Frontal weather (cold, warm, occluded)
A front is where two air masses meet, one warm, one cold. What sets the weather is the slope of that boundary. A steep slope shoves air up fast over a short distance (narrow, rough, showery); a shallow slope lifts it gently over a long one (wide, steady).
| Cold front | Warm front | |
|---|---|---|
| Slope / speed | steep, fast | shallow, slow |
| Width of weather | narrow band | wide band |
| Cloud | cumuliform (Cu, Cb) | lowering Ci, Cs, As, Ns |
| Precipitation | brief heavy showers, thunderstorms | steady, continuous, well ahead |
| Visibility | good, except in showers | poor in cloud and precipitation |
| Temperature at passage | sharp drop | rise |
| After passage | clears quickly | low cloud lingers |
- The wind veers (shifts clockwise) as either kind of front goes past (FAA AWH), so the wind change on its own will not tell them apart; use the cloud, the rain, and the temperature instead.
- The warm-front giveaway is the cloud sequence: high cirrus first, thickening and lowering through cirrostratus (a halo), altostratus (a watery sun), then nimbostratus with steady rain, all hours before the front arrives.
- Occluded front: the faster cold front catches up to the warm front and lifts the warm air off the ground, mixing both types of weather around the low.
- Trowal (TROugh of Warm air ALoft) is the Canadian name for that lifted warm air wrapped around a mature low (TC AIM MET).
Intuition: picture a side-on view. The cold front is a steep wedge shoving in under the warm air, so it throws the air up all at once (towering cumulus, showers) in a thin line. The warm front is a long ramp sliding up over the cold air, so the cloud thickens and lowers slowly and the rain is spread out and steady.
Source note: the cold/warm-front weather (slope, cloud, precipitation, wind veer) 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). The occluded front and the Canadian trowal stay cited to the TC AIM MET.
Trap: cold versus warm is the exam's core confusion. Anchor on the temperature after it passes (a cold front leaves it colder, a warm front leaves it warmer) and on the cloud (heaped for cold, a lowering layered sequence for warm).
References
- TC AIM MET
- FAA Aviation Weather Handbook (FAA-H-8083-28)