Mammatus are pouch-like cloud structures. They're also a rare example of clouds in sinking air-- most clouds form in rising air. Although mammatus most frequently form on the underside of a cumulonimbus, they can develop underneath cirrocumulus, altostratus, altocumulus and stratocumulus.
For a mammatus to form, the sinking air must be cooler than the air around it and have high liquid water or ice content. They derive their name from their appearance, like the bag-like sacs that hang beneath the cloud resemble cow's udders.
Mammatus are long-lived if the sinking air contains large drops and snow crystals since larger particles require greater amounts of energy for evaporation to occur. Over time, the cloud droplets do eventually evaporate and the mammatus dissolve.
Despite popular misconception, mammatus clouds are not a sign that a tornado is about to form. While associated with thunderstorms, mammatus clouds are not necessarily an indicator of severe weather. Mammatus result from the sinking of moist air into dry air. They are in essence upside-down clouds. The sharp boundary of mammatus is much like the sharp boundary of a rising cumulonimbus cloud before an anvil has formed.
Weather plays a bigger role than you may think when it comes to seasonal outdoor allergies.
Why can different types of precipitation be seen on Earth while temperatures remain constant?
Dangerous flash flooding is captured as an arroyo becomes filled with water in Carson Valley, Nevada.
The RealFeel Temperature uses an equation to determine how it actually feels outside.
Knowing what the different advisories, watches, and warnings mean will lead to more informed decision making when a winter storm threatens a particular area.
How can you determine if and when the ice is thick enough for safely going out on?
Seeking shelter in the event of a tornado could save your life, but is there really any safe place to hide?
Driving on a 90-degree angle away from the tornado is a good strategy to follow in order to distance yourself from the tornado.
Supercell thunderstorms have been responsible for major tornadoes that have demolished parts of the U.S.
After a cold, clear winter night without much wind, the ground and nearby tree branches may be covered by tiny, white ice crystals.
Clearfield, PA (1994)
Tornado touched down.
Dakotas & Minn. (1942)
26th-28th, severe freeze with temperature of only degrees F. at Parshall, N.D.
N.E. United States (1950)
Blue sun and moon from forest fires in British Columbia.