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 dcollins@warren-wilson.edu.
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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