The sky over parts of Michigan and Wisconsin looked downright eerie Monday night, as storm clouds began to sag with enormous yellow bubbles that appeared ready to burst and flood the land.
Facebook user Joe Nottage, of Menominee, Michigan, posted the accompanying images with the question: “Can anyone explain this?”
NOAA Meteorologist Jeff Last saw the photo and identified the unique cloud formation, via Twitter, as mammatus. Last also tweeted a similar image captured over Spread Eagle, Wisconsin.
“More mammatus from last eve’s storm. This one from Spread Eagle, Wisc., by Larry Warren,” reads the tweet.
It turns out, these bizarre cloud formations are somewhat rare and mysterious.
The American Meteorological Society describes mammatus clouds as “an intriguing enigma of atmospheric fluid dynamics and cloud physics,” and added that they’re most commonly observed beneath anvil-shaped thunderclouds.
On the rarity of these formations, the AMS stated: “Despite their aesthetic appearance, mammatus have been the subject of few quantitative research studies,” and that “observations have been obtained largely through serendipitous opportunities.”
Weather.com states that mammatus clouds are formed when ice crystals fall from the cumulonimbus cloud’s anvil. “The ice crystals sublimate, or change from ice to water vapor as they fall, causing the surrounding air to thermodynamically cool. The cooled air becomes negatively buoyant and begins to sink, producing the punched-out look indicative of the mammatus cloud.”
To be sure, the precise dynamics are complex, with an end result that can be both menacing and surreal.
Stated Last, the NOAA meteorologist, via email: “They are sometimes ominous in appearance, but are harmless and do not mean that severe weather is necessarily going to occur. In fact, they are usually seen after the worst of a storm has passed.”