Standard atmosphere (ISA)

How do temperature, pressure and density change with altitude?

The ICAO standard atmosphere at any altitude or flight level, on a standard day or a warmer or colder one.

Altitude

Altitude unit

Temperature difference from standard

°C

0 is a standard day; 15 means 15 °C warmer than standard.

Air temperature

-20.7 °C

At FL180 in the standard atmosphere.

The air at this altitude

Pressure: 238.4 hPa · 7.04 inHg
Density: 0.3796 kg/m³ · 31.0%
Speed of sound: 576 kt · 296.5 m/s
Ratios to sea level: σ 0.3099 · δ 0.2353 · θ 0.7594
Altitude in metres: 10,668 m

Temperature and pressure by altitude

Temperature falls steadily up to the tropopause at 36,089 ft and then stays constant, while pressure falls ever more slowly; at 35,000 ft it is -54.3 °C and 238.4 hPa.

Temperature and pressure by altitude
Altitude	Temperature	Pressure	Density	Speed of sound
0 ft	15.0 °C	1013.3 hPa	1.2250 kg/m³	661 kt
5,000 ft	5.1 °C	843.1 hPa	1.0555 kg/m³	650 kt
10,000 ft	-4.8 °C	696.8 hPa	0.9046 kg/m³	638 kt
15,000 ft	-14.7 °C	571.8 hPa	0.7708 kg/m³	626 kt
20,000 ft	-24.6 °C	465.6 hPa	0.6527 kg/m³	614 kt
25,000 ft	-34.5 °C	376.0 hPa	0.5489 kg/m³	602 kt
30,000 ft	-44.4 °C	300.9 hPa	0.4583 kg/m³	589 kt
35,000 ft	-54.3 °C	238.4 hPa	0.3796 kg/m³	576 kt
36,089.2 ft	-56.5 °C	226.3 hPa	0.3639 kg/m³	574 kt
40,000 ft	-56.5 °C	187.5 hPa	0.3016 kg/m³	574 kt
45,000 ft	-56.5 °C	147.5 hPa	0.2371 kg/m³	574 kt
50,000 ft	-56.5 °C	116.0 hPa	0.1865 kg/m³	574 kt
55,000 ft	-56.5 °C	91.2 hPa	0.1466 kg/m³	574 kt
60,000 ft	-56.5 °C	71.7 hPa	0.1153 kg/m³	574 kt
65,000 ft	-56.5 °C	56.4 hPa	0.0907 kg/m³	574 kt

The ICAO standard atmosphere at any altitude or flight level, on a standard day or a warmer or colder one.

How to use

Type the altitude and pick its unit: feet, metres or a flight level. Switching the unit keeps the same altitude.
Leave the temperature difference at 0 for the standard atmosphere, or enter how much warmer (+) or colder (−) the day is than standard.
Read the air temperature, then the pressure, density, speed of sound and the ratios to sea level below it.
The chart shows the whole profile with your altitude marked; its Table tab lists the same values level by level.

Standard atmosphere by flight level

Flight level	Temperature	Pressure	Density
FL000	15.0 °C	1,013.3 hPa	100.0%
FL050	5.1 °C	843.1 hPa	86.2%
FL100	-4.8 °C	696.8 hPa	73.8%
FL180	-20.7 °C	506.0 hPa	57.0%
FL250	-34.5 °C	376.0 hPa	44.8%
FL300	-44.4 °C	300.9 hPa	37.4%
FL350	-54.3 °C	238.4 hPa	31.0%
FL390	-56.5 °C	196.8 hPa	25.8%
FL450	-56.5 °C	147.5 hPa	19.4%
FL600	-56.5 °C	71.7 hPa	9.4%

A flight level is the pressure altitude in hundreds of feet: FL350 is 35,000 ft. Density as a share of sea level (1.225 kg/m³).

How it works

The International Standard Atmosphere (ISA) is an agreed, average picture of the air: 15 °C and 1013.25 hPa at sea level, getting 6.5 °C colder every 1,000 m up to the tropopause at 11,000 m (36,089 ft), then a steady −56.5 °C up to 20,000 m. Aircraft instruments, performance charts and flight levels are all calibrated to it, so an altimeter set to 1013.25 hPa reads the ISA altitude of the pressure around it.

Pressure follows from the weight of the air above: in the lower layer p = p₀ · (T / T₀)^5.2559, where p₀ = 1013.25 hPa and T₀ = 288.15 K, and above the tropopause it falls to 37% (1/e) every 6.3 km. Density comes from the gas law, ρ = p / (R · T) with R = 287.05 J/(kg·K), and the speed of sound from a = √(1.4 · R · T). The ratios δ = p/p₀, θ = T/T₀ and σ = ρ/ρ₀ compare the air with sea level; engine thrust and true airspeed scale with them.

On a real day the air is rarely standard. A temperature difference such as ISA +15 keeps the pressure of the altitude but makes the air warmer, so it is thinner: its density altitude, the standard altitude with the same density, is higher, and aircraft perform as if they were there. The equations and tables are from ISO 2533:1975 and ICAO Doc 7488/3, Manual of the ICAO Standard Atmosphere.

The model covers −5,000 m to 20,000 m of geopotential (pressure) altitude for dry air. Real weather has humidity, inversions and a tropopause that moves with the season and latitude, so actual temperatures can differ by tens of degrees.

Worked example

What are the temperature, pressure and density at FL350 (10,668 m) in the standard atmosphere?

Temperature: T = 15 °C − 6.5 °C/km × 10.668 km = -54.3 °C.
Pressure: p = 1013.25 hPa × (218.81 K / 288.15 K)^5.2559 = 238.4 hPa.
Density: ρ = p / (R · T) = 23,842 Pa / (287.05 × 218.81 K) = 0.3796 kg/m³, 31.0% of sea level.

At FL350 the standard air is -54.3 °C at 238.4 hPa, with 31.0% of the sea-level density.

Frequently asked questions

What is the temperature at 35,000 feet?

In the standard atmosphere it is −54.3 °C at 35,000 ft (FL350), with a pressure of 238 hPa. Real temperatures at cruise level often differ by 10 °C or more.

Why does the temperature stop falling at 11 km?

That is the tropopause. Above it, in the stratosphere, ozone absorbs sunlight and warms the air, so the standard atmosphere keeps the temperature at −56.5 °C from 11 km to 20 km before it starts rising again.

What does ISA +15 mean?

The air is 15 °C warmer than the standard temperature for that altitude. At sea level that is 30 °C instead of 15 °C. The pressure is unchanged, but the air is less dense, so engines, propellers and wings perform worse.

What is the difference between pressure altitude and density altitude?

Pressure altitude is the standard altitude with the same pressure: what an altimeter set to 1013.25 hPa shows. Density altitude is the standard altitude with the same density, which decides aircraft performance. On a standard day they are equal; on a warm day density altitude is higher.

What are σ, δ and θ?

The density ratio σ = ρ/ρ₀, the pressure ratio δ = p/p₀ and the temperature ratio θ = T/T₀, each relative to sea level. Performance formulas use them; for example true airspeed is about equivalent airspeed divided by √σ.

How fast is the speed of sound at cruise altitude?

It depends only on temperature: 661 kt (340 m/s) at sea level, but 573 kt (295 m/s) at −56.5 °C above the tropopause. That is why Mach 0.8 is a lower true airspeed high up.

Is this the same as the US Standard Atmosphere?

Yes, up to 20 km. The US Standard Atmosphere 1976 and ISO 2533 agree up to 32 km; this tool covers the part aircraft and weather balloons use most.

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