What are those oddly beautiful but scary clouds that hang down from thunderclouds at times?.......Likely mammatus clouds. They generate a ton of excitement on social media and are somewhat of a meteorological mystery.
If you live in the Great Plains, there is a decent chance that you have seen them. On June 17th 2016, many people in the Atlanta, Georgia area experienced them too. Numerous photos appeared on
They look amazing in person. We kept saying they looked like little ornaments or balls in the sky. I love them!
By the way, take a look at this video of the type of weather happening at my house (not far from where Mrs. Schnupp took this photography). As an atmospheric scientists familiar with these clouds, I find that the public is very much in awe of these clouds, and some people fear them. So what are they?
The American Meteorological Society's Glossary of Meteorology offers this formal definition,
Mammatus: Hanging protuberances, like pouches, on the undersurface of a cloud.
There are numerous terms for mammatus clouds: mamma, mammatocumulus, or mammory cloud. The word is actually derived from the Latin "mamma," which means breast or udder. A more common usage of the root word in society is "mammogram." Mammatus are typically associated with thunderstorms (often the anvil) but contrary to common thinking, they are not necessarily a "smoking gun" indicator of tornado, large hail or significant wind gusts. The National Weather Service Online School for Weather (a great resource if you are not familiar with it) describes mammatus formation this way:
Clouds with pouch-like downward extensions associated with very active cumulonimbus in latter stages of development. Mammatus occurs when air from the cloud layer descends into the clear air below and the cloud droplets evaporate, cooling down the air and making it descend even faster. Some of the cloud is dragged down, creating the appearance.
This description is somewhat limited in my view. Scientists note that a similar process can occur when ice crystals fall out of the anvil of a cumulonimbus cloud and sublimate (transition directly from ice to water vapor). Like evaporation, sublimation also cools the air. Cooler, denser air causes the air to descend more rapidly. Scholarly research suggests that sublimation is very important (more on that later).
Some have described mammatus as upside down clouds. While I understand that description, the formation of mammatus is more complicated. Dr. David Schultz and colleagues published possibly the most extensive review on mammatus in the Journal of Atmospheric Sciences. They actually reviewed several hypothesized mechanism including: anvil subsidence, sub-cloud sublimation/evaporation processes, detrainment at cloud base, radiative processes, Kelvin-Helmoltz instability/other instabilities, and hydrometeor fallout from the cloud. Noted mesoscale meteorologist Dr. Chuck Doswell has also posited the notion of double diffusive convection, a concept more common to oceanography. The basic premise is that a fluid has constituents that contribute to buoyancy in counter ways.
Kanak and colleagues extended Schultz's work with a set of numerical modeling simulations to evaluate three proposed mammatus causation mechanisms: detrainment at cloud base, hydrometeor fallout from cloud base, and sublimation below cloud base. They concluded that cloud-base detrainment instability (essential the mixing of warm, dry air and cool, saturate air at the base of the cloud) is a necessary condition but is not sufficient alone. They also found that mammatus can form irrespective of fallout but not without the sublimation processes. Their analysis of weather balloon data (soundings) in mammatus environments reveals that the layer below the cloud was typically dry adiabatic (contains relatively little water) and characterized by low relative humidity. These are just fancy ways of saying the process of sublimation can happen.
Ultimately the body of scholarly research suggests that formation, size, shape, and scale of mammatus depends on sub-cloud moisture, its stability, and the terminal velocities of the hydrometeors (droplets, ice crystals) in the mammatus cloud. I also take away from reading the scientific literature that there is still no comprehensive or conclusive theory yet, but we may be close.
