Dear Tom,
Can the relative humidity ever be greater than 100 percent?
S. Wahrman
Dear S.,
Surprisingly, yes, and the condition is known as supersaturation. At any given
temperature and air pressure, a specific maximum amount of water vapor present in the
air will produce a relative humidity of 100 percent, and air in that state is said to be
saturated. Supersaturated air literally contains more water vapor than is needed to
cause saturation.
Airborne water vapor begins to condense onto impurities in the air (such as salt
particles and dust) as the relative humidity of moist air approaches 100 percent; a
cloud (or fog) forms. In absolutely clean air devoid of impurities that ordinarily serve as
condensation surfaces, the humidity can climb to incredible levels of supersaturation
-- 400 to 800 percent -- before condensation begins. Realistically, though, air is never
totally clean.
