Physics Photo of the Week

May 10, 2013

Mountain Cap
Mountains often wear cloud caps due to the
moist air condensing as the air is forced over the higher elevation of the mountain.  This is shown on Mt. Pisgah at Willoughby Lake, Vermont.  The wind was blowing from the right side of the photo.

As the cloud rises over the top of the mountain, the adiabatic (insulated) expansion of the air due to the lower pressure cools the air below the dew point. (See the video clip at lower right).  Because air - a gas - expands as the pressure is lowered, it performs work.  Conservation of energy dictates that the work come from the thermal energy, thus cooling the gas.  As the air descends on the downwind side of the mountain, the adiabatic compression re-warms the air above the dew point and the condensation evaporates.  Sometimes a cloud cap continues to rise forming a cumulus cloud - especially if the air is unstable.  However in this case the ambient air temperature at the undisturbed higher elevation is almost as warm as the rising air.  The heat released by the condensing cloud does not provide enough buoyancy to float above the already warm air.  Thus in this case, the cap cloud is stable.  Watch for unstable clouds produced by mountains in a future Physics Photo of the Week.

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department. These photos feature 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

All photos and discussions are copyright by Donald Collins or by the person credited for the photo and/or discussion.  These photos and discussions may be used for private individual use or educational use.  Any commercial use without written permission of the photoprovider is forbidden.

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