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Why does atmospheric pressure decrease at higher altitudes?

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differential manometer, Torricellian barometer, and siphon — fluid mechanics Figure 1: Schematic representations of (A) a differential manometer, (B) a Torricellian barometer, and (C) a siphon.

In the context of fluid mechanics, atmospheric pressure is defined as the force per unit area exerted by the weight of the entire column of air above a specific location.1

Atmospheric pressure decreases at higher altitudes primarily because there is less air remaining above that point to exert weight. As you ascend, the mass of the overlying atmospheric column diminishes, resulting in a lower force per unit area.12

Furthermore, because the atmosphere is a compressible fluid, it is compressed by its own weight, making it significantly denser near the Earth's surface than at higher altitudes. This variation in density means that the rate of pressure decrease is not uniform; as altitude increases, the air becomes progressively less dense, which influences the vertical pressure gradient. Because of this relationship between pressure and height, instruments like barometers can be used to measure altitude.3124