International Standard Atmosphere (ISA): Lapse Rate, Temperature and Pressure
The International Standard Atmosphere (ISA) is a hypothetical atmospheric model used as a reference for air navigation and aircraft design.
Standard Values
- Mean Sea Level (MSL):
- Temperature: +15°C
- Pressure: 1013.25 hPa
- Density: 1.225 kg/m³
- Troposphere:
- Lapse Rate: Temperature decreases at a rate of 2°C per 1000 ft (or 0.65°C per 100 m).
- Tropopause:
- Altitude: 36,090 ft (11 km).
- Temperature: -56.5°C.
- Lower Stratosphere:
- From the tropopause up to approx. 20 km (65,000 ft), the temperature remains constant (Isothermal) at -56.5°C.
ISA Temperature Calculations
To calculate the standard temperature at a given altitude (in the troposphere):
$T_{ISA} = 15 - (2 \times \frac{Altitude}{1000})$
Examples:
- At 18,000 ft: $15 - (2 \times 18) = 15 - 36 = \mathbf{-21^\circ C}$.
- At 20,000 ft: $15 - (2 \times 20) = 15 - 40 = \mathbf{-25^\circ C}$.
- At 30,000 ft: $15 - (2 \times 30) = 15 - 60 = \mathbf{-45^\circ C}$.
If the actual temperature is different from ISA, it is expressed as a deviation (e.g., ISA +10, ISA -5).
- If at 18,000 ft the actual temperature is -35°C (ISA being -21°C), the deviation is ISA -14°C.
- Note: If the temperature is colder than ISA, the air density will be higher than standard.
Pressure Levels and Flight Levels
Relationship between Flight Levels (FL) and pressure in hPa:
| Flight Level (FL) | Pressure (hPa) |
|---|---|
| FL050 | 850 |
| FL100 | 700 |
| FL140 | 600 |
| FL180 | 500 |
| FL240 | 400 |
| FL300 | 300 |
| FL340 | 250 |
| FL390 | 200 |
| FL450 | 150 |
Lapse Rates
- ELR (Environmental Lapse Rate): It is the actual temperature change with height at a given time and place. The ISA average is 2°C/1000 ft.
- DALR (Dry Adiabatic Lapse Rate): Cooling rate of an unsaturated air parcel that rises. Constant value: 3°C/1000 ft.
- SALR (Saturated Adiabatic Lapse Rate): Cooling rate of a saturated air parcel (100% humidity). Variable value (approx. 1.8°C/1000 ft). It is lower than the DALR because the condensation of water vapor releases latent heat, which warms the parcel and reduces its cooling rate.