Atmospheric stability
Stability is what a lump of air does when shoved upward: keep going up (unstable) or settle back down (stable). That one habit sets the cloud, the ride, the precipitation, and the visibility.
| Unstable air | Stable air | |
|---|---|---|
| Cloud | cumuliform (heaped) | stratiform (layered) |
| Ride | turbulent, bumpy | smooth |
| Precipitation | showery, intermittent | steady, continuous, drizzle |
| Visibility | good | poor |
| Surface wind | gusty | steady |
| Also | thermals | fog, low stratus |
- Unstable means a lifted lump of air stays warmer, and so lighter, than the air around it, so it keeps rising. It turns the air over, piles cloud upward, and that up-and-down mixing carries haze and smoke away, so visibility is good (FAA AWH).
- Stable means a lifted lump of air turns cooler than the air around it, so it settles back down. The air will not rise, cloud can only spread sideways into layers, and haze and smoke stay trapped near the surface, so visibility is poor.
Intuition: unstable air moves up and down (bumps, heaped cloud, showers, clear air); stable air moves sideways (smooth ride, layered cloud, steady drizzle, trapped murk). A temperature inversion is stability at its most extreme and a classic haze and fog trap.
Trap: the visibility is the one students get backwards. Rising and mixing (unstable) clears the air; trapping it down (stable) fouls it.
Source note: the weather theory here is grounded in the FAA Aviation Weather Handbook (FAA-H-8083-28), a free public-domain text; the physics is universal and matches the Canadian material (equivalent to TP 9352). Canada-specific products/rules stay cited to the TC AIM.
References
- FAA Aviation Weather Handbook (FAA-H-8083-28)