Mammatus cloud

Last updated December 09, 2023

Mammatus clouds in Saskatchewan, Canada MammatusCloudsSask.jpg — Mammatus clouds in Saskatchewan, Canada

Mammatus (also called mamma^[1] 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 are formed by cold air sinking down to form the pockets contrary to the puffs of clouds rising through the convection of warm air. These formations were first described in 1894 by William Clement Ley.^[1]^[2]^[3]

Characteristics

Mammatus are most often associated with anvil clouds and also severe thunderstorms. They often extend from the base of a cumulonimbus, but may also be found under altostratus, and cirrus clouds, as well as volcanic ash clouds.^[4] When occurring in cumulonimbus, mammatus are often indicative of a particularly strong storm. 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.^[5] Contrails may also produce lobes but these are incorrectly termed as mammatus.^[1]

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 mammae that spread over hundreds of kilometers to being organized along a line, and may be composed of either unequal or similarly-sized lobes. The individual mammatus lobe average diameters of 1–3 kilometres (0.6–1.9 mi) and lengths on average of 1⁄2 kilometre (0.3 mi). 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 are usually composed of ice, but also can be a mixture of ice and liquid water or be composed of almost entirely liquid water.

True to their ominous appearance, mammatus clouds are often harbingers of a coming storm or other extreme weather system. Typically composed primarily of ice, they can extend for hundreds of miles in each direction and individual formations can remain visibly static for ten to fifteen minutes at a time. They usually appear around, before, or even after severe weather.

Hypothesized formation mechanisms

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.^[4]^[6]

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 following are the proposed mechanisms, each described with its shortcomings:

The anvil of a cumulonimbus cloud 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 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.
Cooling due to hydrometeor fallout is a second proposed formation mechanism. As hydrometeors fall into the dry sub-cloud air, the air containing the precipitation cools due to evaporation or sublimation. 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 adiabatic or latent heating effects. Discounting the thermodynamical effects of hydrometeor fallout, another mechanism proposes that dynamics of the fallout alone are enough to create the lobes. Inhomogeneities in the masses of the hydrometeors 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 Emanuel, is called cloud-base detrainment instability (CDI), which acts very much like convective cloud-top entrainment. 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.
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 buoyant 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 valid for only 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 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. 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.^[7]
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. 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 existing along a static interface cannot necessarily be applied to the interface between two sheared atmospheric flows.
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 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.^[1]^[8]

Related Research Articles

In meteorology, a cloud is an aerosol consisting of a visible mass of miniature liquid droplets, frozen crystals, or other particles suspended in the atmosphere of a planetary body or similar space. Water or various other chemicals may compose the droplets and crystals. On Earth, clouds are formed as a result of saturation of the air when it is cooled to its dew point, or when it gains sufficient moisture from an adjacent source to raise the dew point to the ambient temperature.

Cumulonimbus is a dense, towering vertical cloud, typically forming from water vapor condensing in the lower troposphere that builds upward carried by powerful buoyant air currents. Above the lower portions of the cumulonimbus the water vapor becomes ice crystals, such as snow and graupel, the interaction of which can lead to hail and to lightning formation, respectively. When occurring as a thunderstorm these clouds may be referred to as thunderheads. Cumulonimbus can form alone, in clusters, or along squall lines. These clouds are capable of producing lightning and other dangerous severe weather, such as tornadoes, hazardous winds, and large hailstones. Cumulonimbus progress from overdeveloped cumulus congestus clouds and may further develop as part of a supercell. Cumulonimbus is abbreviated Cb.

<span class="mw-page-title-main">Cumulus cloud</span> Genus of clouds, low-level cloud

Cumulus clouds are clouds that have flat bases and are often described as puffy, cotton-like, or fluffy in appearance. Their name derives from the Latin cumulus, meaning "heap" or "pile". Cumulus clouds are low-level clouds, generally less than 2,000 m (6,600 ft) in altitude unless they are the more vertical cumulus congestus form. Cumulus clouds may appear by themselves, in lines, or in clusters.

<span class="mw-page-title-main">Altocumulus cloud</span> Genus of mid-level cloud

Altocumulus is a middle-altitude cloud genus that belongs mainly to the stratocumuliform physical category 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. However, if the layers become tufted in appearance due to increased airmass instability, then the altocumulus clouds become more purely cumuliform in structure. Like other cumuliform and stratocumuliform clouds, altocumulus signifies convection. A sheet of partially conjoined altocumulus perlucidus is sometimes found preceding a weakening warm front, where the altostratus is starting to fragment, resulting in patches of altocumulus perlucidus between the areas of altostratus. Altocumulus is also commonly found between the warm and cold fronts in a depression, although this is often hidden by lower clouds.

A supercell is a thunderstorm characterized by the presence of a mesocyclone; a deep, persistently rotating updraft. Due to this, these storms are sometimes referred to as rotating thunderstorms. Of the four classifications of thunderstorms, supercells are the overall least common and have the potential to be the most severe. Supercells are often isolated from other thunderstorms, and can dominate the local weather up to 32 kilometres (20 mi) away. They tend to last 2–4 hours.

<span class="mw-page-title-main">Squall line</span> Line of thunderstorms along or ahead of a cold front

A squall line, or more accurately a quasi-linear convective system (QLCS), is a line of thunderstorms, often forming along or ahead of a cold front. In the early 20th century, the term was used as a synonym for cold front. Linear thunderstorm structures often contain heavy precipitation, hail, frequent lightning, strong straight-line winds, and occasionally tornadoes or waterspouts. Particularly strong straight-line winds can occur where the linear structure forms into the shape of a bow echo. Tornadoes can occur along waves within a line echo wave pattern (LEWP), where mesoscale low-pressure areas are present. Some bow echoes can grow to become derechos as they move swiftly across a large area. On the back edge of the rainband associated with mature squall lines, a wake low can be present, on very rare occasions associated with a heat burst.

Cloud physics is the study of the physical processes that lead to the formation, growth and precipitation of atmospheric clouds. These aerosols are found in the troposphere, stratosphere, and mesosphere, which collectively make up the greatest part of the homosphere. Clouds consist of microscopic droplets of liquid water, tiny crystals of ice, or both, along with microscopic particles of dust, smoke, or other matter, known as condensation nuclei. Cloud droplets initially form by the condensation of water vapor onto condensation nuclei when the supersaturation of air exceeds a critical value according to Köhler theory. Cloud condensation nuclei are necessary for cloud droplets formation because of the Kelvin effect, which describes the change in saturation vapor pressure due to a curved surface. At small radii, the amount of supersaturation needed for condensation to occur is so large, that it does not happen naturally. Raoult's law describes how the vapor pressure is dependent on the amount of solute in a solution. At high concentrations, when the cloud droplets are small, the supersaturation required is smaller than without the presence of a nucleus.

In meteorology, convective available potential energy, is the integrated amount of work that the upward (positive) buoyancy force would perform on a given mass of air if it rose vertically through the entire atmosphere. Positive CAPE will cause the air parcel to rise, while negative CAPE will cause the air parcel to sink. Nonzero CAPE is an indicator of atmospheric instability in any given atmospheric sounding, a necessary condition for the development of cumulus and cumulonimbus clouds with attendant severe weather hazards.

A funnel cloud is a funnel-shaped cloud of condensed water droplets, associated with a rotating column of wind and extending from the base of a cloud but not reaching the ground or a water surface. A funnel cloud is usually visible as a cone-shaped or needle like protuberance from the main cloud base. Funnel clouds form most frequently in association with supercell thunderstorms, and are often, but not always, a visual precursor to tornadoes. Funnel clouds are visual phenomena, these are not the vortex of wind itself.

A wave cloud is a cloud form created by atmospheric internal waves.

In meteorology, convective instability or stability of an air mass refers to its ability to resist vertical motion. A stable atmosphere makes vertical movement difficult, and small vertical disturbances dampen out and disappear. In an unstable atmosphere, vertical air movements tend to become larger, resulting in turbulent airflow and convective activity. Instability can lead to significant turbulence, extensive vertical clouds, and severe weather such as thunderstorms.

An arcus cloud is a low, horizontal cloud formation, usually appearing as an accessory cloud to a cumulonimbus. Roll clouds and shelf clouds are the two main types of arcus clouds. They most frequently form along the leading edge or gust fronts of thunderstorms; some of the most dramatic arcus formations mark the gust fronts of derecho-producing convective systems. Roll clouds may also arise in the absence of thunderstorms, forming along the shallow cold air currents of some sea breeze boundaries and cold fronts.

An overshooting top is a dome-like protrusion shooting out of the top of the anvil of a thunderstorm and into the lower stratosphere. When an overshooting top is present for 10 minutes or longer, it is a strong indication that the storm is severe.

An air-mass thunderstorm, also called an "ordinary", "single cell", "isolated" or "garden variety" thunderstorm, is a thunderstorm that is generally weak and usually not severe. These storms form in environments where at least some amount of Convective Available Potential Energy (CAPE) is present, but with very low levels of wind shear and helicity. The lifting source, which is a crucial factor in thunderstorm development, is usually the result of uneven heating of the surface, though they can be induced by weather fronts and other low-level boundaries associated with wind convergence. The energy needed for these storms to form comes in the form of insolation, or solar radiation. Air-mass thunderstorms do not move quickly, last no longer than an hour, and have the threats of lightning, as well as showery light, moderate, or heavy rainfall. Heavy rainfall can interfere with microwave transmissions within the atmosphere.

Atmospheric convection is the result of a parcel-environment instability in the atmosphere. Different lapse rates within dry and moist air masses lead to instability. Mixing of air during the day expands the height of the planetary boundary layer, leading to increased winds, cumulus cloud development, and decreased surface dew points. Convection involving moist air masses leads to thunderstorm development, which is often responsible for severe weather throughout the world. Special threats from thunderstorms include hail, downbursts, and tornadoes.

The maximum parcel level (MPL) is the highest level in the atmosphere that a moist convectively rising air parcel will reach after ascending from the level of free convection (LFC) through the free convective layer (FCL) and reaching the equilibrium level (EL), near the tropopause. As the parcel rises through the FCL it expands adiabatically causing its temperature to drop, often below the temperature of its surroundings, and eventually lose buoyancy. Because of this, the EL is approximately the region where the distinct flat tops, often observed around the upper portions of cumulonimbus clouds. If the air parcel ascended quickly enough then it retains momentum after it has cooled and continues rising past the EL, ceasing at the MPL.

Atmospheric instability is a condition where the Earth's atmosphere is considered to be unstable and as a result local weather is highly variable through distance and time. Atmospheric stability is a measure of the atmosphere's tendency to discourage vertical motion, and vertical motion is directly correlated to different types of weather systems and their severity. In unstable conditions, a lifted thing, such as a parcel of air will be warmer than the surrounding air. Because it is warmer, it is less dense and is prone to further ascent.

Tropical convective clouds play an important part in the Earth's climate system. Convection and release of latent heat transports energy from the surface into the upper atmosphere. Clouds have a higher albedo than the underlying ocean, which causes more incoming solar radiation to be reflected back to space. Since the tops of tropical systems are much cooler than the surface of the Earth, the presence of high convective clouds cools the climate system.

A castellanus is a cloud that displays at least in its upper part cumuliform protuberances having the shape of turrets that give a crenellated aspect. Some of these turrets are higher than they are wide; they have a common base and seem to be arranged in a line. The castellanus characteristic is particularly obvious when the clouds are observed from the side.

References

1 2 3 4 Schultz, David M.; Hancock, Y. (2016). "Contrail lobes or mamma? The importance of correct terminology" (PDF). Weather. 71 (8): 203. Bibcode:2016Wthr...71..203S. doi: 10.1002/wea.2765 .
↑ Anonymous (1975). International Cloud Atlas. Volume I. Manual on the observation of clouds and other Meteors (PDF). World Meteorological Organization. Archived from the original (PDF) on 2017-07-08. Retrieved 2017-05-13.
↑ Ley, William Clement (1894). Cloudland: A study on the structure and characters of clouds. London, England: Edward Stanford. pp. 104–105.
1 2 Schultz, David M.; Kanak, Katharine M.; Straka, Jerry M.; Trapp, Robert J.; Gordon, Brent A.; Zrnić, Dusan S.; Bryan, George H.; Durant, Adam J.; Garrett, Timothy J.; Klein, Petra M.; Lilly, Douglas K. (2006). "The Mysteries of Mammatus Clouds: Observations and Formation Mechanisms". Journal of the Atmospheric Sciences. 63 (10): 2409. Bibcode:2006JAtS...63.2409S. doi: 10.1175/JAS3758.1 . S2CID 53128552.
↑ Lane, Todd P.; Sharman, Robert D.; Trier, Stanley B.; Fovell, Robert G.; Williams, John K. (2012). "Recent Advances in the Understanding of Near-Cloud Turbulence". Bulletin of the American Meteorological Society. 93 (4): 499. Bibcode:2012BAMS...93..499L. doi: 10.1175/BAMS-D-11-00062.1 .
↑ Garrett, Timothy J.; Schmidt, Clinton T.; Kihlgren, Stina; Cornet, Céline (2010). "Mammatus Clouds as a Response to Cloud-Base Radiative Heating". Journal of the Atmospheric Sciences. 67 (12): 3891. Bibcode:2010JAtS...67.3891G. doi: 10.1175/2010JAS3513.1 . S2CID 54938314.
↑ Winstead, Nathaniel S.; Verlinde, J.; Arthur, S. Tracy; Jaskiewicz, Francine; Jensen, Michael; Miles, Natasha; Nicosia, David (2001). "High-Resolution Airborne Radar Observations of Mammatus". Monthly Weather Review. 129 (1): 159–166. Bibcode:2001MWRv..129..159W. doi: 10.1175/1520-0493(2001)129<0159:HRAROO>2.0.CO;2 .
↑ Kanak, Katharine M.; Straka, Jerry M.; Schultz, David M. (2008). "Numerical Simulation of Mammatus". Journal of the Atmospheric Sciences. 65 (5): 1606. Bibcode:2008JAtS...65.1606K. CiteSeerX 10.1.1.720.2477 . doi:10.1175/2007JAS2469.1.

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[contrail-1] 1 2 3 4 Schultz, David M.; Hancock, Y. (2016). "Contrail lobes or mamma? The importance of correct terminology" (PDF). Weather. 71 (8): 203. Bibcode:2016Wthr...71..203S. doi: 10.1002/wea.2765 .

[wmo-2] Anonymous (1975). International Cloud Atlas. Volume I. Manual on the observation of clouds and other Meteors (PDF). World Meteorological Organization. Archived from the original (PDF) on 2017-07-08. Retrieved 2017-05-13.

[3] Ley, William Clement (1894). Cloudland: A study on the structure and characters of clouds. London, England: Edward Stanford. pp. 104–105.

[mystery-4] 1 2 Schultz, David M.; Kanak, Katharine M.; Straka, Jerry M.; Trapp, Robert J.; Gordon, Brent A.; Zrnić, Dusan S.; Bryan, George H.; Durant, Adam J.; Garrett, Timothy J.; Klein, Petra M.; Lilly, Douglas K. (2006). "The Mysteries of Mammatus Clouds: Observations and Formation Mechanisms". Journal of the Atmospheric Sciences. 63 (10): 2409. Bibcode:2006JAtS...63.2409S. doi: 10.1175/JAS3758.1 . S2CID 53128552.

[5] Lane, Todd P.; Sharman, Robert D.; Trier, Stanley B.; Fovell, Robert G.; Williams, John K. (2012). "Recent Advances in the Understanding of Near-Cloud Turbulence". Bulletin of the American Meteorological Society. 93 (4): 499. Bibcode:2012BAMS...93..499L. doi: 10.1175/BAMS-D-11-00062.1 .

[6] Garrett, Timothy J.; Schmidt, Clinton T.; Kihlgren, Stina; Cornet, Céline (2010). "Mammatus Clouds as a Response to Cloud-Base Radiative Heating". Journal of the Atmospheric Sciences. 67 (12): 3891. Bibcode:2010JAtS...67.3891G. doi: 10.1175/2010JAS3513.1 . S2CID 54938314.

[7] Winstead, Nathaniel S.; Verlinde, J.; Arthur, S. Tracy; Jaskiewicz, Francine; Jensen, Michael; Miles, Natasha; Nicosia, David (2001). "High-Resolution Airborne Radar Observations of Mammatus". Monthly Weather Review. 129 (1): 159–166. Bibcode:2001MWRv..129..159W. doi: 10.1175/1520-0493(2001)129<0159:HRAROO>2.0.CO;2 .

[8] Kanak, Katharine M.; Straka, Jerry M.; Schultz, David M. (2008). "Numerical Simulation of Mammatus". Journal of the Atmospheric Sciences. 65 (5): 1606. Bibcode:2008JAtS...65.1606K. CiteSeerX 10.1.1.720.2477 . doi:10.1175/2007JAS2469.1.

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