Air Masses & Fronts
Notes
An air mass is a large body of air with nearly uniform temperature and humidity in the horizontal. It takes on the characteristics of its source region (an extensive, uniform surface under a stable high-pressure area).
Classification
- By latitude: Arctic/Antarctic
(a), Polar (P), Tropical (T), Equatorial (E). - By surface: maritime (m), humid; continental
(c), dry. - Main types:
- cP (continental polar): cold and dry (Siberia, northern Canada); causes cold waves.
- mP (maritime polar): cool and moist (North Atlantic and Pacific).
- cT (continental tropical): hot and dry (Sahara, Thar, Arabia).
- mT (maritime tropical): warm and humid (Gulf of Mexico, Arabian Sea and Bay of Bengal in summer).
- mE (maritime equatorial): warm and very humid.
- A warm air mass moving over a cold surface is stable (fog, stratus); a cold air mass over a warm surface is unstable (showers, cumulus).
Fronts A front is the boundary zone between two air masses of different temperature and density. Frontogenesis is the formation of a front; frontolysis is its dissipation. Fronts are typical of mid-latitudes and are associated with temperate cyclones.
- Cold front: a cold air mass advances and wedges under warm air; a steep slope (about 1:50); cumulonimbus clouds, thunderstorms and heavy rain over a narrow belt; a sharp fall in temperature and a clear sky after.
- Warm front: warm air rises gently over retreating cold air; a gentle slope (about 1:100-1:200); a wide belt of stratus clouds and light, prolonged rain; temperature rises after passage.
- Stationary front: neither air mass advances.
- Occluded front: the faster cold front overtakes the warm front and lifts the warm air off the ground; the end of the cyclone's life.
- Polar front: the boundary between polar easterlies and subtropical westerlies (a zone of cyclone formation).
India: true fronts are rare. The ITCZ (monsoon trough) acts like a front in the monsoon. In winter India is influenced by cP air from Central Asia (dry, cold); in summer by cT air from the Thar and Arabia in May-June; and by mT air in the south-west monsoon.
Key Points
- Air mass: uniform horizontally; classified by latitude (A, P, T, E) and surface (m, c).
- cP: cold and dry; mT: warm and humid.
- Cold front: steep, heavy showers; warm front: gentle, steady rain.
- Occluded front: cold front overtakes warm front.
- Fronts are typical of temperate cyclones.
MCQs
Q1. A continental tropical (cT) air mass is:
(a) Cold and dry
(b) Hot and dry
(c) Warm and humid
(d) Cool and humid
Ans: (b).
Q2. Cumulonimbus clouds and heavy showers are typical of a:
(a) Warm front
(b) Cold front
(c) Stationary front
(d) None of these
Ans: (b).
Q3. An occluded front forms when:
(a) Two warm air masses meet
(b) The cold front overtakes the warm front
(c) A front becomes stationary
(d) A warm front overtakes a cold front
Ans: (b).