Mammatus Clouds Blanket the Skies over Parts of the East Coast

Jon Erdman
Published: June 22,2016
Skies over parts of the East were dotted with mammatus clouds as severe thunderstorms rolled through parts of the mid-Atlantic states on Tuesday evening.
The Weather Channel's Facebook page was peppered with photos of the somewhat rare sight from several states.
An evening stroll on the famous Atlantic City, New Jersey, boardwalk included an impressive sky filled with mammatus.
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Mammatus clouds in Atlantic City, New Jersey, on June 21, 2016.
(Brandon English/Facebook)

Words like "unusual," "distinct" and "beautiful" commonly describe mammatus clouds, which are the round structures that appear to be bulging from the underside of a larger cloud.
It was as if the spire atop this building in Atlantic City was pointing skyward to tell the tourists to look up.

Mammatus clouds over Atlantic City, New Jersey, on June 21, 2016.

They're a fascinating type of cloud, as they form in air that sinks instead of rises. The sinking air must be colder than the surrounding air and have high liquid water or ice content.
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Mammatus clouds are defined as hanging protrusions, like pouches, on the undersurface of a cloud. They often occur on the edges and sloping underside of cumulonimbus and have been observed on both the upshear and downshear sided of a thunderstorm's outflow anvil and typically last around 10 minutes. However, they can also occur with altostratus, altocumulus, stratocumulus clouds and cirrus clouds.
Mammatus clouds are frequently associated with severe weather, but they don't produce severe weather themselves.
Some of the best photos we saw came as the sun was setting, casting different colors on the mammatus.
You can see the subtle mammatus near the left-center of this photo taken in Oxford, Pennsylvania.

A distant thunderstorm near sunset displays a few mammatus clouds on the evening of June 21, 2016, in Oxford, Pennsylvania.
(Wayne Smith/Facebook)

Moments later, the view of the mammatus took on an other-worldly hue, as you might see in your favorite disaster movie.

View of mammatus clouds overhead at dusk in Oxford, Pennsylvania, on June 21, 2016.
(Wayne Smith/Facebook)

Finally, in the final minutes of twilight, we highlighted the faint outlines of lingering mammatus in the far left-center of this photo from Churchville, Maryland.

Outlines of lingering mammatus (highlighted) in Churchville, Maryland, on June 21, 2016.
(Jenn Vetter/Facebook)

Numerous theories have been proposed regarding how they form. When they are observed in the vicinity of severe weather, one theory suggests that their formation can occur when a storm has a very strong updraft. As air is pushed high in the storm, it reaches the top and cools, getting pushed outward as precipitation begins to form. The cool, moist air sinks and then warms again, creating the pouch-like structures on the base of the anvil.
Jonathan Erdman is a senior meteorologist at weather.com and has been an incurable weather geek since a tornado narrowly missed his childhood home in Wisconsin at age 7.