Temperature inversions
Normally air gets colder as you climb. A temperature inversion is a layer where it does the opposite: temperature rises with height, so warmer air sits on top of colder air.
Four ways they form:
| Type | How it forms |
|---|---|
| Radiation (nocturnal) | ground cools on a clear, calm night, chilling the air just above it |
| Subsidence | sinking air warms under a high, forming an inversion aloft |
| Frontal | warm air overrides colder air at a front |
| Valley | cold air is trapped in a valley on a clear, calm night, warmer air above |
An inversion is the most stable layer there is, so it:
- Traps moisture, smoke and haze below it → poor visibility, fog.
- Gives smooth air.
- Often has marked wind shear at its top.
Trap: an inversion means warmer air on top of colder air. It kills mixing, so the murk stays trapped underneath, which is why visibility below an inversion is poor.
Source note: the inversion 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)