Atmospheric Circulation and Weather Systems — Practice

Question 1

Why are the subtropical high-pressure belts (~30°N and S) associated with deserts?

Accepted Answer

The subtropical high-pressure belts form because air that rose at the equator (Equatorial Low/ITCZ) has lost its moisture through heavy rainfall and cooled. This dry air DESCENDS at ~30°N and S, warming as it compresses. Descending, warming air increases its capacity to hold moisture → relative humidity falls → no rain forms. This persistent descent of dry air creates the world's great desert belts at ~30° latitude: Sahara and Thar in the Northern Hemisphere; Kalahari and Australian desert in the Southern Hemisphere.

Question 2

Explain why tropical cyclones cannot form at the equator, and why they are called by different names in different oceans.

Accepted Answer

Tropical cyclones require the Coriolis effect to initiate and maintain their rotation (anticlockwise in NH, clockwise in SH). The Coriolis effect is ZERO at the equator — there is no force to impart rotation to converging air. Without rotation, air simply rises without forming the organised spiral structure of a cyclone. Cyclones therefore form at minimum 5°–8° from the equator (where Coriolis is small but non-zero), typically between 5°–20° latitude. Different names for the same phenomenon: HURRICANE in the Atlantic and NE Pacific, TYPHOON in the NW Pacific, CYCLONE in the Indian Ocean and SW Pacific. All are tropical cyclones — the different names reflect regional meteorological traditions, not different physical phenomena.

Question 3

Describe the global pattern of pressure belts and planetary wind systems, explaining how their seasonal shift drives the Indian Monsoon.

Accepted Answer

The Earth's surface is unevenly heated — the equator receives most solar radiation, creating hot rising air and a persistent low-pressure zone: the ITCZ (Inter-Tropical Convergence Zone). Near 30°N and S, the air that rose at the equator descends, creating subtropical high-pressure belts. Near 60°N and S, relatively warmer sub-polar air rises, creating subpolar low-pressure zones. At the poles, cold dense air sinks, creating polar high-pressure. From these pressure belts, three planetary wind systems operate: (1) Trade winds (30° → equator, NE in NH / SE in SH), (2) Westerlies (30° → 60°, SW in NH), (3) Polar easterlies (90° → 60°, NE in NH). Crucially, these belts SHIFT northward in Northern summer (July) following the overhead sun's apparent movement. The ITCZ, normally near the equator, shifts northward and lies over the Gangetic plains in June-July. This northward shift, combined with intense low pressure over NW India from differential heating of land vs sea, draws moist maritime air from the Indian Ocean northward — the Southwest Monsoon. The Arabian Sea branch hits the Western Ghats and rises (orographic rain); the Bay of Bengal branch reaches NE India and the Ganga plains. In winter, the belts shift south, the ITCZ returns to the equator, and India's winds reverse to dry Northeast Monsoon. The seasonal shift of planetary pressure belts is thus the atmospheric mechanism for India's monsoon.

Practice — Atmospheric Circulation and Weather Systems

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