Air composition and moist air density
Air by volume (dry air gases, plus variable water vapour):
| Gas | Share |
|---|---|
| Nitrogen | ~78% |
| Oxygen | ~21% |
| Argon | 0.93% (most of the remaining ~1%) |
| Carbon dioxide | 0.04% |
| Other trace gases | trace |
| Water vapour | variable, zero to about 5% |
- Water vapour drives most day-to-day weather (PHAK ch.12).
- Most water vapour lies in the lowest layers (AWH 6.1).
- Moist air is less dense than dry air at same pressure and temperature (AWH 8.3.5).
Roles:
- Nitrogen: inert bulk. Dilutes oxygen, stops it burning too fast at the surface (FAA AWH 4.2).
- Oxygen: needed by body and engine.
With height: proportions unchanged, total pressure falls, so each breath holds less oxygen.
Trap: nitrogen ~78%, oxygen ~21%, not the other way round.
References
- FAA Aviation Weather Handbook (FAA-H-8083-28)
- FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25), ch.12