How does high humidity reduce the oxygen in the
atmosphere? How do changes in relative humidity occur?
The amount of oxygen in the atmosphere is fairly constant. Relative
humidity is a measure of how much water vapor is in the air - which is not
constant.. As water enters the air by evaporation, the percentage of the
other gasses - including oxygen goes down.
Said another way, the amount of oxygen in a breath of moist air is less
(percentage wise and in actual number of molecules taken into the lung) than
in dry air. It takes more breaths to get the same amount of oxygen into
your body. We often mistakenly say the air is "heavy" when it is humid. It
is actually lighter, but contains more water vapor making is more work to
obtain the required oxygen.
The actual amount of oxygen in the atmosphere does not decrease except in
High humidity (whether relative humidity or absolute humidity)
has so little effect on the amount of oxygen in the
air that the change is virtually imperceptible.
Relative humidity is a measure of how close to saturation the air is
and is dependent on temperature. Absolute humidity is the mass of
water in the air. The humidity increases in the air by evaporation
from the ground and plants or by precipitation. If the absolute humidity
remains constant, the relative humidity decreases with increasing
temperature and increases with decreasing temperature.
David R. Cook
Atmospheric Physics and Chemistry Section
Environmental Research Division
Argonne National Laboratory
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Update: June 2012