Dew point

The dew point or dewpoint of a given parcel of air is the temperature to which the parcel must be cooled, at constant barometric pressure, for the water vapor component to condense into water, called dew. When the dew point temperature falls below freezing it is called the frost point, instead creating frost or hoar frost by deposition.

The following graph shows the maximum percentage of water vapor that can exist in air at sea level across a range of temperatures. Note that with higher temperatures the equilibrium partial pressure of water vapor increases thus more water evaporates. The behavior of water vapor does not depend on the presence of air. The formation of dew would occur at the dew point even if the only gas present was water vapor.

The dew point determines relative humidity. When the relative humidity is high, the dew point is closer to the current air temperature. If the relative humidity is 100%, the dew point will be equal to the current temperature. As relative humidity falls, the dew point becomes lower, given the same air temperature.

Humans tend to react with discomfort to high dewpoints. Those accustomed to continental climates often begin to feel discomfort when the dew point reaches between 15 and 20 °C (59 to 68 °F). Most inhabitants of these areas will consider dewpoints above 21 °C (70 °F) to be oppressive. Some consider a dewpoint above 10 °C (50 °F) to be uncomfortable.

Here is a formula to calculate the dew point in degrees Celsius to within ±0.4 ℃. It is valid for

0 ℃ < T < 60 ℃

0.01 < RH < 1.0

0 ℃ < T_d < 50 ℃

where

T = temperature in degrees Celsius

RH = is the relative humidity as a fraction (not percent)

T_d = the dew point temperature to be calculated

The formula is:

$T_{d}={\frac {b\ \gamma (T,RH)}{a-\gamma (T,RH)}}$

where

$\gamma (T,RH)={\frac {a\ T}{b+T}}+\ln RH$

and

a = 17.27

b = 237.7 ℃

ln is the natural logarithm.

See also

• Heat index
• Wind chill
• Relative humidity

External links

• What is the dew point?
• NOAA Dew point
• dew point formula
• dew point calculation
• Windows program for Heat Index, Dew Point, etc
• How dew point, heat index, Real Feel Temperature differ