Vertical structure of the atmosphere
The Earth's atmosphere is divided into several layers, based on the vertical temperature profile.
Source: ICAO Doc 7488 Manual of the ICAO Standard Atmosphere (3rd ed.); WMO; NASA.
Layers — from the surface upwards
| Layer | Altitude | Temperature trend | Significance |
|---|---|---|---|
| Troposphere | 0 — ~8–18 km | Temperature falls with altitude (~0.65 °C/100 m) | Weather happens here |
| Stratosphere | up to ~50 km | Temperature rises with altitude (ozone-layer UV absorption) | Stable; airliners fly just below or in the lower strat. |
| Mesosphere | up to ~85 km | Temperature falls again | Meteor entry layer |
| Thermosphere | up to ~600 km | Temperature rises steeply (solar absorption) | Contains the ionosphere — important for radio propagation |
| (Exosphere) | > 600 km | Transition to space | — |
In ascending order from the surface: troposphere, stratosphere, mesosphere, thermosphere (often equated in aviation contexts with the ionosphere).
Troposphere — the "weather layer"
Most weather phenomena occur in the troposphere. Properties:
- Temperature falls with altitude (lapse rate ~0.65 °C/100 m).
- Holds 80 % of atmospheric mass and 99 % of water vapour.
- Vertical convection and mixing possible → clouds, precipitation, wind.
- Height varies: pole ~7-9 km, equator ~16-18 km.
Tropopause — the boundary
The tropopause is the boundary between troposphere and stratosphere, marked by constant temperature (isothermal) or slight temperature rise (inversion).
Inversion or isothermal layers can always be found at the tropopause — it is a barrier to vertical motion.
The height of the tropopause changes with seasons and the geographic latitude:
- Summer and low latitudes (equator): higher (up to 18 km).
- Winter and high latitudes (pole): lower (down to 7-9 km).
- ISA value: 11 000 m (= ~36 090 ft).
Stratosphere
- Temperature rises with altitude (from ~-56.5 °C at tropopause to ~0 °C at 50 km).
- Cause: ozone layer (~15-35 km) absorbs UV and warms the air.
- Very stable layer — barely any vertical motion → no weather as in troposphere.
- Airliners fly just below (FL340-FL400) or in the lower stratosphere (Concorde flew at 18-20 km).
Mesosphere
- Temperature falls again (to -90 °C at 85 km).
- Most meteorites burn up here.
- Above civilian flight.
Thermosphere and ionosphere
- Temperature rises steeply (to +2000 °C at 600 km) by direct solar radiation.
- Ionosphere (part of the thermosphere, 60-1000 km): layers ionised by solar radiation.
- Important for radio navigation: the ionosphere reflects HF and MF waves (sky wave) → enables long-range radio (see Subject 060 radio-wave propagation).