The Atmosphere
The Atmosphere is the foundation domain: the exam tests the International Standard Atmosphere (ISA), the vertical structure of the atmosphere, and how temperature, pressure and density change with height - material you reuse in altimetry, stability and aircraft performance questions.
ISA and its numbers
- Sea level: 15 °C, 1013.25 hPa, density 1.225 kg/m³.
- Troposphere: temperature falls at 1.98 °C per 1000 ft (6.5 °C/km).
- Above 36,090 ft (11 km): isothermal at −56.5 °C up to 65,600 ft (20 km).
Structure
- Troposphere: about 80% of the mass and virtually all water vapour and weather; its lapse rate is the ELR.
- Tropopause: the boundary; its height depends on latitude and season - roughly 55,000 ft over the equator, 28,000 ft at the poles, higher in summer. It is discontinuous at the polar front, which is why tropopause breaks appear near jet streams.
- Stratosphere: contains the ozone layer, temperature rises with height, very stable - hence smooth cruising.
- Mesosphere, thermosphere and ionosphere lie above; they matter for radio propagation and very high-altitude operations, not for day-to-day ATPL weather.
Worked example
At FL350 in ISA, temperature = 15 − (1.98 × 35) = 15 − 69.3 = −54.3 °C. If the aircraft is above the ISA tropopause at 36,090 ft, the answer would instead be the constant −56.5 °C; the trap is applying the lapse rate above the tropopause.
Traps that cost marks
- Continuing the lapse rate above 36,090 ft.
- Treating tropopause height as a fixed flight level.
- Placing the ozone layer in the troposphere.
- Forgetting that the tropopause is higher over the equator and in summer, and that it is lower over cold air.