The amount of water vapor in the air can be
expressed in many different ways. Some variables used to quantify and
calculate moisture in the air include vapor pressure (e),
saturation vapor pressure (es), mixing ratio
(w), saturation mixing ratio, (ws),
dew point (Td), and relative humidity (RH).
- Vapor pressure (e): The pressure exerted by water
vapor in the air. Atmospheric pressure (p) is the
sum of the air pressure and vapor pressure where p>>e.
- Saturation vapor pressure (es):
How much water the air can hold at any temperature. Saturation
vapor pressure increases with rising temperature. Note that at
below freezing temperatures, the saturation vapor pressure is
greater over water than over ice.
- Mixing Ratio (w): The amount of water vapor in a
given volume of air.
- Saturation mixing ratio, (ws):
The amount of water vapor in a given volume of air saturated with
respect to a plane surface of water to the mass of the dry air.
- Dew point (Td): The temperature to
which the air must be cooled at constant pressure in order for it
to become saturated with respect to plane surface of water. Also, frost
point is the temperature to which the air must be cooled at
constant pressure in order for it to become saturated with respect
to plane surface of ice. Dew point is also the temperature at
which saturation mixing ratio (ws)
with respect to water becomes equal to the actual mixing ratio (w).
- Relative Humidity (RH): With respect to water, it
is the ratio (expressed as a percentage) of the actual mixing
ratio (w) to the saturation mixing ratio (ws)
with respect to water at the same temperature and pressure.
|Since the mixing ratio and saturation mixing ratio are
dependent on temperature and pressure, you can use the Goff-Gratch
Equation to calculate relative humidity as follows, based on the dew
Or, to calculate relative humidity based on the frost point:
Two examples for dew and frost point:
1) For liquid condensate (i.e. dew), to calculate RH based upon dew
point temperature and ambient temperature (measurements
in deg C ):
Tv = dew point temperature
Ta = ambient temperature
Assuming the condensate is liquid, fore "dew points":
With Tv = 0, and Ta =10
RH= ((6.1078 * exp ((17.7017 * Tv)/(243.7096 + Tv)))/
(6.1078 * exp ((17.7017 * Ta)/(243.7096 + Ta)))) * 100;
2) For solid condensate (i.e. frost, Tv=0C or below), for
"frost points", change the coefficients to:
RH = ((6.1078 * exp ((22.0438 * Tv)/(243.7096 + Tv)))/
(6.1078 * exp ((22.4038 * Ta)/(243.7096 + Ta)))) * 100;
For the same Tv and Ta, 0, and 10 as above, but assuming the
condensate is frost and not liquid dew, RH = 45.18%, slightly lower.
|Ahrens, C. D., 1994: Meteorology Today: An
Introduction to Weather, Climate, and the Environment, West
Publishing Company, 591 pp.
Byers, H. R., 1974: General Meteorology, McGraw-Hill, 461
Wallace, J. M. and P. V. Hobbs, 1977: Atmospheric Science An
Introductory Survey. Academic Press, 467 pp.