Thinner air with height
Air has weight, and the pressure at any point is the weight of all the air above it. As you climb, there is less air overhead, so pressure falls. Near sea level it drops by about 1 hectopascal for every 8 meters (27 feet), or roughly 3.5% per 1,000 feet, and the decline slows with height.
The barometric formula
The International Standard Atmosphere models the lower atmosphere (the troposphere, up to 11 km or 36,089 ft) with a temperature that falls by 6.5 °C per kilometer. Pressure then follows:
p = p₀ × (1 − 0.0065 × h ÷ T₀)^5.256
where p₀ is the sea-level pressure (1,013.25 hPa standard), T₀ the sea-level temperature in kelvin (288.15 K, or 59 °F, standard) and h the altitude in meters. Above 11 km the temperature stays roughly constant, and the calculator switches to an exponential decay.
Worked example
Denver, at 5,280 feet, on a standard day:
| Result | Value |
|---|---|
| Air pressure | 834.3 hPa |
| In inches of mercury | 24.64 inHg |
| Share of sea-level pressure | 82.3% |
| Equivalent oxygen at sea level | 17.2% |
| Standard temperature | 40.2 °F |
Each breath in Denver contains about 18% fewer oxygen molecules than at sea level.
Reference values
| Altitude | Pressure | Share of sea level |
|---|---|---|
| Sea level | 1,013 hPa (29.92 inHg) | 100% |
| 5,280 ft (Denver) | 834 hPa (24.64 inHg) | 82% |
| 10,000 ft | 697 hPa (20.58 inHg) | 69% |
| 14,000 ft (Colorado fourteeners) | 595 hPa (17.58 inHg) | 59% |
| 29,032 ft (Everest) | about 314 hPa | 31% |
On Everest the real pressure is a little higher, about 330 to 340 hPa, because the atmosphere over the tropics is warmer than the standard model.
Why it matters
- Health: above about 8,000 feet, some people develop altitude sickness. Climbing slowly lets the body adapt.
- Cooking: water boils at a lower temperature, so food takes longer to cook and baking recipes need adjustments.
- Aviation: altimeters are barometers. Pilots set the local sea-level pressure so the instrument reads true altitude.
- Weather: high and low pressure systems shift the actual pressure by a few percent around these standard values.
The model gives a standard estimate. For the real pressure at your location right now, use a barometer or the station pressure from a local weather service.