Air masses and fronts

The essentials in two minutes

1. An air mass keeps the memory of its origin (polar or tropical, maritime or continental); heated from below, it becomes unstable; cooled from below, it becomes stable.
2. Post-frontal polar maritime air is flying air: cold aloft, heated below, dry, crystal clear.
3. Warm front: a gentle ramp (1/100 to 1/300), layered clouds, light and lasting rain, announced 24 to 36 h ahead by cirrus, then by the halo (12 to 24 h).
4. Cold front: a steep wedge, cumulonimbus, gusts, violent showers, little warning; you must be on the ground an hour before the line, because the gust front runs ahead of it.
5. As it passes, the wind veers (S, SW, W, NW), the pressure falls then rises quickly, the temperature rises then drops by 3 to 8 °C.
6. An occlusion is the sign of an ageing frontal system: lasting rain, little post-frontal air.
7. Before the front, barrier effect south of the Alps and foehn to the north; after it, barrier effect to the north-west, and the southern French Alps sheltered but windy; the best flying slot is the dry post-frontal air, the day after the cold front.
8. For hiking: stratiform rain (6 to 24 h), postpone; convective rain (10 to 45 min per shower), work around the windows.