What is an air mass? Describe its chief characteristics.

Definition and source

• Bergeron’s idea of an air mass is air that sits long enough over a uniform surface—ocean, snow, desert, or rainforest—to take that surface’s heat and moisture signature.
• Source regions are therefore extensive and relatively flat: subtropical oceans, continental interiors, ice caps, and hot deserts, not a broken hill district.
• The mass then moves as a unit in the general circulation, carrying that signature over new land or sea.

Chief physical characteristics

• Horizontal homogeneity of temperature and specific humidity is the defining trait; vertical lapse rates decide whether the mass is stable or unstable.
• Continental polar (cP) air is cold and dry; maritime tropical (mT) is warm and moist; continental tropical (cT) is hot and dry; maritime polar (mP) is cool and moist; Arctic and equatorial types extend the same logic.
• Stability, visibility, and cloud type follow: a stable cP mass may mean clear winter days in north India after a western disturbance exits; mT monsoon air means deep moisture and convective cloud.
• Pressure and wind inside the mass are secondary; the sharp weather change sits at the front where two masses meet.

Modification and Indian relevance

• As an air mass leaves its source it is modified by the new surface: cP air crossing the Mediterranean and Iran toward India in winter warms aloft in parts and still arrives as a cold wave on the Gangetic plain.
• Monsoon mT air from the Arabian Sea and Bay of Bengal is the summer opposite: high humidity, conditional instability, and orographic lift on Western Ghats and Himalaya.
• A fair description therefore names size, source, homogeneity, classification letters, stability, and the fact that characteristics change in transit.