Winds and Air Masses

Wind is air moving from high to low pressure, deflected by Earth's rotation (Coriolis effect). Planetary winds shape global climate; local winds shape regional weather. NDA tests both types plus cyclones.

Global Pressure Belts

Earth has alternating pressure belts caused by uneven heating and the Coriolis effect:

Belt	Latitude	Pressure
Equatorial Low (Doldrums)	0°–5°	Low — rising hot air
Subtropical High (Horse Latitudes)	30° N & S	High — descending dry air; world's hot deserts here
Subpolar Low	60° N & S	Low — temperate cyclones form here
Polar High	90° N & S	High — cold descending air

Planetary (Permanent) Winds

• Trade Winds: Subtropical High (30°) → Equatorial Low (0°). NE in N hemisphere, SE in S hemisphere. Steady, reliable — used by ships in sailing era.
• Westerlies: Subtropical High (30°) → Subpolar Low (60°). SW in N hemisphere, NW in S hemisphere. Strongest in Southern Ocean ("Roaring Forties", "Furious Fifties", "Shrieking Sixties").
• Polar Easterlies: Polar High (90°) → Subpolar Low (60°). Cold, dry, weaker than the other two.

Local Winds

Wind	Region	Type
Loo	Northern Plains of India (May–June)	Hot, dry summer wind
Chinook	Rocky Mountains, USA/Canada	Warm, dry, descending — "snow eater"
Foehn	Northern Alps	Warm, dry, descending (same physics as Chinook)
Mistral	S France, Rhone valley	Cold, dry, strong northerly
Sirocco	Sahara → Mediterranean	Hot, dusty, southerly
Bora	Adriatic coast	Cold, dry, strong NE
Harmattan	West Africa	Hot, dry, dusty NE trade wind
Khamsin	Egypt	Hot, dusty southerly

Cyclones — Tropical and Temperate

Tropical cyclones form over warm tropical oceans (≥ 27 °C). Different names by region: hurricanes (Atlantic, NE Pacific), typhoons (NW Pacific), cyclones (Indian Ocean, S Pacific). India is hit by Bay of Bengal cyclones (more frequent) and Arabian Sea cyclones (less frequent but increasing).

Temperate (extratropical) cyclones form in mid-latitudes along the polar front where cold polar air meets warm tropical air. Different mechanism, larger but less intense than tropical cyclones.

Jet streams are fast, narrow air currents in the upper troposphere (9–16 km), flowing west to east at 100–300 km/h. They steer weather systems and affect aircraft routes.

NDA PYQ Examples

Frequently Asked Questions

What is the Coriolis effect?

An apparent deflection of moving objects (winds, currents) due to Earth's rotation — to the right in the Northern Hemisphere and to the left in the Southern. It is zero at the equator and maximum at the poles.

Why do trade winds blow towards the equator?

Air rises at the warm equatorial low and descends at the subtropical high (30°). Surface air then flows from 30° back to 0°. The Coriolis effect deflects this flow into easterly trade winds.

What is the difference between a hurricane, typhoon and cyclone?

Same phenomenon, different names by region. Hurricane = Atlantic and NE Pacific. Typhoon = NW Pacific. Cyclone = Indian Ocean and S Pacific. All are tropical cyclones.

Why are there hot deserts at 30° latitude?

The subtropical high pressure belt at ~30° has descending dry air → very little rainfall. World's major hot deserts (Sahara, Arabian, Australian, Kalahari, Atacama) lie around this latitude.

What is a jet stream?

A fast (100–300 km/h) narrow air current in the upper troposphere flowing west to east. Important for: aircraft fuel savings (with the stream), and weather steering (jet streams guide cyclones and fronts).

Related Topics

NDA GeographyAtmospheric Structure NDA GeographyClouds and Precipitation NDA GeographyWorld Climatic Types NDA GeographyIndian Climate and Monsoon NDAGeography — Full Syllabus