Barometric Atmospheric Pressure by Altitude

Atmospheric pressure vs. altitude

The International Standard Atmosphere (ISA) describes how pressure drops with altitude through P = P₀·(1 − 0.0065·h/T₀)^5.255, where P₀ = 101.325 kPa (sea-level pressure), T₀ = 288.15 K (15 °C), and the lapse rate runs at 6.5 °C per kilometer in the troposphere. Near the surface pressure drops by roughly 12% per 1000 m, and by about 5500 m it has fallen to half. Some reference points: sea level 101.3 kPa, São Paulo (760 m) ≈ 92.7 kPa, Serra paulistana ≈ 950 hPa, and La Paz (3640 m) ≈ 65 kPa, which is low enough to bring on altitude sickness in visitors who haven't acclimatized. This is the formula that altimeters, weather models and climbing tables all rely on.

Applications

You'll find it behind aircraft barometric altimeters (QNH reference), smartphone barometric GPS (where pairing pressure with GPS pulls vertical accuracy down to ±1 m), mountaineering (estimating boiling point and how much supplemental oxygen you'll need), weather forecasting (synoptic maps reduce pressure to sea level), and engineering (HVAC and combustion derate at altitude as air density falls).

FAQ

Why does the formula stop working above ~11 km? That's where the troposphere ends. Above the tropopause the lapse rate shifts (the lower stratosphere is roughly isothermal), so the ISA switches to a different equation for each layer.

Does temperature matter? It does. A warmer air column is less dense, so the real pressure at a given altitude can run 1-3% above the ISA value. That's the reason altimeters get a QNH calibration before each flight.

Why does water boil faster in the mountains? Lower pressure drags the boiling point down. Water boils at 100 °C at sea level but at around 88 °C in La Paz, which is exactly why food up there takes longer to cook through.