Is there less oxygen in the air at altitude?

The fraction is the same: up to about 80 km air is about 20.95% oxygen by volume. What falls is the total pressure, so each breath holds fewer oxygen molecules (a lower partial pressure). At the summit of Everest the pressure is about a third of sea level and the oxygen partial pressure only about 6.6 kPa, like breathing 6.5% oxygen at sea level. The tool's equivalent oxygen figure is exactly this conversion.

Why does water boil below 100 °C in the mountains?

Water boils when its vapour pressure equals the surrounding pressure. Lower pressure means a lower boiling point: about 88 °C in Lhasa and about 70 °C on Everest. The tool uses both the empirical Antoine equation and the Clausius–Clapeyron equation, which agree within 1 °C. Cooking reactions roughly halve in speed for every 10 °C drop, so high-altitude cooking is much slower and pressure cookers are common.

How does the standard atmosphere differ from real weather?

ISA is an annual-mean, mid-latitude reference: 15 °C and 1013.25 hPa at sea level, cooling 6.5 °C per km in the troposphere, and isothermal from 11 to 20 km. The real atmosphere varies with latitude, season and weather; the tropopause is lower over the poles in winter and higher in the tropics. Aviation uses ISA to define pressure altitude and calibrate altimeters, and engineers use it as a common baseline.

How should I read dew point, humidity and feels-like temperatures?

Dew point is the temperature at which air would saturate if cooled at constant pressure, and it tracks mugginess better than relative humidity: above 20 °C it feels humid, above 24 °C oppressive. The heat index combines temperature and humidity using the US National Weather Service regression; above 32 °C calls for extreme caution and above 41 °C means danger of heat illness. Wind chill only applies below 10 °C with some wind.

What do conditional instability and CAPE mean?

Unsaturated rising air cools at the dry adiabatic rate of about 9.8 K/km; once saturated, released latent heat slows the cooling to the moist rate of about 4–7 K/km. If the environment's lapse rate lies between the two, air rises on its own only after being lifted to saturation (LCL) and past the LFC: that is conditional instability. CAPE is the total buoyant work from the LFC to the EL; above 1000 J/kg strong convection is possible, and √(2·CAPE) bounds the updraft speed.

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🏔️Standard Atmosphere Calculator - Pressure by Altitude, Boiling Point, Dew Point

The tool follows the seven layers of the International Standard Atmosphere (ISA 1976) to give temperature, pressure, density, speed of sound and viscosity at any altitude from 0 to 86 km, and works backwards from a measured pressure to altitude. The boiling page shows the boiling point of water at altitude, a rough cooking-time factor and the equivalent oxygen level. The humidity page turns temperature and relative humidity (or dew point) into dew point, absolute humidity, wet-bulb temperature, heat index, wind chill and cloud base. The last page lifts an air parcel along dry and moist adiabats, marks LCL, LFC, EL and CAPE, and lets you release the parcel to watch it rise or sink.

Free · No sign-up · Runs in your browserLast updated: 2026-10-29

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How to use Atmosphere & Altitude Calculator

1On the standard atmosphere tab, drag the altitude or tap a place (Lhasa, Everest, airliner cruise and more)
2Optionally enter a measured pressure and QNH to get pressure altitude and the altimeter reading
3Switch to the boiling or humidity tabs and enter temperature, humidity and wind
4On the lapse-rate tab pick an atmosphere, adjust surface temperature, dew point and lapse rates, then press Release

FAQ

Is there less oxygen in the air at altitude?: The fraction is the same: up to about 80 km air is about 20.95% oxygen by volume. What falls is the total pressure, so each breath holds fewer oxygen molecules (a lower partial pressure). At the summit of Everest the pressure is about a third of sea level and the oxygen partial pressure only about 6.6 kPa, like breathing 6.5% oxygen at sea level. The tool's equivalent oxygen figure is exactly this conversion.
Why does water boil below 100 °C in the mountains?: Water boils when its vapour pressure equals the surrounding pressure. Lower pressure means a lower boiling point: about 88 °C in Lhasa and about 70 °C on Everest. The tool uses both the empirical Antoine equation and the Clausius–Clapeyron equation, which agree within 1 °C. Cooking reactions roughly halve in speed for every 10 °C drop, so high-altitude cooking is much slower and pressure cookers are common.
How does the standard atmosphere differ from real weather?: ISA is an annual-mean, mid-latitude reference: 15 °C and 1013.25 hPa at sea level, cooling 6.5 °C per km in the troposphere, and isothermal from 11 to 20 km. The real atmosphere varies with latitude, season and weather; the tropopause is lower over the poles in winter and higher in the tropics. Aviation uses ISA to define pressure altitude and calibrate altimeters, and engineers use it as a common baseline.
How should I read dew point, humidity and feels-like temperatures?: Dew point is the temperature at which air would saturate if cooled at constant pressure, and it tracks mugginess better than relative humidity: above 20 °C it feels humid, above 24 °C oppressive. The heat index combines temperature and humidity using the US National Weather Service regression; above 32 °C calls for extreme caution and above 41 °C means danger of heat illness. Wind chill only applies below 10 °C with some wind.
What do conditional instability and CAPE mean?: Unsaturated rising air cools at the dry adiabatic rate of about 9.8 K/km; once saturated, released latent heat slows the cooling to the moist rate of about 4–7 K/km. If the environment's lapse rate lies between the two, air rises on its own only after being lifted to saturation (LCL) and past the LFC: that is conditional instability. CAPE is the total buoyant work from the LFC to the EL; above 1000 J/kg strong convection is possible, and √(2·CAPE) bounds the updraft speed.

Features

Seven-layer ISA 1976 with a temperature profile and log-scale pressure and density curves
Temperature, pressure, density, speed of sound, viscosity, gravity and mean free path at once
Altitude from pressure, including an altimeter reading corrected with QNH
Boiling point by Antoine and Clausius–Clapeyron, equivalent oxygen and altitude zones
Dew point, relative and absolute humidity, wet-bulb, heat index, wind chill and cloud base
Dry and moist adiabatic lapse rates, LCL, LFC, EL, CAPE and CIN with a parcel release animation

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