Air Masses, Fronts and Frontal Depressions
Air Masses, Fronts and Frontal Depressions is the synoptic-scale domain: source regions and air-mass classification, frontal types and slopes, and the life cycle of the polar-front depression.
Air masses
Classified by source region and surface: cA, cP, cT, mP, mT. They modify as they travel. A cold air mass over a warm sea becomes unstable with showers; a warm air mass over a cold surface becomes stable with stratus, mist and drizzle.
Fronts
- Warm front: slope about 1:150; cloud sequence cirrus → cirrostratus → altostratus → nimbostratus; steady rain, poor visibility; wind veers after passage and temperature rises.
- Cold front: steeper, about 1:50 to 1:100; a narrow belt of cumulonimbus with squally gusts and heavy showers; rapid clearing, pressure rising, temperature falling, wind veering.
- Occluded: the warm sector is lifted off the ground; mixed cloud and rain.
- Stationary: little movement.
Frontal depression life cycle
Wave development on the polar front → mature open wave with warm and cold sectors → occlusion as the cold front catches the warm front → decay. The warm sector has the poorest visibility; the cold sector has the showers.
Worked example
A warm front with a 1:150 slope has cloud base at 1500 ft. Distance to the surface front: 1500 × 150 = 225,000 ft ≈ 37 NM. So the cloud arrives roughly 37 NM ahead of the front; at 25 kt of frontal speed that is several hours before the rain starts.
Traps
- Reversing the warm front cloud sequence.
- Believing cold fronts are gentle and slow.
- Forgetting that wind veers after both fronts in the Northern Hemisphere.
- Assuming an occluded front contains no warm air - it is warm air aloft.