Natural Science Forum / Biology / Biology / July 2004

The short answer is that exhaled air is saturated with water --100%
relative humidity.

The long answer:

Relative humidity depends critically on temperature.  Air in the lungs
is essentially saturated with water at 37 C, that is, it has a vapor
pressure of about 47 mm Hg or a water content of about 44 mg/liter.
(Note to purists: the vapor pressure is slightly reduced by the salt
content of the body fluids.) Most of that water is supplied by
evaporation from the membranes lining the nose.  As a result, these
surfaces are cooled a bit.  When you exhale, the breath passes over
these cooled surfaces and loses some of its moisture, thus conserving
at least some of the body water. This effect is rather small in
humans, but some desert animals make very effective use of it to
conserve water.

Still, the air you exhale is saturated (100% relative humidity) or
very close to it and is still at a temperature that is usually warmer
than ambient (unless it really is a very hot day!).  So as the exhaled
air cools to the ambient temperature, it is supersaturated.  If it is
cool enough or if the gas is contained, it will form a visible cloud
or leave visible condensation on any surface it touches.  Usually,
though, the water vapor disperses and dilutes rapidly enough into the
surrounding atmosphere to prevent condensation. So breathing on a
piece of glass leaves it wet while breathing normally into the
atmosphere usually doesn't result in visible condensation unless the
air is very cold.