The atmosphere

The standard atmosphereCopy Anchor Link

The International Standard Atmosphere is a table of layers, each with a constant lapse rate: the temperature falls 6.5 K a kilometre to 11 km, holds to 20 km, then rises, and so on to 86 km. Above that it is held isothermal, to a ceiling of 120 km.

Within a layer the pressure follows from the hydrostatic equation. A layer whose temperature changes gives a power law; one that holds gives an exponential. standard_atmosphere walks the layers up to the altitude, so the pressure at the top of one is the bottom of the next.

The dayCopy Anchor Link

A real day is warmer or colder than the standard one. temperatureOffsetK warms the air at the standard pressure, so a hot day at the same altitude has the same pressure but thinner air, and a faster speed of sound.

The waterCopy Anchor Link

The humidity is relative, as a weather report gives it: a share of what the air could hold at its temperature. What it could hold is the saturation pressure, from the Magnus fit of Alduchov and Eskridge, over liquid water even below freezing: the droplets a sudden expansion makes are supercooled, and freeze far too slowly to matter in the tenth of a second they live.

From the vapour pressure follow:

• the density, the vapour being lighter than the dry air it replaces, so humid air is a little thinner;
• the mixing ratio, kilograms of vapour a kilogram of dry air;
• the dew point, the Magnus fit inverted: where the air would condense if cooled at its pressure.

The vapor reads the dew point to know how far an expansion has to cool the air before it fogs.

The inletCopy Anchor Link

ram_pressure is the stagnation pressure over the static one that an inlet recovers. Below Mach 1 it is the isentropic ratio. Above, a supersonic inlet's shocks lose some, as MIL-E-5007 schedules it: 1 − 0.075 (M − 1)^1.35 of it is kept.