6.1.6 (HL)—Atmospheric pressure and altitude

Gravity pulls atmospheric molecules toward Earth's surface. With increasing altitude there is less air above, so pressure and density decrease; oxygen remains a similar percentage of air, but its partial pressure falls.

Worked example: for a 500 m climb, ΔT ≈ -(500 m / 100 m) × 1°C = -5°C. If the starting temperature is 20°C, the standard-lapse-rate estimate is about 15°C.

The lapse rate is an approximate tropospheric temperature relationship, not a formula for gas pressure; actual temperature profiles vary with weather and atmospheric layer.

Lower oxygen partial pressure at altitude does not mean the oxygen percentage has fallen by the same amount.