The Standard Atmosphere (ISA) in Numbers

According to the FAA Aviation Weather Handbook, the properties of the atmosphere fluctuate continuously. Engineers and meteorologists therefore need a fixed reference: the standard atmosphere. It is a hypothetical distribution of temperature, pressure and density with height. By international agreement it serves as the reference for calibrating altimeters and for aircraft performance calculations. You will meet it as the International Standard Atmosphere (ISA) or the ICAO standard atmosphere.

The ISA in five values

• At mean sea level the pressure is 1013.25 hPa.
• At mean sea level the temperature is 15 °C.
• In the troposphere the temperature falls by 6.5 °C per 1,000 m.
• The tropopause lies at a pressure altitude of 11,000 m (36,089 ft).
• At the tropopause the temperature is −56.5 °C.

The values come from the table “Selected Properties of the Standard Atmosphere” in the FAA Aviation Weather Handbook. What pressure altitude means is explained further down, in the section on the altimeter. The International Commission for Air Navigation set the 15 °C at sea level for the International Standard Atmosphere in 1924.

The handbook describes the troposphere like this: it begins at the Earth’s surface and extends to about 11 km (36,000 ft). Almost all weather occurs in it. The tropopause is the boundary between the troposphere and the layer above it.

Rules of thumb for flying

In the cockpit you work in feet. The FAA handbook gives rules of thumb for this: in the ISA the temperature falls by about 2 °C per 1,000 ft, up to the tropopause. Above that it is taken as constant at first.

Pressure falls with height: the higher you are, the less air lies above you. According to the FAA handbook, the pressure at 18,000 ft is only about half its value at sea level. EASA gives a rule of thumb: near sea level, 30 ft corresponds to a pressure drop of 1 hPa.

Why the real atmosphere differs

The FAA Aviation Weather Handbook points out that the 6.5 °C per kilometre is an average, so the exact value seldom occurs. In the troposphere the temperature sometimes stays constant or even rises with height.

According to the same handbook, the tropopause is not at 11 km everywhere either. The troposphere gets lower from the equator to the poles and is higher in summer than in winter. At the equator it reaches about 18 to 20 km, at 50° latitude 9 km and at the poles 6 km.

What your altimeter does with it

According to the FAA handbook, an altimeter measures the air pressure where it is and shows an altitude derived from it. It is calibrated to the standard atmosphere. Pressure altitude is an atmospheric pressure expressed as the altitude that corresponds to that pressure in the standard atmosphere.

According to the FAA handbook, the indicated altitude is correct only if the sea level pressure is standard, the sea level temperature is standard, and both fall with height at the standard rate. You compensate for a departure from standard pressure with the pressure setting on the altimeter. Which errors remain is explained on the related page about altimeter errors.

What the EASA syllabus says

In the PPL(A) theoretical knowledge syllabus the standard atmosphere appears in Meteorology as “ICAO standard atmosphere”. Under Instrumentation the syllabus lists it for the altimeter as “Standard atmosphere”, together with “The different barometric references (QNH, QFE and 1013.25)“.