Air Masses, Fronts and Frontal Weather
This domain tests source regions and air-mass classification, the structure and slope of the different fronts, and the change of wind, temperature, pressure, cloud and visibility across each one. Questions are frequently 'what happens after passage'.
Air masses
- Source regions: continental polar (cold dry, stable), maritime polar (cool moist, unstable over land), maritime tropical (warm moist, haze), continental tropical (hot dry), arctic/antarctic (very cold, very stable).
- Modification along track: warming from below destabilises and gives showers; cooling from below stabilises and gives stratus, mist and fog.
- Winter continental polar air gives frost, fog and excellent visibility above the haze; maritime tropical air gives drizzle and poor visibility.
Front structures
- A front is the boundary between two air masses. Cold air is the denser, wedge-shaped mass.
- Slope: a cold front is steep, about 1:100 (often 1:50 to 1:100); a warm front is shallow, about 1:150 to 1:200. This single fact explains the narrow band of cold-front weather and the wide band of warm-front weather.
- Types: cold, warm, cold occlusion (cold air behind undercuts the cool air), warm occlusion, stationary.
Cold front passage
- Cloud: narrow belt of cumulus and cumulonimbus, sometimes a squall line ahead.
- Weather: heavy showers, hail, thunder, gusty surface wind.
- Wind veers, temperature falls sharply, pressure falls then rises steadily, visibility improves markedly behind the front.
Warm front passage
- Cloud sequence ahead: cirrus, cirrostratus, altostratus, nimbostratus.
- Weather: continuous rain or drizzle, low cloud base, poor visibility, possible icing in the frontal zone.
- Wind veers, temperature rises, pressure falls then levels, visibility remains moderate to poor in the warm sector.
Occluded fronts
- Mix the two: extensive cloud, continuous precipitation, often a complex wind and temperature sequence; the triple point may spawn a secondary low.
Worked example
Cirrus with a base of 25,000 ft appears overhead and the warm front slope is 1:150. The horizontal distance to the surface front is 25,000 × 150 ft = 3,750,000 ft ≈ 617 nm. At 300 kt ground speed the front arrives in about two hours - so the cirrus sighting is a long-range forecast, not a warning of imminent rain.
Traps that cost marks
- Expecting showers from a warm front, or steady rain from a cold front.
- Forgetting the wind veers on both fronts in the northern hemisphere.
- Reversing the front slopes, or assuming the surface front has already arrived when high cloud appears.