Dew point

The dew point is the temperature to which a given parcel of humid 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.

The dew point is associated with relative humidity. A high relative humidity indicates that the dew point is closer to the current air temperature. Relative humidity of 100% indicates the dew point is equal to the current temperature and the air is maximally saturated with water. When the dew point remains constant and temperature increases, relative humidity will decrease.^[1]

The dew point is an important statistic for general aviation pilots, as it is used to calculate the likelihood of carburetor icing and fog, and estimate the height of the cloud base.

1 Constant pressure
2 Varying pressure
3 Comfort range
4 Calculating the dew point
- 4.1 Simple approximation
- 4.2 Closer approximation
5 See also
6 References
7 External links

Constant pressure

This graph shows the maximum percentage (by mass) of water vapor that can exist in air at sea level across a range of temperatures. The behavior of water vapor does not depend on the presence of other gases in air. The formation of dew would occur at the dew point even if the only gas present is water vapor.

At a given barometric pressure, 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 temperature rises without changing this mole fraction, the dew point will remain unchanged; however, the relative humidity will go down accordingly (and water continues to condense at the same temperature). Increasing the mole fraction, i.e., making the air more humid, would bring the relative humidity back up to its initial value. In the same way, decreasing the mole fraction after a temperature drop brings the relative humidity back down to its initial level. For this reason, the same relative humidity on a day when it's 80°F, and on a day when it's 100°F will imply that a higher fraction of the air on the hotter day consists of water vapor than on the cooler day, i.e., the dew point is higher.

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. Considering New York and Denver, for 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.

Comfort range

Humans tend to react with discomfort to a high dew point (i.e., greater than 15 °C (59 °F)). When the air temperature is high, the body's thermoregulation uses evaporation of its perspiration (sweat) to cool down, with the cooling effect directly related to how fast the perspiration evaporates. The rate perspiration can evaporate depends on how much moisture is in the air and how much moisture the air (at the same pressure and temperature) can hold. If the air already is saturated with moisture, perspiration will not evaporate. The body's cooling system will produces perspiration in an effort to keep the body at its normal temperature even after the rate it is producing sweat is less than the evaporation rate. So even a body at rest (i.e., not generating additional body heat by exercising or manual labor) can become coated with sweat on humid days. (Heat stroke occurs when the body's perspiration system gives up, so the skin of a heat stroke victim is typically dry.) It is the unevaporated sweat that tends to make one feel uncomfortable in humid weather.

But the air that affects comfort is not the air where the thermometer and humidity meters are located. It is the air that is touching one's body. As that portion of air is warmed by body heat, it will rise and be replaced with other air. By moving the air away from one's body faster, as with a natural breeze or a fan, the sweat will evaporate faster, making perspiration more effective, i.e., you feel cooler. The more unevaporated perspiration, the greater the discomfort.

A wet bulb thermometer also uses evaporative cooling, so it provides a good analog for use in evaluating comfort level.

Discomfort also exists when dealing with low dew points (i.e., below −30 °C (−22 °F)). The drier air can cause skin to crack, become irritated more easily and will dry out the respiratory paths.

Lower dew points, less than 10 °C (50 °F), correlate with lower ambient temperatures and the body requires less cooling. A lower dew point can go along with a high temperature only at extremely low relative humidity (see graph below), allowing for relative effective cooling.

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

Dew point °C	Dew point °F	Human perception^[1]	Rel. humidity at 32 °C (90 °F)
> Higher than 26 °C	> Higher than 80 °F	Severely high. Even deadly for asthma related illnesses	65% and higher
24 – 26 °C	75 – 80 °F	Extremely uncomfortable, fairly 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 edge	44% – 52%
16 – 18 °C	60 – 64 °F	OK for most, but all perceive the humidity at upper edge	37% – 46%
13 – 16 °C	55 – 59 °F	Comfortable	38% – 41%
10 – 12 °C	50 – 54 °F	Very comfortable	31% – 37%
< 10 °C	< 49 °F	A bit dry for some	30%

Calculating the dew point

Graph of the dependence of the dewpoint upon air temperature for several levels of relative humidity. Based on the August–Roche–Magnus approximation.

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

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

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:

a = 17.271

b = 237.7 °C

This expression is based on the August–Roche–Magnus approximation for the saturation vapor pressure of water in air as a function of temperature.^[2] It is considered valid for

0 °C < T < 60 °C

1% < RH < 100%

0 °C < T_d < 50 °C

Simple approximation

There is also a very simple approximation that allows conversion between the dew point, the dry-bulb temperature and the relative humidity. This approach will be accurate to within about ±1 °C as long as the relative humidity is above 50%.

The equation is:

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

$RH = 100 - 5 (T - T_d). \,$

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 approach, 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.^[3]

In Fahrenheit,

$Tf_d = Tf - \frac {100 - RH} {2.778}$

For example, a relative humidity of 100% means dew point is the 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.

TF_d is in degrees Fahrenheit; RH same as above.

Closer approximation

A calculation used by NOAA is:^[4]

$\begin{align} e_\text{s} & = 6.112 \exp \left( {17.67T \over T+243.5} \right) \\[8pt] e_\text{w} & = 6.112 \exp \left( {17.67T_\text{w} \over T_\text{w} + 243.5} \right) \\[8pt] e & = e_\text{w} - p_\text{sta} \left(T-T_\text{w}\right) 0.00066 \left[1 + (0.00115 T_\text{w}) \right] \\[8pt] RH & = 100 {e \over e_\text{s}} \\[8pt] T_\text{d} & = {243.5 \ln(e/6.112) \over 17.67 - \ln(e/6.112)} \end{align}$

where:

RH is relative humidity and $T_\text{d}$ is dew point in degrees Celsius

$T$ and $T_\text{w}$ are the dry-bulb and wet-bulb temperatures respectively in degrees Celsius

$e_\text{s}$ is the saturated water vapor pressure, in units of millibar, at the dry-bulb temperature

$e_\text{w}$ is the saturated water vapor pressure, in units of millibar, at the wet-bulb temperature

$e$ is the actual water vapor pressure, in units of millibar

$p_\text{sta}$ is "station pressure" (absolute barometric pressure at the site that humidity is being calculated for) in units of millibar (which is also hPa).

For greater accuracy use the Arden Buck equation to find the water vapor pressures.

References

↑ ^1.0 ^1.1 Horstmeyer, Steve (2006-08-15). "Relative Humidity....Relative to What? The Dew Point Temperature...a better approach". Steve Horstmeyer, Meteorologist, WKRC TV, Cincinnati, Ohio, USA. http://www.shorstmeyer.com/wxfaqs/humidity/humidity.html. Retrieved 2009-08-20.
↑ "MET4 AND MET4A CALCULATION OF DEW POINT". Paroscientific, Inc. 4500 148th Ave. N.E. Redmond, WA 98052. 2007-09-13. http://www.paroscientific.com/dewpoint.htm.
↑ 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
↑ http://www.srh.noaa.gov/images/epz/wxcalc/rhTdFromWetBulb.pdf

External links

What is the dew point?
Dew point definition NOAA Glossary
Dew point formula
Often Needed Answers about Temp, Humidity & Dew Point from the sci.geo.meteorology Usenet newsgroup

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