A cloud is a suspension of small particles in the atmosphere. These particles could be little drops of liquids or ice crystals that contain water. Clouds appear in the atmosphere due to the saturation of cooled air. There are three primary types of cloud, with ten cloud types of different species and genres. Mammatus clouds, also called mammatocumulus clouds, are pouch-like structures that form beneath a thunderstorm’s anvil.
Mammatus is from the Latin word mamma, translating to breast or udder, and this cloud type is named as such because Mammatus cloud formations look like breasts.
Meteorologists consider Mammatus clouds to be strange and rare cloud forms. Keep on reading to discover more about the characteristics of Mammatus clouds and the cause of their existence.
A Mammatus cloud looks like a protruding pouch in the sky. Others refer to it as cow udders or breasts, just like its name means. People, including meteorologists, find Mammatus clouds fascinating. They form in various sizes, ranging from barely visible to well-defined cloud forms.
Also, you can only see them clearly at sunset or in areas with low sunlight. The sunset illuminates the lobes of Mammatus clouds, creating striking, beautiful billows in the sky. Mammatus clouds are not a type or genre of cloud. Instead, they are an extension of other cloud types.
Researchers noticed that Mammatus clouds form on the thunderstorm anvil4, the spreading top of a cumulonimbus cloud, up and down its shear. Later on, they found Mammatus formations in other cloud types in the 20th century. For example, German meteorologists found Mammatus clouds formation in altostratus, altocumulus, cirrus clouds, and volcanic ash clouds.
There's a common misconception that Mammatus clouds are a sign of severe storm weather, but there is no concrete proof that they are a sign of severe weather or thunderstorms. However, heavy rainfall and thunderstorms often follow sightings of Mammatus clouds.
Science has several different theories to explain the formation of Mammatus clouds1, sharing that the formation of Mammatus clouds varies according to the environment and atmosphere. However, researchers are confident that Mammatus clouds cannot form independently. Instead, Mammatus clouds form on the underside of different cloud types, and we will examine them in this article.
First, here are some of the theories:
Wagner, Ludlam, and Scorer established the large-scale anvil subsidence in 1948, 1953, and 1972. The theory states that hazy anvil air flows in a horizontal position above a layer of unsaturated air. It causes the unsaturated air and clouds to sink. They thought the Mammatus cloud shapes happened because the cloud had reached its level of neutral buoyancy.
The air sinks, and the cloudy layer warms at a moist-adiabatic lapse rate, while unsaturated air beneath it gets warm at a dry-adiabatic lapse rate. The disparity leads to higher warming in the sub-cloud air than in the main cloudy area. Then, the convective overturn occurring at the underside of the anvil might result in the saturated air dropping in a pouch-like shape of Mammatus clouds.
Troeger proposed sub-cloud evaporation as a method of Mammatus cloud formation in 1922. Letzmann, Hlad, Ludlam, and Scorer contributed to this theory in 1930, 1944, 1948, and 1958 respectively. The hypothesis stated that frozen crystals, water droplets, snowdrops, and other mixtures of hydrometeors drop from the cloud's underside into sub-saturated air before it cools down after subliming and evaporating.
It is during this evaporation process that it forms Mammatus clouds. In the case of large cumulonimbus clouds, the cloud base descends lower, and the edges of the cloud might return upwards. Its ascendance might create Mammatus clouds.
Unlike large-scale anvil subsidence, some proof points support this theory. For example, radar-based theories claim that the small particles and narrow size distribution at the base of radar-observed Mammatus clouds show that evaporation occurred within the Mammatus clouds.
Also, Letzmann's hypothesis states that the cooling caused by the evaporation of water drops beneath the cloud base, combined with the sinking of the cumulonimbus anvil, might be the reason for Mammatus clouds’ inversion.
The lack of local homogeneity in hydrometeors can lead to inhomogeneities in air moving upwards. And it causes a lack of uniformity in sinking hydrometeors. The scale of the downdraft enlarges, and Mammatus-like shapes may happen as a result when coupled with the hydrodynamic effects of frictional drag along the edge of its condensation shaft.
Although this action may be why anvils subside, this fallout mechanism does not necessitate thermodynamic instability, a requirement for other suggested causes of Mammatus clouds. However, sublimation and evaporation take place during the descent of hydrometeor lobes.
Unfortunately, there is a loophole in this theory. The upwards motions in Mammatus clouds are sometimes more immense than the speed at which hydrometeors fall.
So, the components of hydrometeors are not the primary cause of Mammatus clouds. Therefore, this hypothesis concludes that local-scale hydrometeor fallout is impossible because it would simply mean that all clouds will have components of Mammatus clouds.
Agee claimed that Rayleigh-Taylor instability was the cause of Mammatus clouds. Rayleigh-Taylor instability happens on the interface between two incompressible liquids when the liquid with more density pushes against the one with lesser density. However, we can’t apply it to stratified airflows because of interfacial instability.
When we apply this theory to clouds, one part of the cloud will be fluid, while the other will be the sub-saturated air below the cloud that bears the fluid. However, the lapse rate cannot occur in an open atmosphere after following the calculations required to produce the effects of Rayleigh-Taylor instability. So, the cause of Mammatus clouds isn't Rayleigh-Taylor instability.
Researchers refer to Mammatus clouds as sinking air upside convection instead of warm clouds rising. The hypothesis surrounding it states that Mammatus clouds happen because of cellular convection. However, Rayleigh-Bernard convection is from the static instability that occurs from heating the quiescent homogeneous fluid's base.
In Mammatus clouds, the cooling does not happen on a flat boundary. There is no heat transfer because of the cloud's evaporation or vertical motion. Schaefer and Day's hypothesis included Rayleigh-Bernard's convection, which is plausible because areas with Mammatus clouds can have strong temperature gradients.
Berg was the first to speculate that Mammatus might be because of Kelvin-Helmholtz instability. Kelvin-Helmholtz instability happens within a stable and layered fluid with a strong upwards wind gradient. It raises questions regarding the formation of Mammatus clouds in unstable static areas- what happened to Mammatus formed in unstable static areas?
Since this question lacks answers, we should conclude that Kelvin-Helmholtz instability might not be the cause of Mammatus clouds.
However, in 2004, Petre and Verlinde produced substantial evidence regarding the relationship between the release of Kelvin-Helmholtz instability and Mammatus. So, we cannot apply it to the formation of Mammatus clouds because it occurs in a slightly unstable environment.
Mammatus clouds form at the underside of six cloud types. They are:
Cumulonimbus clouds, also called thunderstorms, are a body of clouds that are present in the entirety of the troposphere. Researchers often refer to it as a thunderstorm's anvil because it has the shape of an anvil. The cumulonimbus cloud is responsible for rainfall.
Furthermore, large cumulonimbus clouds produce severe storms, tornadoes, and large hail. Therefore, it is responsible for precipitation in many areas all over the earth2.
Researchers recognize cumulonimbus clouds as the largest cloud that can form in the sky. Cumulonimbus clouds form through convection over warm and unstable air. They start developing in small sections called small cumuli and can grow to the size of large powerhouses.
There are three species of the cumulonimbus cloud. They are cumulonimbus calvus, cumulonimbus capillatus, and cumulonimbus incus. Cumulonimbus calvus has a puffy cumulus cloud top with water droplets. The water drops of cumulonimbus calvus clouds do not turn into ice crystals.
Cumulonimbus capillatus, on the other hand, has a fibrous cumulus cloud top. However, its water droplets are partially frozen. Also, the cumulonimbus incus has a fibrous cloud top with a prominent anvil shape. It forms the shape of an anvil by growing outwards, away from the top of the troposphere.
Altocumulus clouds are small patches of mid-level clouds and represent cool, non-turbulent weather. The altocumulus cloud's composition includes ice and water. There are various types and sizes of altocumulus clouds.
Unlike other types of clouds, altocumulus clouds have diverse methods of formation. For example, breaking the altostratus cloud often leads to the formation of altocumulus clouds. Also, wet air chilled by little turbulence leads to the formation of an altocumulus cloud.
Altostratus clouds are layered, wide sheets of mid-level clouds in the sky. They are a composition of water and frozen crystals. Sunrays peak through its cloud sheets easily.
Cirrostratus clouds are responsible for the formation of altostratus. They form when a layer of cirrostratus lowers from a higher level. Furthermore, they are an indication of warm air.
Cirrus clouds, also known as ice clouds, are a composition of ice crystals. Their composition is fibrous, thin, and wispy-like. Cirrus clouds form at high altitudes when warm air rises, depositing water vapor on rocky and metallic particles. They form in stable and unstable layers of the atmosphere.
Furthermore, the frozen crystals contain ice nuclei like mineral dust, metallic particles, and other biological materials. Cirrus clouds absorb the earth’s thermal radiation, but thicker cirrus clouds may increase net cooling by solar radiation reflection5.
Stratocumulus clouds are low-level clouds that occur in subtropical and polar ocean regions. They are a common cloud that is properly visible when there is little or no sunlight. It is a feature of dull weather, often present in all-weather situations.
Sightings of stratocumulus clouds don’t necessarily mean rain or snow will fall. However, there could be slight showers of rainfall and snow. There are four species and seven varieties of stratocumulus clouds3.
It is a high-level cloud with a patchy appearance. Cirrocumulus clouds are part of the three primary clouds that form at high altitudes. People also call it mackerel sky because its patchy pattern looks like the scales of mackerel fish. Its composition includes frozen crystals and a small amount of supercooled water. It is important to note that cirrocumulus clouds are a transition phase between cirrus and cirrostratus clouds6.
Cirrocumulus clouds have four species and two varieties. They don't last long in the atmosphere because each cloudlet is a transitional phase in an area of cirrus clouds. However, cirrocumulus clouds occur when a part of the cumulonimbus anvil breaks off. Also, they have some levels of transparency. For example, they switch between red, orange, and yellow at sunset and sunrise.
Related: Why is the sky yellow?
Mammatus clouds are primarily sinking cold air that forms beneath a cumulus cloud. It is a beautiful phenomenon that forms on the underside of thunderstorm clouds. However, they are not the cause or indicators of severe thunderstorms and tornadoes. Instead, most cumulus clouds cause and indicate a storm and heavy rain.
Most clouds have unique features that make them recognizable. For example, the next time you are outside at sunset, you will recognize Mammatus clouds by their breast-like shapes.
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