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.
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