Air Masses, Fronts and Frontal Weather
Air masses
An air mass takes on the character of its source region: cP (continental polar - cold dry), mP (maritime polar - cold moist), mT (maritime tropical - warm moist), cT (continental tropical - hot dry). Classification also uses stability (stable/unstable).
Fronts
A cold front has a steep slope (~1:50), fast movement, narrow band of heavy showers, squalls, and a sharp wind shift; cumulonimbus is common. A warm front has a shallow slope (~1:150), slow movement, wide band of stratiform cloud and steady rain; altostratus/nimbostratus dominate. An occluded front forms when a cold front catches a warm front; complex weather. A stationary front shows little movement.
Frontal passage indicators
- Cold front: pressure rises, temperature falls, wind veers sharply, visibility improves after passage.
- Warm front: pressure falls then steadies, temperature rises, wind backs, visibility may drop.
Worked example
A cold front approaching: expect pre-frontal cumulonimbus, heavy showers, gusty winds, and a rapid wind veer at passage; post-frontal, cold unstable air gives scattered showers and good visibility.
Traps
Cold front = narrow, heavy, short; warm front = wide, steady, long. Students often swap the slope ratios. Occluded fronts are not "no weather" - they can be active.