Air masses
An air mass is a large body of air that has taken the temperature and moisture of the region it sat over (its source region).
The name has two letters, moisture first, temperature second:
- Moisture: c = continental (dry, land), m = maritime (moist, ocean).
- Temperature: A = Arctic (very cold), P = Polar (cold), T = Tropical (warm).
| Type | Character (temperature, moisture) |
|---|---|
| cA | very cold, dry |
| cP | cold, dry |
| mP | cool, moist |
| mT | warm, moist (humid) |
| cT | hot, dry |
Stability is not fixed by the name: an air mass turns stable or unstable through modification as it moves over a new surface (FAA AWH):
- Cold air over a warmer surface → heated from below → lapse rate steepens → unstable (convection, showers, good visibility).
- Warm, moist air over a colder surface → cooled from below → stable (stratiform cloud, fog, drizzle, poor visibility).
Source note: the air-mass classification (cA, cP, mP, mT, cT) and the modification rule are 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).
Trap: the moisture letter is lower-case and comes first (moist vs dry); the temperature letter is upper-case and comes second. And do not read stability off the name: warm, moist mT air flowing over a colder surface is cooled from below and turns stable (fog, low stratus), while cold cP/cA air moving over a warmer surface is heated from below and turns unstable.
References
- FAA Aviation Weather Handbook (FAA-H-8083-28)