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Encyclopedia > Dew point

The dew point (or dewpoint) is the temperature 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. Graph of atmospheric dewpoints across a range of temperatures. ... For other uses, see Temperature (disambiguation). ... Look up air in Wiktionary, the free dictionary. ... Atmospheric pressure is the pressure caused by the weight of air above any area in the Earths atmosphere. ... Water vapor or water vapour (see spelling differences), also aqueous vapor, is the gas phase of water. ... For other uses, see Condensation (disambiguation). ... Dew on a spider web Dew is water in the form of droplets that appears on thin, exposed objects in the morning or evening. ... Look up Saturation in Wiktionary, the free dictionary. ...

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. Frost on black pipes Frost is a solid deposition of water vapor from saturated air. ... Frost on black pipes Frost is a solid deposition of water vapor from saturated air. ... Deposition is the process by which, in sub-freezing air, water vapor changes directly to ice without first becoming a liquid. ...

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. Humidity is the amount of water vapor in air. ... A hygrometer used to measure the humidity of air. ...

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. Atmospheric pressure is the pressure caused by the weight of air above any area in the Earths atmosphere. ... The mole fraction is one way of expressing the relative concentration of a given species. ... Specific humidity is a method of expressing the amount of aqueous vapor in air by using a ratio of aqueous vapor to dry 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. A general aviation scene at Kemble Airfield, England. ... Carburetor, carburettor, carburator, carburetter icing is an icing condition which can affect any carburetor under certain atmospheric conditions. ... For other uses, see Fog (disambiguation). ...

The graph above shows the maximum percentage (by mass) 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 other gasses in 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. A burette, an apparatus for carrying out acid-base titration, is an important part of equilibrium chemistry. ... In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. ...

### 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 consists of water vapor than in New York. Atmospheric pressure is the pressure caused by the weight of air above any area in the Earths atmosphere. ... The mole fraction is one way of expressing the relative concentration of a given species. ... Specific humidity is a method of expressing the amount of aqueous vapor in air by using a ratio of aqueous vapor to dry air. ...

### 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. Absolute humidity is the mass of water vapor in a given volume of air or gas, expressed by weight and usually measured in grams per cubic meter, though grains per cubic foot has also been used in the United States. ...

## Human reaction to high dew points

Humans tend to react with discomfort to high dew points, as a high dew point corresponds with a high ambient temperature (causing the body to perspire and produce sweat) and/or a high relative humidity (which inhibits the evaporation of sweat, by which the body is cooled); as a result the body may overheat, resulting in discomfort. SWEAT is an OLN/TSN show hosted by Julie Zwillich that aired in 2003-2004. ...

A lower dew point, meanwhile, corresponds to a lower ambient temperature or lower relative humidity, either of which allow the body to more effectively regulate its temperature to avoid overheating.

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. Regions containing a continental climate exist in portions of Northern Hemisphere continents, and also at higher elevations in certain other parts of the world. ...

Dew Point

Temp. °C

Dew Point

Temp. °F

Human Perception Relative Humidity

Air Temp 90°F (32.2°C)

>24°C >75°F Extremely uncomfortable, oppressive 62%
21 - 24°C 70 - 74°F Very Humid, quite uncomfortable 52% - 60%
18 - 21°C 65 - 69°F Somewhat uncomfortable for most people at upper limit 44% - 52%
16 - 18°C 60 - 64°F OK for most, but everyone perceives the humidity at upper limit 37% - 46%
13 - 16°C 55 - 59°F Comfortable 31% - 41%
10 - 12°C 50 - 54°F Very comfortable 31% - 37%
<10°C <49°F Feels like the western US a bit dry to some 30%

## Record high dew point

The highest recorded dew point was 35°C (95°F), reported in Dhahran, Saudi Arabia at 3 p.m. July 9, 2003. The temperature was 42.2°C (108°F) resulting in an apparent temperature or Heat Index of 77.7°C (172°F).[2] Road to Dhahran (Picture taken from Khobar way) Dhahran ([[Arabic language الظهران al-Dahrān), or Dharan is a city in Saudi Arabia. ... The heat index (HI) or humidex is an index that combines air temperature and relative humidity to determine an apparent temperature â€” how hot it actually feels. ...

## Calculating the dew point

A well-known approximation used to calculate the dew point Td given the relative humidty RH and the actual temperature T of air is:

where

$gamma(T,RH) = frac {a T} {b+T} + ln (RH/100)$

where the temperatures are in degrees Celsius and "ln" refers to the natural logarithm. The constants are: For other uses, see Celsius (disambiguation). ... The natural logarithm, formerly known as the hyperbolic logarithm, is the logarithm to the base e, where e is an irrational constant approximately equal to 2. ...

a = 17.27
b = 237.7 °C

This expression is based on the "Magnus" (or "Magnus-Tetens") approximation for the saturation vapor pressure of water in air as a function of temperature.[3] It is considered valid for In chemistry, saturation has four different meanings: In physical chemistry, saturation is the point at which a solution of a substance can dissolve no more of that substance and additional amounts of that substance will appear as a precipitate. ...

0 °C < T < 60 °C
1% < RH < 100%
0 °C < Td < 50 °C

### 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 dry-bulb temperature is the temperature of air measured by a thermometer freely exposed to the air but shielded from radiation and moisture. ...

The equation is:

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

or

RH = 100 − 5(TTd)

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 Td 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

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 °F.

Tfd is in degrees Fahrenheit; RH same as above.

## References

1. ^ [Steve] (2006-08-15). Relative Humidity....Relative to What? The Dew Point Temperature...a better approach. Steve Horstmeyer, Meteorologist, WRKC TV, Cincinnati, Ohio, USA.
2. ^ Burt, Christopher C. (2004). Extreme Weather: A Guide & Record Book. W. W. Norton & Company, 28. ISBN 0393326586.
3. ^ Webpage maintained by Paroscientific, Inc., retrieved September 13, 2007.
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

Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 227th day of the year (228th in leap years) in the Gregorian calendar. ...

Results from FactBites:

 Observed Dew Point Temperature: indicates the amount of moisture in the air (288 words) Dew points indicate the amount moisture in the air. The higher the dew points, the higher the moisture content of the air at a given temperature. Dew point temperature is defined as the temperature to which the air would have to cool (at constant pressure and constant water vapor content) in order to reach saturation.
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