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 hoarfrost by deposition.

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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.

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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.

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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.

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Here is a formula to calculate the dew point in degrees Celsius to within ?0.4 deg C. It is valid for

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:0 deg C < T < 60 deg C

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:0.01 < RH < 1.0

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:0 deg C < Td < 50 deg C

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where

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:T = temperature in degrees Celsius

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

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:Td = the dew point temperature to be calculated

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The formula is:

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:

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T_d = rac {b gamma(T,RH)} {a - gamma(T,RH)}

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where

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:

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gamma(T,RH) = rac {a T} {b+T} + ln RH

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and

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:a = 17.27

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:b = 237.7 deg C:ln is the natural logarithm.

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Temperature: Temperature is the physical property of a system which underlies the common notions of "hot" and "cold"; the material with the higher temperature is said to be hotter. Temperature is a measure of the average kinetic energy of the particles in a sample of matter. In other words, temperature is a meas...

Barometric pressure: REDIRECT atmospheric pressure...

Water vapor: Water vapor or water vapour, also aqueous vapour, is the gas phase of water. On the Earth, water vapor is one state of the water cycle within the hydrosphere. Water vapor can be produced from the evaporation of liquid water or from the sublimation of ice. Under normal atmospheric conditions, wate...

Dew point related Images and Photos (experimental)

Point	Point Break	Point Lobos	Point Break (1991) (BD)
Point Blank (DVD)	On Point	West Point (DVD)	West Point
Rond Point	Insight: Dew Drop	Match Point	North Point