What is Atmospheric Pressure?
Air pressure is the force exerted by the atmospheric air on the surface of the planet. It changes with altitude and temperature. The higher the elevation, the smaller is the mass of the air overlying the ground. Also, atmospheric pressure increases with the increase in temperature.
The units of pressure are Pascals (symbol: Pa). This calculator also supports hPa, kPa, atm, mmHg, inHg, psi, bar, and mbar.
How to Calculate Air Pressure at Altitude
The barometric formula relates air pressure to altitude:
Where:
- h — altitude in meters
- P — air pressure at altitude h
- P₀ — pressure at the reference level (sea level: 101,325 Pa = 1 atm)
- T — temperature at altitude h, in Kelvins
- g — gravitational acceleration = 9.80665 m/s²
- M — molar mass of air = 0.0289644 kg/mol
- R — universal gas constant = 8.31432 J/(mol·K)
Example Calculation
- Choose altitude: 4,000 m
- Reference pressure P₀: 1 atm = 101,325 Pa
- Air temperature: 30°C = 303.15 K
- Apply formula: P = 101,325 × e^(−9.80665 × 0.0289644 × 4000 / (8.31432 × 303.15))
- Result: ≈ 64,557 Pa
FAQs
Why does water boil earlier at higher altitude?
Water boils at lower temperatures at high altitudes because atmospheric pressure is reduced. Boiling occurs when the vapor pressure equals the ambient pressure. A lower ambient pressure means a lower boiling point. At 4,000 ft (1,219 m), water boils at about 204°F (95.5°C) instead of 212°F (100°C).
What is the standard sea-level pressure?
The standard sea-level pressure is defined as 101,325 Pa (1 atm), equivalent to 1,013.25 hPa, 760 mmHg, 29.92 inHg, or 14.696 psi.
What is the difference between metric and imperial units?
In metric (SI), altitude is measured in meters (m) and temperature in degrees Celsius (°C). In the US/Imperial system, altitude is measured in feet (ft) and temperature in degrees Fahrenheit (°F). This calculator supports both systems and converts internally to SI for the barometric formula.
How accurate is the barometric formula?
The barometric formula provides a good approximation for well-mixed air below about 86 km. It assumes a uniform temperature, which is an approximation — in reality, temperature varies with altitude (lapse rate). For precise atmospheric modeling, the International Standard Atmosphere (ISA) uses different lapse rates for different altitude layers.