About Maximum Specific Humidity and Temperature in the Atmosphere
Water vapor (gas) is typically between 1 and 2% of the total atmospheric gas, with the
highest values in the warm humid tropics. Humidity can be expressed as grams of water
vapor per kilogram of atmospheric gas (g/kg or part per thousand, ppt). This measure of
humidity is referred to as the specific humidity, which is NOT the measure of humidity
given on the nightly newscast (that would be relative humidity).
Specific humidity is a useful measure of the amount of water vapor gas in the
atmosphere for a number of reasons. Specific humidity is not dependent upon the volume
of gas involved, and therefore can be used to directly compare parcels of atmosphere that
might be at different pressures and temperatures. A kilogram of gas is a kilogram of gas,
regardless of its volume (a pound of feathers weighs the same as a pound of lead).
When the atmosphere is completely saturated with water vapor to its maximum
possible extent, the specific humidity is at its highest possible value for that given
pressure and temperature. This is the saturation specific humidity (or maximum specific
humidity). If more water vapor than the maximum allowed is somehow stuffed into
atmosphere, that excess water vapor will condense as liquid water until the concentration
of water vapor (gas) is reduced to the saturation level. Just like dissolving salt in water,
there's a maximum level of salt saturation possible in the water for a given temperature
and pressure.
Another way of looking at the saturation specific humidity is the humidity level at
which condensation (water vapor turning to liquid water) begins, for a given pressure and
temperature.
The table and graph show the variation of saturation specific humidity as a function of
temperature (and implicitly, altitude) for the standard atmosphere. Note that these values
are not "data", they were generated from a model (function) of the standard atmosphere.
Saturation specific humidity increases with increasing temperature, consistent with
personal experience! And as felt and observed, a small increase in temperature at high
temperatures produces a large change in the maximum possible humidity. The
relationship looks exponential; how well does an exponential model fit these numbers?
We hope to uncover real measurements from experiments soon.
Reference: Aguado E and Burt JE (1999), Understanding Weather and Climate; Prentice
Hall, Saddle River, New Jersey, 474 pp
Saturation Specific Humidity versus Temperature in the Atmosphere
Temp. (° C)
Spec. Hum. (g/kg)
-40
0.1
-30
0.3
-20
0.75
-10
2
0
3.5
5
5
10
7
15
10
20
14
25
20
30
26.5
35
35
40
47