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 contains more water vapor than is needed to cause saturation. Airborne water vapor begins to condense onto impurities in the air (such as dust) as the relative humidity of moist air approaches 100 percent; a cloud (or fog) forms. In 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.
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Write to: ASK TOM WHY, 2501 Bradley Pl., Chicago, IL 60618 or: asktomwhy@wgntv.com
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