The ISA Standard Atmosphere

Temperature falls steadily, then stops. In the troposphere it drops about 1.98°C per 1000 ft, close enough to 2°C for mental arithmetic. At the tropopause, 36 090 ft in the standard atmosphere, the fall stops and the stratosphere holds a constant −56.5°C. Watch the temperature trace go vertical at that height.

Pressure and density fall faster than temperature, and they never stop falling. Both drop roughly exponentially, so by 18 000 ft the pressure is already about half its sea-level value while the temperature has only dropped a third of the way to the tropopause. That is why altitude hurts engine power long before it feels cold.

Density is what the aeroplane actually feels, and it depends on both. ρ = P/RT, so a hot day at a given pressure altitude gives thinner air, less lift, less thrust and a longer take-off run. The density altitude readout is the height at which the standard atmosphere would give the density you actually have.

The speed of sound follows temperature alone. a = √(γRT), so it falls with temperature through the troposphere and then holds constant in the stratosphere, which is why an aircraft cruising at a fixed Mach number flies a fixed true airspeed once it is above the tropopause.

• Try this: Press Climb and watch which of the three traces bends first. Where does temperature stop falling, and why?
• Try this: Hold at 18 000 ft. What fraction of sea-level pressure is left, and is the temperature down by the same fraction?
• Try this: Set the deviation to +30°C on a hot day. What happens to the density and to the density altitude?