Dewpoint or Dew Point

It is the temperature to which a given parcel of 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 point.

When the dew point temperature falls below freezing it is often called the frost point, as the water vapor no longer creates dew but instead creates frost or hoarfrost by deposition.

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 that the dew point is equal to the current temperature (and the air is maximally saturated with water). When the dew point stays constant and temperature increases, relative humidity will decrease.

At a given barometric pressure, independent of temperature, the dew point indicates the mole fraction of water vapor in the air, and therefore determines the specific humidity of the air.

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.


A superficial explanation for cloud formation, regarding the process of how water vapor in air condenses when cooling, is "cold air cannot hold as much water as warm air". While this can be said to be true in some sense, actually water vapor will begin condensing as soon as its temperature drops below its dew point, quite regardless of the presence or absence of any other gasses. We could say as well that "a cold vacuum cannot hold as much water as a warm vacuum".

A more precise way of explaining the same facts would be to say that whenever water vapor and liquid water coexist, there is some evaporation and some condensing. For a given partial pressure of water vapor, there exists a dew point. If the actual temperature is higher than the dew point, evaporation is faster than condensation, so some liquid water will evaporate and decrease the overall temperature; if the temperature is lower than the dew point, some water vapor will condense (if there are some liquid or solid surfaces for it to condense upon) and increase the temperature.

Constant pressure

At a given barometric pressure, independent of temperature, 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 barometric pressure rises without changing this mole fraction, the dew point will rise accordingly, and water condenses at a higher temperature. Reducing the mole fraction, i.e. making the air dryer, will bring the dew point back down to its initial value. In the same way, increasing the mole fraction after a pressure drop brings the dew point back up to its initial level. For this reason, the same dew point in New York and Denver (which is at a much higher altitude) will imply that a higher fraction of the air in Denver, CO consists of water vapor than in New York, NY.

Varying pressure

At a given temperature but independent of barometric pressure, the dew point indicates the absolute humidity of the air. If the temperature rises without changing the absolute humidity, the dew point will rise accordingly, and water condenses at a 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 temperature drop brings the dew point back up to its initial level. Coming back to the New York - Denver 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 also be the same in those cities.

Source: Wikipedia
Posted: How Ruey