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Weathercatch: Mammatus clouds – odd, eerie and very photogenic

Mammatus clouds, nicknamed udder clouds, they are pouchlike protuberances on the underside of storm clouds. (Henry Moore / Washington State University)
Mammatus clouds, nicknamed udder clouds, they are pouchlike protuberances on the underside of storm clouds. (Henry Moore / Washington State University)
By Nic Loyd and Linda Weiford Washington State University

Anyone lucky enough to see mammatus clouds can’t turn away. Reminiscent of bubble wrap sliding across the sky, they’re a remarkable sight.

These floating bands of lobes make occasional appearances over the Pacific Northwest, as they did above Pullman on Feb. 12. Fittingly nicknamed “udder clouds,” they are pouchlike protuberances on the underside of storm clouds, but occasionally form beneath other cloud types and even volcanic ash clouds.

It’s believed that mammatus clouds (pronounced me-MATT-es) occur when cold, dense air sinks – in essence “punching” through the storm cloud and creating a band of sacklike structures. While their appearance means that storms and turbulence are nearby, the clouds themselves don’t produce stormy weather.

Not only do these clouds look strikingly different than other cloud types, they are also formed differently because they are produced in sinking air instead of rising air. And though they fascinate laypersons and scientists alike, less is known about them because of their scarcity.

Calling them “an intriguing enigma of atmospheric fluid dynamics and cloud physics,” lead researchers from the University of Oklahoma and the National Oceanic and Atmospheric Administration attempted to demystify mammatus clouds in a 2006 study published in the Journal of Atmospheric Sciences.

“Their photogenic characteristics and their very occurrence raises many interesting scientific questions that challenge our conventional views of clouds and hydrometeors,” they wrote.

One thing we know for certain is that in the right light, these upside down bubbles look dramatic and ghostly. Their color is normally grayish blue, the same as the host cloud, but illumination from a rising or setting sun can make them glow in gold or pink.

And one more thing: A belt of mammatus clouds can extend dozens of miles across the sky. As their ice crystals and water droplets evaporate, they can dissipate quicker than other kinds of clouds.

In other words, you’ll want to snap some photos while you can.

Nic Loyd is a meteorologist with Washington State University’s AgWeatherNet. Linda Weiford is a WSU news writer and weather geek. Contact: or

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