Table of Contents
Atmospheric Pressure and Altitude
Atmospheric pressure decreases with altitude because there is less air above pressing down. At sea level, standard pressure is 1013.25 hPa (14.696 psi). At the summit of Mount Everest (8,849 m), pressure drops to about 337 hPa, roughly one-third of sea level. This decrease follows a roughly exponential pattern described by the barometric formula.
Understanding pressure at altitude is essential for aviation (altimeters work by measuring pressure), mountaineering (oxygen levels for climbers), weather forecasting (correcting station pressure to sea level), cooking (boiling points change with pressure), and engineering of pressurized systems.
Barometric Formula
Where P0 is sea level pressure, L is temperature lapse rate (0.0065 K/m), h is altitude in meters, T0 is sea level temperature in Kelvin, g = 9.80665, M = 0.0289644 kg/mol (molar mass of air), and R = 8.31447 J/(mol·K).
Standard Atmosphere Pressure
| Altitude (m) | Pressure (hPa) | % of Sea Level | Temp (°C) |
|---|---|---|---|
| 0 | 1013.25 | 100% | 15.0 |
| 1,000 | 898.76 | 88.7% | 8.5 |
| 3,000 | 701.21 | 69.2% | -4.5 |
| 5,500 | 505.39 | 49.9% | -20.8 |
| 8,849 | 314.42 | 31.0% | -42.5 |
FAQ
Why does pressure decrease with altitude?
Air pressure at any point equals the weight of the air column above it. Higher up, there is less air above, so the pressure is lower. The relationship is approximately exponential because air is compressible: lower layers are compressed by the weight above them.
How does this affect cooking?
Lower pressure means water boils at a lower temperature. At 2,000 meters, water boils at about 93°C instead of 100°C. This means food takes longer to cook by boiling. Pressure cookers restore higher pressure to speed cooking at altitude.
What is the standard atmosphere?
The International Standard Atmosphere (ISA) is a model of how pressure, temperature, and density vary with altitude. It assumes sea level conditions of 15°C, 1013.25 hPa, and a temperature lapse rate of 6.5°C per kilometer up to the tropopause at 11 km. It is used as a reference for aircraft performance calculations.