Mammatus cloud
Encyclopedia
Mammatus, also known as mammatocumulus (meaning "mammary cloud" or "breast cloud"), is a meteorological
term applied to a cellular pattern of pouches hanging underneath the base of a cloud
. The name mammatus, derived from the Latin mamma (meaning "udder" or "breast"), refers to a resemblance between the characteristic shape of these clouds and the breast of a woman.
, but may also be found under altocumulus
, altostratus
, stratocumulus
, and cirrus
clouds, as well as volcanic ash clouds. In the United States, sky gazers may be most familiar with the very distinct and more common cumulonimbus mammatus. When occurring in cumulonimbus, mammatus are often indicative of a particularly strong storm or maybe even a tornadic
storm. Due to the intensely sheared environment in which mammatus form, aviator
s are strongly cautioned to avoid cumulonimbus with mammatus.
Mammatus may appear as smooth, ragged or lumpy lobes and may be opaque or translucent. 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. They usually are composed of ice, but also can be a mixture of ice and liquid water or be composed of almost entirely liquid water.
Despite varying hypothesized formation mechanisms, there is one environmental trend that remains common through all of the formation mechanisms; that is, that across the anvil cloud/sub-cloud air boundary there exist sharp gradient
s in temperature, moisture and momentum (wind shear
), which strongly influence interactions therein. The following are the proposed mechanisms, each described with its shortcomings:
This plenitude of proposed formation mechanisms shows, if nothing else, that the mammatus cloud is generally poorly understood. Detailed observations of the cloud have been meager and usually occur only by chance, since mammatus do not pose a meteorological threat to society. However, scientists' lack of understanding of the phenomenon exhibits that there are microphysical cloud processes that remain to be researched.
Meteorology
Meteorology is the interdisciplinary scientific study of the atmosphere. Studies in the field stretch back millennia, though significant progress in meteorology did not occur until the 18th century. The 19th century saw breakthroughs occur after observing networks developed across several countries...
term applied to a cellular pattern of pouches hanging underneath the base of a cloud
Cloud
A cloud is a visible mass of liquid droplets or frozen crystals made of water and/or various chemicals suspended in the atmosphere above the surface of a planetary body. They are also known as aerosols. Clouds in Earth's atmosphere are studied in the cloud physics branch of meteorology...
. The name mammatus, derived from the Latin mamma (meaning "udder" or "breast"), refers to a resemblance between the characteristic shape of these clouds and the breast of a woman.
Characteristics
Mammatus are most often associated with the anvil cloud and also severe thunderstorms. They often extend from the base of a cumulonimbusCumulonimbus cloud
Cumulonimbus is a towering vertical cloud that is very tall, dense, and involved in thunderstorms and other inclement weather. Cumulonimbus originates from Latin: Cumulus "Heap" and nimbus "rain". It is a result of atmospheric instability. These clouds can form alone, in clusters, or along a cold...
, but may also be found under altocumulus
Altocumulus cloud
Altocumulus is a cloud belonging to a class characterized by globular masses or rolls in layers or patches, the individual elements being larger and darker than those of cirrocumulus and smaller than those of stratocumulus. Like other cumulus clouds, altocumulus signifies convection...
, altostratus
Altostratus cloud
Altostratus is a cloud belonging to a class characterized by a generally uniform gray to bluish-gray sheet or layer, lighter in color than nimbostratus and darker than cirrostratus. The sun can be seen through thin altostratus, but thicker layers can be quite opaque...
, stratocumulus
Stratocumulus cloud
A Stratocumulus cloud belongs to a class of clouds characterized by large dark, rounded masses, usually in groups, lines, or waves, the individual elements being larger than those in altocumuli, and the whole being at a lower altitude, usually below 2,400 m...
, and cirrus
Cirrus cloud
Cirrus clouds are atmospheric clouds generally characterized by thin, wispy strands, giving them their name from the Latin word cirrus meaning a ringlet or curling lock of hair...
clouds, as well as volcanic ash clouds. In the United States, sky gazers may be most familiar with the very distinct and more common cumulonimbus mammatus. When occurring in cumulonimbus, mammatus are often indicative of a particularly strong storm or maybe even a tornadic
Tornado
A tornado is a violent, dangerous, rotating column of air that is in contact with both the surface of the earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. They are often referred to as a twister or a cyclone, although the word cyclone is used in meteorology in a wider...
storm. Due to the intensely sheared environment in which mammatus form, aviator
Aviator
An aviator is a person who flies an aircraft. The first recorded use of the term was in 1887, as a variation of 'aviation', from the Latin avis , coined in 1863 by G. de la Landelle in Aviation Ou Navigation Aérienne...
s are strongly cautioned to avoid cumulonimbus with mammatus.
Mammatus may appear as smooth, ragged or lumpy lobes and may be opaque or translucent. 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. They usually are composed of ice, but also can be a mixture of ice and liquid water or be composed of almost entirely liquid water.
Hypothesized formation mechanisms
A large range of types of mammatus clouds exist, each with distinct properties and in distinct environments. Accordingly, there are multiple hypothesized formation mechanisms. Each is discussed below, but each focuses on proposed formation of mammatus in cumulonimbus anvil clouds specifically. Many of these mechanisms shed light on formation in other cloud forms, but they will not be discussed further.Despite varying hypothesized formation mechanisms, there is one environmental trend that remains common through all of the formation mechanisms; that is, that across the anvil cloud/sub-cloud air boundary there exist sharp gradient
Gradient
In vector calculus, the gradient of a scalar field is a vector field that points in the direction of the greatest rate of increase of the scalar field, and whose magnitude is the greatest rate of change....
s in temperature, moisture and momentum (wind shear
Wind shear
Wind shear, sometimes referred to as windshear or wind gradient, is a difference in wind speed and direction over a relatively short distance in the atmosphere...
), which strongly influence interactions therein. The following are the proposed mechanisms, each described with its shortcomings:
- The anvil of a cumulonimbus cloudCumulonimbus cloudCumulonimbus is a towering vertical cloud that is very tall, dense, and involved in thunderstorms and other inclement weather. Cumulonimbus originates from Latin: Cumulus "Heap" and nimbus "rain". It is a result of atmospheric instability. These clouds can form alone, in clusters, or along a cold...
gradually subsides as it spreads out from its source cloud. As air descends, it warms. However, the cloudy air will warm more slowly (at the moist adiabatic lapse rate) than the sub-cloud, dry air (at the dry adiabatic lapse rate). Because of the differential warming, the cloud/sub-cloud layer destabilizesConvective instabilityIn meteorology, convective instability or stability of an airmass refers to its ability to resist vertical motion. A stable atmosphere makes vertical movement difficult, and small vertical disturbances dampen out and disappear...
and convectiveConvectionConvection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....
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 hydrometeorPrecipitation (meteorology)In meteorology, precipitation In meteorology, precipitation In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation...
fallout and cloud-base subsidence, thus rendering it unclear as to whether either process is occurring. - That said, cooling due to hydrometeorPrecipitation (meteorology)In meteorology, precipitation In meteorology, precipitation In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation...
fallout is a second proposed formation mechanism. As hydrometeors fall into the dry sub-cloud air, the air containing the precipitation cools due to evaporationEvaporationEvaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid....
or sublimationSublimationSublimation may refer to:* Sublimation , the change from solid to gas without entering liquid phase* Sublimation , the transformation of emotions* Sublimation , a music album by Canvas Solaris-See also:...
. Being now cooler than the environmental air and unstable, they descend until in static equilibrium, at which point a restoring force curves the edges of the fallout back up, creating the lobed appearance. One problem with this theory is that observations show that cloud-base evaporation does not always produce mammatus. This mechanism could be responsible for the earliest stage of development, but other processes (namely process 1, above) may come into play as the lobes are formed and mature. - 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. However, this strict temperature environment is not always present.
- The above processes specifically relied on the destabilization of the sub-cloud layer due to adiabaticAdiabatic processIn thermodynamics, an adiabatic process or an isocaloric process is a thermodynamic process in which the net heat transfer to or from the working fluid is zero. Such a process can occur if the container of the system has thermally-insulated walls or the process happens in an extremely short time,...
or latent heatLatent heatLatent heat is the heat released or absorbed by a chemical substance or a thermodynamic system during a process that occurs without a change in temperature. A typical example is a change of state of matter, meaning a phase transition such as the melting of ice or the boiling of water. The term was...
ing effects. Discounting the thermodynamicalThermodynamicsThermodynamics is a physical science that studies the effects on material bodies, and on radiation in regions of space, of transfer of heat and of work done on or by the bodies or radiation...
effects of hydrometeor fallout, another mechanism proposes that dynamicsThermodynamicsThermodynamics is a physical science that studies the effects on material bodies, and on radiation in regions of space, of transfer of heat and of work done on or by the bodies or radiation...
of the fallout alone are enough to create the lobes. Inhomogeneities in the masses of the hydrometeorsPrecipitation (meteorology)In meteorology, precipitation In meteorology, precipitation In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation...
along the cloud-base may cause inhomogeneous descent along the base. Frictional drag and associated eddy-like structures create the lobed appearance of the fallout. The main shortcoming of this theory is that vertical velocities in the lobes have been observed to be greater than the fall speeds of the hydrometeors within them; thus, there should be a dynamical downward forcing, as well. - Another method, that was first proposed by Kerry EmanuelKerry EmanuelKerry Emanuel is an American professor of meteorology currently working at the Massachusetts Institute of Technology in Cambridge. In particular he has specialized in atmospheric convection and the mechanisms acting to intensify hurricanes. He coined the term "hypercane" in 1994. In 2007, he was...
, is called cloud-base detrainment instability (CDI), which acts very much like convective cloud-top entrainmentEntrainment (meteorology)Entrainment is a phenomenon of the atmosphere which occurs when a turbulent flow captures a non-turbulent flow. It is typically used to refer to the capture of a wind flow of high moisture content, or in the case of tropical cyclones, the capture of drier air....
. In CDI, cloudy air is mixed into the dry sub-cloud air rather than precipitating into it. The cloudy layer destabilizes due to evaporative cooling and mammatus are formed. Issues with this mechanism are that it fails to explain why mammatus are located in specific locations in the anvil rather than under the entire anvil, and it fails to explain how the mixing originates. - Clouds undergo thermal reorganization due to radiative effects as they evolve. There are a couple of ideas as to how radiation can cause mammatus to form. One is that, because clouds radiatively cool (Stefan-Boltzmann law) very efficiently at their tops, entire pockets of cool, negatively buoyantBuoyancyIn physics, buoyancy is a force exerted by a fluid that opposes an object's weight. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus a column of fluid, or an object submerged in the fluid, experiences greater pressure at the bottom of the...
cloud can penetrate downward through the entire layer and emerge as mammatus at cloud-base. Another idea is that as the cloud-base warms due to radiative heating from land surface's longwave emission, the base destabilizes and overturns. This method is only valid for optically thick clouds. However, the nature of anvil clouds is that they are largely made up of ice, and are therefore relatively optically thin. - Gravity waveGravity waveIn fluid dynamics, gravity waves are waves generated in a fluid medium or at the interface between two media which has the restoring force of gravity or buoyancy....
s 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 updraftVertical draftAn updraft or downdraft is the vertical movement of air as a weather related phenomenon. One of two forces causes the air to move. Localized regions of warm or cool air will exhibit vertical movement. A mass of warm air will typically be less dense than the surrounding region, and so will rise...
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. Furthermore, time and size scales for gravity waves and mammatus do not match up entirely. Gravity wave trains may be responsible for organizing the mammatus rather than forming them. - Kelvin-Helmoltz (K-H) instabilityKelvin-Helmholtz instabilityThe Kelvin–Helmholtz instability, after Lord Kelvin and Hermann von Helmholtz, can occur when velocity shear is present within a continuous fluid, or when there is sufficient velocity difference across the interface between two fluids. One example is wind blowing over a water surface, where the...
is prevalent along cloud boundaries and results in the formation of wave-like protrusions (called Kelvin-Helmoltz 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. Still, the main downfall with this theory is that K-H instability occurs in a stably stratified environment, and the mammatus environment is usually at least somewhat turbulent. - Rayleigh–Taylor instability is the name given to the instability that exists between two fluids of differing densities, when the denser of the two is atop the less dense fluid. Along a cloud-base/sub-cloud interface, the denser, hydrometeor-laden air could cause mixing with the less-dense sub-cloud air. This mixing would take the form of mammatus clouds. The physical problem with this proposed method is that an instability that exists along a static interface cannot necessarily be applied to the interface between two shearedWind shearWind shear, sometimes referred to as windshear or wind gradient, is a difference in wind speed and direction over a relatively short distance in the atmosphere...
atmospheric flows. - The last proposed formation mechanism is it arises 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 method has not proven to be observationally sound and is viewed as generally insubstantial.
This plenitude of proposed formation mechanisms shows, if nothing else, that the mammatus cloud is generally poorly understood. Detailed observations of the cloud have been meager and usually occur only by chance, since mammatus do not pose a meteorological threat to society. However, scientists' lack of understanding of the phenomenon exhibits that there are microphysical cloud processes that remain to be researched.