Dew point

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The dew point is the temperature to which a given parcel of humid air must be cooled, at constant barometric pressure, for water vapor to condense into water. The condensed water is called dew. The dew point is a saturation temperature.

The dew point is associated with relative humidity. A high relative humidity indicates that the dew point is closer to the current air temperature. Relative humidity of 100% indicates the dew point is equal to the current temperature and the air is maximally saturated with water. When the dew point remains constant and temperature increases, relative humidity will decrease.[1]

The dew point is an important statistic for general aviation pilots, as it is used to calculate the likelihood of carburetor icing and fog, and estimate the height of the cloud base.

Contents

Constant pressure

At a given barometric pressure, the dew point indicates the mole fraction of water vapor in the air, or, put differently, determines the specific humidity of the air. If the temperature rises without changing this mole fraction, the dew point will remain unchanged; however, the relative humidity will go down accordingly (and water continues to condense at the same temperature). Increasing the mole fraction, i.e., making the air more humid, would bring the relative humidity back up to its initial value. In the same way, decreasing the mole fraction after a temperature drop brings the relative humidity back down to its initial level. For this reason, the same relative humidity on a day when it's 80°F, and on a day when it's 100°F will imply that a higher fraction of the air on the hotter day consists of water vapor than on the cooler day, i.e., the dew point is higher.

Varying pressure

At a given temperature but independent of barometric pressure, the dew point indicates the absolute humidity of the air. If the pressure rises without changing the absolute humidity, the dew point will rise accordingly, and the water may condense at the higher pressure. Reducing the absolute humidity will bring the dew point back down to its initial value. In the same way, increasing the absolute humidity after a pressure drop brings the dew point back up to its initial level. Considering New York and Denver, for example, this means that if the dew point and temperature in both cities are the same, then the mass of water vapor per cubic meter of air will be greater in New York.

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