Dew point

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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 as the water vapor no longer creates dew but instead creates frost or hoarfrost by deposition. The graph to the right 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 were water vapor.

For a given level of barometric pressure but independent from temperature, the level of dew point indicates the mole fraction of water vapor in the air, or put differently, determines the specific humidity of the air. If the level of barometric pressure rises without changing this mole fraction, then so will the dewpoint. Reducing the mole fraction 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 consists of water vapor than in New York.

On the other hand, for a given level of temperature but independent from barometric pressure, the level of dew point indicates the absolute humidity of the air. If the level of temperature rises without changing the absolute humidity, then so will the dewpoint. 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 weight of water vapor per cube meter of air will also be the same in those cities.

The dew point is associated with relative humidity. A high relative humidity indicates that 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. Given a constant dew point, an increase in temperature will lead to a decrease in relative humidity. It is for this reason that equatorial climates can have low relative humidity, yet still feel humid.

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

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

0 °C < T < 100 °C

0.01 < RH < 1.0

0 °C < T_d < 50 °C

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 °C

ln is the natural logarithm.

For a derivation of the above formula as well as an error estimation of it, see e.g. [1].

There is also a very simple approximation which allows you to convert in your head between the dew point, the dry bulb temperature and the relative humidity, which is accurate to within about ±1 °C as long as the relative humidity is above 50%. This can be expressed as a simple rule of thumb: for every 1 °C difference in the dew point and dry bulb temperatures, the relative humidity decreases by 5%, starting with RH=100% when the dew point equals the dry bulb temperature. In equations this is:

$T_d = T - \frac {(100 - RH)} {5}$

R H = 100 - 5(T - T d)

where in this case RH is in percent, and T and T_d are in degrees Celsius. The derivation of this, a discussion of its accuracy, comparisons to other approximations, and more information on the history and applications of the dew point are given in the Bulletin of the American Meteorological Society ^[1].

In Fahrenheit

$Tf_d = Tf - \frac {(100 - RH)} {3.333}$

For example, a relative humidity of 100% means dew point is same as air temp. For 90% RH dew point is 3 degrees Fahrenheit lower than air temp. For every 10 percent lower, dew point drops 3 deg.

Tf_d is in Fahrenheit RH same as above.

Dew On Webs	Dew On plants

Mount Damavand - Iran	Mount Damavand - Iran

[edit] See also

Meteorological data and variables
Atmospheric pressure \| Baroclinity \| Cloud \| Convection \| CAPE \| CIN \| Dew point \| Heat index \| Humidex \| Humidity \| Lightning \| Pot T \| Precipitation \| Sea surface temperature \| Surface solar radiation \| Surface weather analysis \| Temperature \| Theta-e \| Visibility \| Vorticity \| Wind chill \| Water vapor \| Wind

[edit] References

^ M. G. Lawrence, "The relationship between relative humidity and the dew point temperature in moist air: A simple conversion and applications", Bull. Am. Meteorol. Soc., 86, 225-233, 2005