What this calculator does
Given a sea-level temperature and an altitude, this calculator returns the temperature at that altitude using the 1976 U.S. Standard Atmosphere (ISA) model. It also computes air pressure, density, and the speed of sound at altitude, identifies which atmospheric layer you're in (troposphere through mesosphere), shows how your conditions deviate from the ISA standard, and applies wind-chill (apparent temperature) if relevant. Includes presets for common flight levels (FL100-FL430), famous mountains (Everest, Kilimanjaro), and regional landmarks (Burj Khalifa, Jebel Jais).
The ISA (International Standard Atmosphere) model
ISA is a mathematical model that describes the average global atmosphere as a function of altitude. It's the reference for aviation altimeter calibration, aircraft performance certification, and engineering design. The 1976 version (NOAA-S/T 76-1562) extends from sea level to 1,000 km, with the troposphere through mesosphere layers used for practical altitudes. Standard sea-level conditions: 15 °C (288.15 K), 101,325 Pa pressure, 1.225 kg/m³ density. Real atmospheres deviate ±20 °C from ISA at any altitude depending on latitude, season, and weather.
The 7 atmospheric layers
Troposphere (0-11 km, -6.5 °C/km): heated from below by Earth's surface, weather happens here. Tropopause (11-20 km, isothermal at -56.5 °C): the lid of the troposphere, jet streams flow here. Lower Stratosphere (20-32 km, +1 °C/km): ozone starts absorbing UV. Upper Stratosphere (32-47 km, +2.8 °C/km): ozone layer concentration peaks at ~25 km. Stratopause (47-51 km, isothermal at -2.5 °C). Lower Mesosphere (51-71 km, -2.8 °C/km): meteors burn up here. Upper Mesosphere (71-86 km, -2 °C/km): the coldest part of the atmosphere (down to -86 °C at the mesopause). Above 86 km is the thermosphere where temperature rises again but air is so thin you'd freeze, not burn.
Non-standard sea-level temperature
If you input a sea-level temperature different from 15 °C (e.g., 35 °C in a Dubai summer), the calculator applies the delta uniformly across the atmosphere - in the troposphere this gives the familiar T(h) = T₀ - 0.0065 × h. This is the aviation 'ISA + N °C deviation' convention used in airline performance charts, and matches what Omni Calculator and other reference tools produce. So 30 °C at sea level produces -41.5 °C at the tropopause (11 km), which is 15 °C warmer than standard ISA throughout. The 'ISA deviation' indicator in the result panel confirms this offset at your chosen altitude.
Aviation applications
Pilots use ISA for: (1) altimeter setting - the altimeter assumes ISA pressure, so corrections (QNH, QFE, QNE) are needed for real-world weather. (2) Performance charts - aircraft climb rate, fuel burn, and engine power are tabulated against 'ISA + N °C' deviation. A 'hot day' (ISA + 15 °C) reduces takeoff thrust and required runway distance. (3) True airspeed calculation - the indicated airspeed must be corrected for air density at altitude. At FL350 the air density is ~30% of sea level, so a Mach 0.85 cruise = ~875 km/h true airspeed vs ~280 km/h indicated. The 'Comparison at common altitudes' panel above shows temperatures at typical flight levels for quick reference.
Frequently asked questions
Using the ISA standard model with 15 °C at sea level, the temperature at FL350 (35,000 ft / 10,668 m) is approximately -54.3 °C (-65.7 °F). This is within the troposphere just below the tropopause, where temperature stops dropping at around -56.5 °C. Real flight conditions vary ±15-20 °C from this standard depending on latitude, season, and weather.
The troposphere (0-11 km) is heated FROM BELOW by Earth's surface absorbing solar radiation and emitting infrared. As you climb higher, you get farther from this heat source, so the air gets colder. The standard lapse rate is -6.5 °C per kilometer (-3.56 °F per 1,000 ft). This is why mountain tops are colder than valleys, why airliners need heated cabins, and why frost forms on overpasses before roads (more exposed surface area cooling).
The stratosphere (11-47 km) contains the ozone layer (peak concentration at ~25 km), which absorbs the Sun's ultraviolet radiation. This UV absorption directly heats the surrounding air molecules. Above the troposphere, the source of heat shifts from surface infrared (below) to UV absorption (above), so the temperature gradient reverses - it climbs from -56.5 °C at the tropopause to -2.5 °C at the stratopause (47 km).
ISA + 15 °C means the actual sea-level temperature is 15 degrees warmer than the ISA standard (15 °C + 15 °C = 30 °C). Pilots use this notation in performance charts because hot days reduce engine thrust, increase takeoff distance, and decrease climb rate. A 'hot and high' airport (high elevation + ISA+) is the most demanding for aircraft performance. Enter your actual sea-level temperature in the calculator and the 'ISA deviation' indicator will tell you how far you are from standard at any altitude.
Often colder. Using ISA with 15 °C sea-level temperature, Everest summit (8,848 m) shows about -42 °C. In reality, summit temperatures range from -20 °C in summer to -60 °C in winter due to seasonal sea-level variation, wind chill (jet stream winds at the summit can reach 280 km/h), and humidity. The calculator's wind-chill feature adds the apparent temperature when wind speed is entered.
Sources
- U.S. Standard Atmosphere, 1976 (NOAA-S/T 76-1562)— NOAA, NASA, US Air Force
- International Standard Atmosphere - ICAO Doc 7488— International Civil Aviation Organization
- PDAS - The Public Domain Aeronautical Software ISA Calculator— Public Domain Aeronautical Software
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