It might not seem like it, but air has weight. Anything with mass has weight, and we know air has mass because
(for example) we can feel it when the wind blows.
The total weight of the atmosphere exerts a pressure of about
14.7 pounds per square inch at sea level. You don't notice this weight, however, because you are used to it.
If you live in Denver, Colorado, which is at an elevation of about 5,000 feet, then about 85% of the atmosphere
is above you, resulting in an air pressure of about 12.5 pounds
per square inch. At the top of Mount Everest (over 29,000 feet), only 30% of the atmosphere lies above you,
leaving an air pressure of only 4.4 pounds per square inch.
Some people think air does not have weight since a parcel of air (say, air in a thin, lightweight balloon) just 'floats' around.
This is because the parcel of air is embedded in surrounding air that has the same density (mass per unit volume).
But imagine that we remove that parcel of air from the atmosphere...the atmosphere would then weigh very slightly less. If we could precisely
measure the barometric pressure everywhere on the surface of the Earth, we would see a very tiny decrease in the weight of the atmosphere
(the barometric pressure). If we could remove 50% of the mass of the atmosphere, the global-average barometric pressure at the
Earth's surface would also be reduced by 50%.
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