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Myth: Warm Air “Holds more Water than Cooler Air”

16 Sep 2008, 11:54 pm

Since the advent of the Internet, there has been an unfortunate tendency for individuals to misrepresent basic scientific concepts via blogs, wikis, forums, and various other forms of communication since the digital age.

Of great concern are individuals to whom the public is dependent upon, especially during times of crisis, who perhaps unintentionally misrepresent basic concepts of Meteorology. Such as “Warm air ‘holds’ more water than cooler air.”

Let us examine a few basic concepts.

A. Molecules of water can exist in three (and only three) physical states ( a.k.a. phases).

  1. liquid
  2. solid
  3. vapor

B. Water molecules undergo phase changes from one physical state to another, such as evaporation, or condensation.

C. Air within the planetary boundary layer will always have molecules of water present, even in Earth’s driest deserts.

D. A given column of air is composed of water molecules in various physical states throughout its height, and those molecules of water are traversing their various states within the column, based upon the stresses forced upon that column.

E. The determining factor of ‘making a cloud’ is condensation, or a phase change of a water molecule from water vapor to liquid.

F. For condensation to occur, a given volume of air must cool to the dew point temperature (at constant pressure).

G. The dew point temperature is an important concept in itself, because it represents the temperature to which a given parcel of air must cool for saturation to occur (for water vapor to initiate a phase change into liquid (or condense)).

H. Of the phase changes, evaporation occurs more frequently at higher temperatures. This has ABSOLUTELY NOTHING WHATSOEVER to do with the air being able to ‘hold’ more water because of higher temperatures. Evaporation occurs more frequently when an air parcel is at higher temperatures because the molecules are moving more quickly.

I. There are many other factors involved in evaporation rates of a parcel (such as purity, and phases), however a parcel is always undergoing a battle between net evaporation, and net condensation. As stated above, if a volume of air drops below the dew point temperature, there is a net condensation and water droplets will form.

The important thing for now is to understand the basic concept that air has ABSOLUTELY NO HOLDING CAPACITY WHATSOEVER. Therefore the supposed paradigm that warm air “holds more water” than cooler air is completely false. If you hear your source for weather information make such a statement, hit him over the head with a HAMweather bat, and point him in the right direction.



One response to “Myth: Warm Air “Holds more Water than Cooler Air”

  1. Hotter air does indeed have a holding capacity for water in the following sense. Air with a dry bulb temp of 70 deg F holds 110 grains of water vapor at 100% humidity, while air with a dry bulb of 80 deg F holds 156 grains of water at 100 percent humidity (for example). This can be found by looking at a psychrometric chart. If you want to understand the relationship between dry bulb, wet bulb, dew point, and other air properties, just find a web site that explains the psychrometric chart.

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