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Atmospheric pressure is the pressure exerted by the air in Earth’s atmosphere on a given surface. It is not the same as the mass of gases but is caused by the weight of the vertical column of air above a unit area.
Air has mass, and due to gravity, this mass exerts force downward. Imagine a column of air extending from the ground up to the edge of the atmosphere. The total weight of all gases in this column pressing on a unit surface area is what we call atmospheric pressure. At sea level, this pressure is about 101.3 kPa or 1 atmosphere (atm), which is equivalent to 760 mm of mercury (Hg) in a barometer.
The important factor here is weight, not just mass. Mass only tells us how much matter is present, while weight is the force due to gravity acting on that mass. Since atmospheric pressure is a measure of force per unit area, it is directly linked to the weight of gases above the surface.
If we compare the options:
The mass of a vertical column of gases is not sufficient because pressure depends on force (weight), not just mass.
A horizontal column does not determine pressure, as air pressure is uniform in all horizontal directions.
The weight of the vertical column of gases is correct, because this force per unit area defines atmospheric pressure.
The combined option is incorrect because only the vertical weight matters.
Thus, atmospheric pressure is determined by the weight of the vertical column of gases acting on the surface.
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