The atmosphere and stability

Nearly all weather is one question: does a parcel of air that starts rising keep rising? The lapse rate answers it, and everything else follows.

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What it is made of

Four gases make up 99.998% of the atmosphere: nitrogen, oxygen, argon and carbon dioxide. Water vapour is not on that list, and that is worth noticing — the constituent responsible for almost every weather phenomenon you will ever brief is a trace component that varies wildly from place to place.

The five layers

The boundary between the troposphere and the stratosphere is the tropopause — and it is not just a naming detail. It is a lid: rising air that reaches it stops, which is why mature thunderstorms spread sideways into an anvil rather than continuing up.

The standard atmosphere

15 °C and 29.92 inHg at sea level, with temperature falling about 2 °C per 1,000 ft. It is an average across all latitudes, seasons and altitudes, which means it describes almost no real day.

That is the point of it. Performance charts, altimetry and airspeed are all defined against a fixed reference, and the gap between that reference and today is exactly what density altitude and pressure altitude measure.

Lapse rate, and the two exceptions

Temperature normally falls with height. In the troposphere it does not always: it can hold steady or even rise.

An isothermal layer is one where temperature stays constant with height. A temperature inversion is one where it increases — the atmosphere upside down.

Inversions are worth knowing by their consequences rather than their definition. Warm air sitting on cold air is enormously stable, so nothing mixes vertically. That traps whatever is underneath: haze, smoke and moisture accumulate under the lid and visibility goes down while the sky above stays clear. It is also a classic wind-shear setup, because a strong wind can run above the inversion while the surface stays calm — and you fly through the change in seconds.

Stable or unstable, and what each looks like

Stability is the answer to one question: a parcel of air is nudged upward — does it keep going, or sink back? If the surrounding air cools quickly with height, the rising parcel stays warmer than its surroundings and keeps rising. That is unstable. If the surroundings cool slowly, the parcel quickly becomes colder than its surroundings and sinks back. That is stable.

This table is the single most useful thing in this lesson, because it runs backwards too. A ceiling of flat grey stratus and five miles in haze is telling you the air is stable before you have read a single forecast — and that what is there now will likely still be there in three hours, because stable air does not clear itself out.

Where you meet it next

• Clouds — stability written in the sky
• Thunderstorms — instability taken to its conclusion
• Fog — stability plus moisture at the surface
• Air masses and fronts — where the lift comes from

Sources

Composition, the layers, the standard atmosphere and lapse rates are from the FAA Aviation Weather Handbook (FAA‑H‑8083‑28) and the Pilot's Handbook of Aeronautical Knowledge.