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 that the dew point is equal to the current temperature (and the air is maximally saturated with water). When the dew point stays constant and temperature increases, relative humidity will decrease.
A more precise way of explaining the same facts would be to say that whenever water vapor and liquid water coexist, there is some evaporation and some condensing. For a given partial pressure of water vapor, there exists a dew point. If the actual temperature is higher than the dew point, evaporation is faster than condensation, so some liquid water will evaporate and decrease the overall temperature; if the temperature is lower than the dew point, some water vapor will condense (if there are some liquid or solid surfaces for it to condense upon) and increase the temperature.
Lower dew points (< 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 to 68 °F). Most inhabitants of these areas will consider dew points above 21 °C (70 °F) to be oppressive.
|Dew Point °C||Dew Point °F||Human Perception||Rel. Humidity at 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 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||31% - 41%|
|10 - 12°C||50 - 54°F||Very comfortable||31% - 37%|
|<10°C||<49°F||A bit dry for some||30%|
A well-known approximation used to calculate the dew point Td given the relative humidity RH and the actual temperature T of air is:
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. It is considered valid for
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:
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 .
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.
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