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Name: Shruti R.
Status: N/A
Age: 10
Location: N/A
Country: N/A
Date: 3/22/2004


Question:
Why are the amounts of gases in the earth's atmosphere usually shown as percentages of dry air?


Replies:
Shruti,

Besides the gases that comprise air, air can also contain variable amounts of water vapor -- characterized by the term, "relative humidity." Because the amount of water vapor held in air can change, it is simpler and more sensible to express component gas percentages on the basis of dry air.

Regards,
ProfHoff 830


Because the amount of water vapor that can exist in a mixture of atmospheric gases can vary from zero (dry gas) to 25 mm Hg @ 25 C. (saturated vapor) it is customary to make comparisons in the absence of any water vapor. Otherwise the percentages would 'jump' around depending upon how much water vapor was present.

Vince Calder


Is your question about dry air vs. wet, or about percentage vs. pressure in pounds-per- square-inch, or other?

Percentage rather than pressure, because we like to envision parts of a familiar whole which we consider "normal" or "1".

Dry rather than wet, because the amount of water is totally variable with relative humidity and temperature.

On a hot, wet day in the tropics, the relative humidity is 100% and the temperature is 100 degrees Fahrenheit (around 40 degrees C), so the partial pressure of water is 49mmHg, or 6% of the air. That's enough to change the whole amount of air we're fractioning, and mess up our answers. On a cold, dry day at the south pole, say -10 deg F and 10% humidity, water would be 0.58 mmHg or 0.07% of the air. That is just about nothing.

And it is different every day.

So we leave all water out, to make the composition question a simple one. The amounts of the other gasses are relatively quite constant, and that is what we are intuitively asking about with the question of atmospheric composition.

One also must decide between percentage of pressure (same as counting up molecules), and percentage of weight (the molecule weight of each gas is different than the other gasses).

hope that answers it-

Jim Swenson



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