atmospheric pressure

Meaning: Atmospheric pressure comes from the weight of the column of air above a point. The standard sea-level value is 1 atm = 101.3 kPa ≈ 1.01 × 10⁵ Pa = 760 mmHg = 760 torr. It falls with altitude because less air lies above, and at a given point it pushes equally in all directions.

Equation: P = Patm + ρgh, where P is the absolute pressure at depth h in a fluid open to the air (Pa), Patm is the atmospheric pressure at the surface (Pa), ρ is the fluid density (kg/m³), g is 9.8 m/s², and h is the depth (m). Gauge pressure = P − Patm.

On the MCAT: Expect a mercury barometer, where the column height balances the air pressure (760 mm at 1 atm), or an open-tube manometer, where the gas pressure equals Patm plus or minus ρgh. Altitude questions ask you to predict that the boiling point drops (a liquid boils when its vapor pressure equals the external pressure) and that the partial pressure of O₂ drops while its fraction of the air, about 21%, stays nearly the same.

Common confusion: Absolute pressure includes Patm, and gauge pressure does not. A gauge reading of zero therefore still means about 1 atm absolute. Add Patm whenever a question asks for total pressure.