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 water vapor 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 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 is 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.
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.

Contents

Explanation

Constant pressure

Varying pressure

Human reaction to high dew points

Record high dew point

Calculate the dew point

Simple approximation

See also

References

External links

Explanation

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. With higher temperatures, the equilibrium partial pressure of water vapor increases and 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. Dew point is a monotonic function of the partial pressure of water vapor, so dew point can be determined from partial pressure of water vapor alone, and vice versa.

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. 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.

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. 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.

Human reaction to high dew points

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.
Dew Points

20+C (68F) Oppressive

18C (64F) Sticky

16C (61F) Humid

13C (55F) Comfortable

10C (50F) Refreshing^[1]

Dew PointTemp. °F	Human Perception	Relative HumidityAir Temp 90°F
75°+	Extremely uncomfortable, oppressive	62%
70° - 74°	Very Humid, quite uncomfortable	52% - 60%
65° - 69°	Somewhat uncomfortable for most peopleat upper limit	44% - 52%
60° - 64°	OK for most, but everyone perceives the humidityat upper limit	37% - 46%
55° - 59°	Comfortable	31% - 41%
50° - 54°	Very comfortable	31% - 37%
49°or lower	Feels like the western USa bit dry to some	30%

^[2]

Record high dew point

The highest recorded dew point was 95°F, reported in Dhahran, Saudi Arabia at 3 p.m. July 8, 2003. The temperature was 108°F resulting in an apparent temperature of 172°F.^[3]

Calculate the dew point

Formula to calculate the dew point in degrees Celsius to within ±0.4 °C.
The formula is:
:$$
T_d = rac {b gamma(T,RH)} {a - gamma(T,RH)}

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

and
:''a'' = 17.27
:''b'' = 237.7 °C
:ln is the natural logarithm.
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
For a derivation of the above formula as well as an error estimation of it, see e.g. [1].

Simple approximation

There is also a very simple approximation which allows conversion 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%.
The equation is:
:$$
T_d = T - rac {(100 - RH)} {5}

or
:$$
RH = 100 - 5 (T - T_d)

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.''

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 ^[4].
In Fahrenheit
:$$
Tf_d = Tf - rac {(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.
Alternatively, there are a number of moisture sensing retail products available which measure relative humidity.

Dew On Webs	Dew On plants
DewPointWetWeb.jpg	DamavandDew2.jpg
Mount Damavand - Iran	Mount Damavand - Iran

See also

★ Condensation

★ Heat index

★ Hydrocarbon dew point

★ Humidity

★ Relative humidity

References

1. Mark Margarit, WHDH-TV Boston Channel 7 Weather
2. Relative Humidity....Relative to What? The Dew Point Temperature...a better approach
3.
Extreme Weather: A Guide & Record Book, , Christopher C., Burt, W. W. Norton & Company, ,
4. 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

External links

★ What is the dew point?

★ NOAA Dew point

★ dew point formula

★ Windows program for Heat Index, Dew Point, etc

★ Dew point evaporates on KPRC newscast ("The dew point is absolutely worthless information")

★ Often Needed Answers about Temp, Humidity & Dew Point from the sci.geo.meteorology Usenet newsgroup