The International Standard Atmosphere, or ISA, is a reference model that describes how air pressure, temperature, and density change with altitude. Pilots, engineers, and meteorologists use it as a common starting point because real weather changes from day to day. In aviation, ISA helps predict aircraft performance, altimeter readings, engine power, and lift.
The basic idea is that as an aircraft climbs, the surrounding air becomes colder, thinner, and lower in pressure until it reaches the tropopause.
Key Facts
- ISA sea level standard pressure is 1013.25 hPa = 29.92 inHg = 101325 Pa.
- ISA sea level standard temperature is 15°C = 288.15 K.
- Troposphere lapse rate in ISA is about 6.5°C per 1000 m or about 2°C per 1000 ft.
- Temperature below the tropopause can be estimated by T = 15°C - 6.5°C/km times altitude in km.
- Pressure decreases with altitude because there is less air above pushing down.
- Air density decreases with altitude, and lower density reduces lift, thrust, and engine power.
Vocabulary
- International Standard Atmosphere
- A reference model of average atmospheric pressure, temperature, and density at different altitudes.
- Lapse Rate
- The rate at which air temperature decreases with increasing altitude in a layer of the atmosphere.
- Tropopause
- The boundary near the top of the troposphere where temperature stops decreasing rapidly with altitude.
- Air Density
- The mass of air in a given volume, which affects lift, drag, and engine performance.
- Pressure Altitude
- The altitude shown when an aircraft altimeter is set to the standard pressure of 29.92 inHg or 1013.25 hPa.
Common Mistakes to Avoid
- Assuming temperature always decreases at the same rate forever is wrong because the ISA lapse rate changes at the tropopause, where temperature becomes nearly constant for a layer.
- Treating pressure and density as the same quantity is wrong because pressure is force per area while density is mass per volume, although both usually decrease with altitude.
- Forgetting to convert feet to meters is wrong because ISA formulas often use meters or kilometers, and mixing units gives incorrect temperature or pressure estimates.
- Assuming aircraft perform better at high altitude just because there is less drag is wrong because thinner air also reduces lift, propeller thrust, and engine power.
Practice Questions
- 1 Using the ISA lapse rate of 2°C per 1000 ft, estimate the standard temperature at 10,000 ft if sea level temperature is 15°C.
- 2 An aircraft climbs from sea level to 30,000 ft. Using 2°C per 1000 ft, estimate the temperature drop and the standard temperature at 30,000 ft.
- 3 Explain why a runway at high elevation can require a longer takeoff distance than a runway at sea level, even when the airplane and weather appear normal.