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The dew point is the temperature at which the air becomes saturated, meaning it reaches 100 percent relative humidity. (Relative humidity is the amount of moisture in the air, expressed as a percentage of how much moisture the air can hold at a given temperature.) If the temperature falls below the dew point, water vapor within the air begins to condense (changes from a gas to a liquid). The dew point is so-named because it's the temperature below which dew forms on a cold surface.
The concept of dew point rests on the premise that cold air can hold less water vapor than can warm air. Consider air that at 77° Fahrenheit (25° Celsius) has a relative humidity of 56 percent. If that temperature is lowered to 59° Fahrenheit (15° Celsius) and the amount of moisture held constant, the air will have 100 percent relative humidity. Therefore, the dew point of that air is 59° Fahrenheit.
If a thin layer of air contacts a surface and is chilled to its dew point, dew will form on the surface. Fog and clouds develop when large volumes of air are cooled to their dew point.
Sources: Engelbert, Phillis. The Complete Weather Resource, vol. 1, pp. 49-50; Ludlum, David M. The Audubon Society Field Guide to North American Weather, p. 610; World Book Encyclopedia, vol. p. 176.
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