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What is the dew point and how does dew form?

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...

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