Dew Point

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

The dew point is related with relative humidity. The dew point is closer to the current air temperature is indicated by the high relative humidity. The dew point is equal to the current temperature and air is saturated with water is indicated by 100% relative humidity. Relative humidity will decrease, when the dew point remains constant and temperature increases. 

Dew point in aviation

The dew point is used to calculate the carburetor icing, and fog and also used estimate the base of height of the cloud base; dew point is important for pilots in general aviation. Dew point is used to indicate the absolute humidity of the air at given temperature but independent of barometric pressure. The dew point will rise and relative will lower, if the temperature rises without changing the absolute humidity. To bring back the dew point to its initial value, the absolute humidity should be reduced. In the same way, by increasing the absolute humidity after a temperature drop will bring the dew point back down to its initial level. The same relative humidity on a day when it is 80 degree F, and on a day when it is 100 degree F will involve the higher fraction of air on the hotter day consists of water vapor than on the cooler day, it shows the dew point is higher.

The dew point will indicate the mole fraction of water vapor in the air or determines the specific gravity of air at a given barometric pressure but independent of temperature. The dew point will rise if the pressure rises without changing this mole fraction. To bring the dew point to the initial value, make the air less humid. In the same way, after the pressure drop increase the mole fraction to bring back the dew point to initial level. Considering New York (33 ft elevation) and Denver (5,130 ft elevation), for example, 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 same, but the mole fraction of water vapor in the air will be greater in Denver.

Questions:

• What is dew point?
• The dew point will rise if the pressure rises without changing the mole fraction? True or false