Physics Photo of the Week

December 4, 2009

Thermal Inversion
On the morning of September 13, 2007 I noticed the rising smoke from a fire just east of Warren Wilson College in front of the Four Brothers' Knobs.  The smoke, however, did not rise very far.  The smoke was trapped by a thermal inversion.

Normally, air at higher altitudes is cooler than air at lower altitudes.  Warm smoke thus can become more bouyant as it rises and reaches surrounding air that is cooler and more dense than the smoke.  During nights where there is little wind, the cooler air settles into the valleys and falls below a layer of warmer air.  The warm air on top of the layer of cool air is called the thermal inversion.  See the PPOW for February 18, 2005 for another example of smoke trapped by an inversion.

During most days when the sun heats the earth surface in the valleys, convection breaks up the inversion and any smoke  can then  reach high altitudes and dissipate.  If the nights are windy, the wind also breaks up the inversion.  Inversions tend to form during very quiet nights when the cool air sinks to the valley floor.  See the drawing at right.

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.

Click here to see the Physics Photo of the Week Archive.

Observers are invited to submit digital photos to: