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Name: Jim
Status: other
Grade: other
Location: MI
Country: N/A
Date: 7/15/2005

We always hear about "relative humidity," which I understand. My question is one of "absolute" humidity. In ballpark terms, can you tell me how much water might be suspended in, say, a "cubic meter" of atmosphere at, say 50%RH with an ambient temperature of 70°F at sea level with a barometric pressure of, say, 30"? A pint? A gallon? A tablespoon? I do not have a clue! (I pulled these constants out of the air, so to speak, so do not hesitate to change them to facilitate your answer.)

Relative humidity is the percentage of water vapor in the air compared to the vapor pressure of water, at the temperature stated. You can find a table, and formulas for the vapor pressure on the web site: as well as many others. If you know the pressure in mm of Hg you can convert to atmospheres using the conversion: 760 mm of Hg = 1 atm. You can then use the ideal gas law: PV = nRT where P= pressure in atmospheres, V = volume in liters, n = number of moles = gm / MW(= 18 for water), R = 0.082 and T = temperature in kelvins (kelvins = Celsius + 273.15). So you have all the information to determine how many grams of water per liter of air there is at 100% relative humidity. You can then just divide that by a factor of "2" of 50% relative humidity. etc. The barometric pressure of the atmosphere does not enter into the calculation. It is the vapor pressure of water that is important.

Vince Calder

Dear Jim-

Your question about the amount of water in the atmosphere is an interesting one. People are usually surprised when they see the answer. Below is a representation of all the earth's water, reduced to fit in a one-liter jar...(slightly larger than a quart jar)

Table 1-3. World's water supply relative to 1-liter total total.
Source                 Amount (ml)             Percent of Total
Oceans*                973 or 4 cups               97.0
Icecaps/glaciers       21 or 1.5 tablespoons        2.1
Groundwater^           6.1 or .5 tablespoon         0.6
Saline lakes           0.08 or 2 drops              0.1
Freshwater lakes       0.09 or 2 drops              0.1
Soil moisture          0.05 or 1 drop               0.005
Atmosphere             0.01 or 1/5 drop             0.001
Rivers                 not enough to be measured    0.0001

^down to 13,000 ft

(The above table from an EPA web is the link: )

As you can see from the table, all the fresh water in the world amounts to about 3 percent of the total water, and more than 2 percent of that is locked up in polar ice caps and glaciers. So all the rest of the fresh water is less than one percent.

The water in the atmosphere is not evenly distributed, and the amount of moisture the air can hold is dependent on several factors, including temperature. The warmer the temperature, the more moisture the air can hold. So the air in the tropics holds more moisture than the air in the polar regions.

If all the moisture in the air were evenly distributed, and then condensed out, it would amount to between one and two inches deep on the earth. This value is called the "precipitable water" in the atmosphere, and over the United States can range as high as 2.5 inches or rarely, a little higher in the summertime east of the Rocky Mountains. We have weather charts that depict the precipitable water in the atmosphere, and this gives us clues as to where precipitation may occur.

Wendell Bechtold, meteorologist
Forecaster, National Weather Service
Weather Forecast Office, St. Louis, MO

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