Physics Photo of the Week

February 8, 2013

"Smoking" Mailbox
On a sunny, dewey morning in the fall this neighbor's mailbox appeared to be smoking - as if a bomb was hidden in the mailbox ready to blow up!  Not to worry, however.  On closer inspection this "smoke" is merely mist from the heavy dew that formed overnight on the metal.   This dew is evaporating in the bright sunlight.  What is more impressive is that this mailbox was covered with heavy frost on the morning of this photo.  The frost was melted by the direct sunlight.

The mist rising from wet objects in the bright Sun is quite common.  It is more often called "steam" rather than smoke or mist. However, it is definitely not steam.  Steam is water in the gaseous phase after the water has boiled.  At ambient pressures steam has to be 100 deg C (212 deg F) in order to exist as steam.  Steam is invisible.  This is mist, a cloud of microscopic water droplets as opposed to gaseous steam.  The droplets are so tiny (about 1/1000 to 1/100 millimeter in diameter) that they are carried by gentle air currents.  The air friction prevents them from falling.  The mist is visible because the tiny droplets scatter light.  In this picture the Sun is off the upper right in the photograph.

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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