Physics Photo of the Week

Warren Wilson College

February 18, 2005

On February 16, 2005 outside my office window I saw this smoke plume from an apparent brush fire near, but not on, Warren Wilson College campus.  What is interesting to note is how some of the smoke spread horizontally at a fairly low elevation.  This is due to an inversion layer close to ground level.  Normally, the higher air is cooler than lower air, so when smoke rises (hot air rises), it becomes more bouyant than the surrounding cooler air.  During an inversion, however, the higher elevations contain warmer air, which is less dense, hence the warm smoke loses its bouyancy and cannot rise any further.  The smoke is trapped by the layer of warm air.  Inversions are bad for trapping air pollution - the pollutants cannot disperse and are trapped in the valleys. 

The photo at right shows the smoke plume a few minutes later - when the source has subsided a bit.  The smoke has not dispersed, but just spread out further - trapped under the inversion layer.  Notice the Red Barn in both photos.