EASA/GCAA Part-66 · Module 8 — Basic Aerodynamics · 8.1 Physics of the Atmosphere

The International Standard Atmosphere (ISA)

Objective: climb through the ISA and watch temperature, pressure and density change. Sea level is 15 °C and 1013.25 hPa; temperature lapses ≈1.98 °C per 1000 ft to the tropopause (36 090 ft, −56.5 °C), then holds constant. Add a non-standard temperature to see how density altitude climbs on a hot day.

Climb through the atmosphere

ISA profiles — T, P, ρ vs altitude

Pressure altitude 0 ft

Temp deviation from ISA 0 °C

Temperature

15.0°C

Pressure

1013hPa

Density

1.225kg/m³

Density altitude

0ft

T = 15 − 1.98·(h/1000) °C · ρ = P/(R·T) · DA ≈ PA + 118.8·ΔT_ISA

The ISA is the agreed reference every altimeter and performance chart is built on. In the troposphere (up to 36 090 ft) temperature falls steadily and pressure and density fall roughly exponentially. Above the tropopause the lower stratosphere is isothermal at −56.5 °C, while pressure keeps dropping. Density is what aerodynamics and engines actually feel — and it depends on temperature as well as pressure. Add a positive temperature deviation (a hot day) and the air thins: the density altitude rises above the pressure altitude, and the aircraft performs as if it were higher up.

Try this: Climb to 36 090 ft and watch the temperature stop falling at −56.5 °C.
Try this: At a fixed altitude, add +30 °C — density falls and density altitude jumps by roughly 3500 ft.
Try this: Note pressure roughly halves by ~18 000 ft.
Try this: Set ΔT = 0 and confirm sea level reads 15 °C, 1013 hPa, 1.225 kg/m³.