Atmospheric Pressure

The Earth's entire atmosphere is itself a fluid, made of air, and — exactly like the liquid column of the previous section — it exerts a downward force on anything beneath it because of its own weight. The pressure due to this force is called atmospheric pressure: at any given point, it is the weight, per unit cross-sectional area, of the column of air starting at that point and extending all the way up to the very top of the atmosphere.

Because a taller column of air sits above a point at a lower altitude than above a point at a higher altitude, atmospheric pressure is clearly greatest at the Earth's surface (i.e. at sea level), and it steadily decreases as one moves upward, since the height (and hence the weight) of the remaining air column above keeps shrinking. The atmospheric pressure measured at sea level is called the normal atmospheric pressure. The density of air in the atmosphere itself also decreases with increasing height, and becomes negligible beyond an altitude of roughly 8 km — so, for practical purposes, the entire weight of the atmosphere producing sea-level pressure can be thought of as coming from an (idealised, uniform-density) air column about 8 km tall. …