Air mass codes & modification
Air mass: large body of air with the temperature and moisture of its source region. Water warms and cools more slowly than land, so maritime sources have smaller diurnal and seasonal temperature variation than continental (FAA AWH).
Name: two letters, moisture first, temperature second.
| Position | Letter | Meaning |
|---|---|---|
| 1: moisture | c | continental: dry, land |
| 1: moisture | m | maritime: moist, ocean |
| 2: temperature | A | Arctic: very cold |
| 2: temperature | P | Polar: cold |
| 2: temperature | T | Tropical: warm |
| Type | Temperature | Moisture |
|---|---|---|
| cA | very cold | dry |
| cP | cold | dry |
| mP | cool | moist |
| mT | warm | moist (humid) |
| cT | hot | dry |
Source notes (FAA AWH):
- Tropical air is common in summer over the Gulf Coast states and the eastern third of North America.
- cT: dry air pumped north off the Mexican Plateau. Can cause drought if stagnant over the Midwest.
- mT: tropical air moved north over cooler water.
- cA: forms mostly in winter over Arctic ice and snow (AWH 11.2.1.1).
Only these five are classified. mA (maritime Arctic) seldom, if ever, forms (FAA AWH).
Modification: air mass turns stable or unstable over a new surface. Stability not fixed by air-mass code (FAA AWH).
| Air | Surface | Effect | Result | Weather | Cite |
|---|---|---|---|---|---|
| Cold | Warmer | Heated from below, lapse rate steepens | Unstable | Convection, cumuliform cloud, showers, good visibility | FAA AWH |
| Warm, moist | Colder | Cooled from below | Stable | Stratiform cloud, fog, drizzle, poor visibility | TC sample exam TP 13014 Q62 |
| Polar air over | Effect | Result |
|---|---|---|
| Ocean | Picks up warmth and moisture | Modifies toward mP |
| Land | Picks up warmth, stays dry | Remains cP |
Lake effect (FAA AWH 11.2.2.1):
- Cold, dry, stable polar air crosses relatively warm, ice-free lake, heated and moistened from below.
- Bands of cumuliform cloud and showers over and downwind of lake.
Trap: moisture letter lower-case, first (moist vs dry); temperature letter upper-case, second. Never read stability off the name: warm, moist mT over colder surface turns stable (fog, low stratus); cold cP/cA over warmer surface turns unstable.
References
- FAA Aviation Weather Handbook (FAA-H-8083-28)
- TC sample exam TP 13014 Q62