Gases Shown as Percent of Dry Air
Name: Shruti R.
Why are the amounts of gases in the earth's atmosphere usually shown as
percentages of dry air?
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.
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.
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
hope that answers it-
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Update: June 2012