Composition of the atmosphere
The Earth's atmosphere is a gas mixture surrounding the aircraft and hosting all weather phenomena. The main components are stable over long periods.
Source: ICAO Annex 3 Meteorological Service for International Air Navigation; WMO standard.
Dry air — main constituents (by volume)
| Gas | Share | Note |
|---|---|---|
| Nitrogen (N₂) | ~78.084 % | Inert, the largest component — about 78 % of air is nitrogen |
| Oxygen (O₂) | ~20.946 % | Vital, combustible |
| Argon (Ar) | ~0.934 % | Noble gas, inert |
| Carbon dioxide (CO₂) | ~0.04 % (rising) | Greenhouse gas |
| Others (Ne, He, Kr, H₂, Xe …) | < 0.01 % | Traces |
The percentage of oxygen changes with increasing altitude within the troposphere as constant — the mixing ratio stays the same, only the total density falls. Consequence: at altitude there is less O₂ per volume → hypoxia risk (see Subject 040).
Water vapour — the variable component
Beyond "dry air", the real atmosphere contains water vapour in variable amounts (0 to ~4 % by volume). Water vapour is responsible for most weather phenomena in the atmosphere:
- Clouds, precipitation, fog — all depend on water-vapour content.
- The water vapour content in tropospheric air always depends on the current weather situation at a certain position.
How water enters the atmosphere
Water enters our atmosphere mainly by evaporation and plant transpiration:
- Evaporation from oceans, lakes, rivers (main source, ~90 %).
- Plant transpiration through leaf stomata.
- Sublimation from snow and ice (smaller share).
States of water
Water in the atmosphere can exist in gaseous, liquid or solid state:
- Gaseous: water vapour (invisible).
- Liquid: cloud droplets, raindrops.
- Solid: ice crystals, snowflakes, hail.
Phase transitions
| Transition | Name | Energy |
|---|---|---|
| Liquid → Gas | Evaporation | absorbs heat (cools) |
| Gas → Liquid | Condensation | releases heat |
| Solid → Liquid | Melting | absorbs heat |
| Liquid → Solid | Freezing | releases heat |
| Solid → Gas | Sublimation | absorbs heat |
| Gas → Solid | Sublimation (deposition) — water vapour to ice | releases heat |
Examples:
- Evaporation is the transition from liquid to gaseous.
- Condensation is the transition from gaseous to liquid.
- The transition from water vapor (gas) to ice (solid) is called sublimation.
- The state change from gaseous to liquid releases thermal energy — condensation heat is a main driver of thunderstorms and hurricanes.
Greenhouse Gas Bulletin*; ICAO Annex 3; NASA Atmosphere Composition; FAA-H-8083-25B PHAK Ch. 12.*