayleigh–Bénard convection, where differential heating (cooling at the top and heating at the bottom) of a layer causes convective overturning. However, in this case of mammatus, the base is cooled by thermodynamical mechanisms mentioned above. As the cloud base descends, it happens on the scale of mammatus lobes, while adjacent to the lobes, there is a compensating ascent. This plenitude of proposed formation mechanisms shows, if nothing else, that the mammatus cloud is generally poorly understood.

Quick Facts

• According to the WMO International Cloud Atlas, mamma is a cloud supplementary feature rather than a genus, species or variety of cloud.
• Kelvin–Helmholtz (K–H) instability is prevalent along cloud boundaries and results in the formation of wave-like protrusions (called Kelvin-Helmholtz billows) from a cloud boundary.
• The name mammatus is derived from the Latin mamma (meaning "udder" or "breast").
• The distinct "lumpy" undersides form as cold air sinks, creating pockets that contrast with the rising puffs of clouds caused by the convection of warm air.
• Mammatus are not in the form of K-H billows, thus, it is proposed that the instability can trigger the formation of the protrusions, but that another process must form the protrusions into lobes.

Source material: Wikipedia - "Mammatus cloud". Adapted and summarized for DiscoverScroll. Original contributors are credited through the linked Wikipedia article. Read original on Wikipedia. CC BY-SA 4.0. Changes were made from the original.