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What is ISA in Aviation? (International Standard Atmosphere)

Updated: February 29, 2024

The International Standard Atmosphere (ISA)

The International Standard Atmosphere (ISA) is a standard model used in aviation to define the behavior of the Earth's atmosphere under standard conditions. It provides a benchmark for temperature, pressure, density, and other atmospheric properties at different altitudes. The ISA serves as a reference for aircraft performance calculations, flight planning, and aircraft design.

The ISA is based on a set of assumptions about the average atmospheric conditions around the world. It is divided into different layers, each with its own temperature and pressure characteristics. These layers are defined by the altitude range in which they are applicable. The ISA assumes a mean sea level temperature of 15 degrees Celsius (59 degrees Fahrenheit) and a sea level pressure of 1013.25 hectopascals (hPa) or 29.92 inches of mercury (inHg).

One of the key purposes of the ISA is to provide a standard against which aircraft performance can be measured. By defining the expected atmospheric conditions at different altitudes, the ISA allows pilots and aircraft manufacturers to determine the performance capabilities of an aircraft, such as its maximum altitude, climb rate, and fuel consumption.

The Layers of the ISA

The ISA is divided into several layers, each with its own set of temperature and pressure characteristics. These layers are defined by altitude ranges and are as follows:

Troposphere (up to 11 km or 36,090 feet): This is the layer closest to the Earth's surface and is where most weather phenomena occur. In the troposphere, temperature decreases with altitude at a rate of approximately 6.5 degrees Celsius per kilometer (3.6 degrees Fahrenheit per 1,000 feet).
Stratosphere (11 km to 50 km or 36,090 feet to 164,040 feet): The stratosphere is characterized by a stable temperature profile, with temperature remaining relatively constant or even increasing with altitude. This is due to the presence of the ozone layer, which absorbs and dissipates solar radiation.
Mesosphere (50 km to 85 km or 164,040 feet to 278,870 feet): In the mesosphere, temperature once again decreases with altitude. This layer is known for its extremely low temperatures and is where meteors burn up upon entry into the Earth's atmosphere.
Thermosphere (85 km and above or 278,870 feet and above): The thermosphere is the outermost layer of the atmosphere and is characterized by high temperatures. However, due to the low density of air molecules in this layer, the actual temperature is not felt as heat. Instead, it is the layer where the auroras occur and where spacecraft orbit the Earth.

Applications of the ISA

The ISA has several practical applications in aviation. Some of these include:

Aircraft Performance Calculations: The ISA provides the standard conditions against which aircraft performance is calculated. By using the temperature and pressure values from the ISA, pilots can determine the aircraft's maximum altitude, rate of climb, fuel consumption, and other performance parameters.
Flight Planning: When planning a flight, pilots need to take into account the expected atmospheric conditions along the route. By referring to the ISA, pilots can estimate the fuel requirements, calculate the expected time en route, and make informed decisions regarding the flight plan.
Aircraft Design: The ISA is also used in the design and certification of aircraft. Engineers use the standard atmospheric conditions to simulate various flight scenarios and test the aircraft's performance under different conditions. This ensures that the aircraft meets the required safety and performance standards.

It is important to note that the actual atmospheric conditions may deviate from the standard ISA due to factors such as weather patterns, local geography, and time of day. Therefore, pilots and aircraft manufacturers make adjustments to their calculations and designs based on the actual conditions before each flight.


The International Standard Atmosphere (ISA) is a valuable tool in aviation that provides a reference for the behavior of the Earth's atmosphere under standard conditions. It allows pilots to calculate aircraft performance, plan flights, and design aircraft. By understanding the layers of the ISA and its applications, aviation professionals can make informed decisions and ensure the safety and efficiency of their operations.

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