Physics Photo of the Week

February 10, 2006

Disk Clouds - Photos by Leah Quintal

Leah Quintal, a December 2005 graduate of Warren Wilson college, sent me these photos of strange disk-like clouds rolling over the Great Craggy Mountains near Warren Wilson College on January 17, 2006. These two photos were taken about 1 minute apart. The image on the right looks north-east from the area of the Red Barn and the left-hand image is looking almost due north from the same spot, but zoomed-in for a closer look at the strange clouds. Leah says the weather was very warm the day of these wierd clouds, so the wind is most likely from the south.

There are clearly two types of clouds shown in Leah's photographs. The puffy clouds hovering over the mountain tops are easy to explain: As warm, moist air is forced to flow over the mountain, it is cooled by encountering lower pressure from the higher altitude. The sudden expansion due to the lower pressure is essentially insulated (adiabatic). Adiabatic expansion of air leads to cooling - enough that clouds form because warm weather is associated with high humidity. Furthermore, the condensation of the water vapor into clouds release heat (latent heat of condensation). This heat of condensation imparts more bouyancy to the cloud, so that it rises further giving the "puffy" appearance over the mountain ridges.

The cuspy disk appearance of the clouds at the higher elevation is more difficult to explain - I've never seen them like this before, but here is my hypothesis: I believe they are lenticular clouds. Lenticular clouds are named for their resemblance to lenses, and are caused by waves in the wind strata at the high elevations. See PPOW for April 15, 2005 and April 9, 2004 with an explanatory drawing. If anyone knows a better explanation both Leah and I would be glad to hear about it. Furthermore, if anyone sees a similar cloud formation, grab your camera and snap away. With a tripod and taking a photo every 10 sec, we can play the series back and get a time lapse image.

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department. These photos feature an interesting phenomena in the world around us. Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation. Atmospheric phenomena are especially welcome. Please send any photos to dcollins@warren-wilson.edu.

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