The wind is the breath of the earth. — Anonymous
9.1 Pressure and Winds
Atmospheric pressure decreases with altitude. Horizontal pressure differences drive wind.
Pressure belts (idealised):
- Equatorial low (ITCZ) — rising air, convergence
- Subtropical highs (~30° N/S) — descending dry air
- Subpolar lows (~60° N/S)
- Polar highs
Coriolis force deflects winds: right in the Northern Hemisphere, left in the Southern Hemisphere.
9.2 Planetary Winds and Pressure Systems
| Wind system | Region | Character |
|---|---|---|
| Trade winds | 0°–30° | Steady, NE (NH) and SE (SH) |
| Westerlies | 30°–60° | Variable, SW (NH) |
| Polar easterlies | 60°–90° | Cold, dry |
Cyclones (low pressure): air converges and rises — tropical cyclones, temperate cyclones.
Anticyclones (high pressure): air descends — clear, stable weather.
9.3 Jet Streams and Local Winds
Jet streams — narrow bands of strong upper-atmosphere winds (e.g. subtropical jet over north India in winter). They steer weather systems.
Local winds:
- Land and sea breezes — coastal areas
- Mountain and valley breezes — Himalayas
- Loo — hot, dry summer wind in north India
- Mistral, Bora, Chinook, Foehn — regional examples worldwide
9.4 Tropical and Extra-Tropical Cyclones
Tropical cyclones form over warm oceans (>26°C). Indian names (e.g. Amphan, Fani) are assigned by the IMD. They bring storm surges, flooding, and wind damage — especially on the east coast (Odisha, Andhra Pradesh, Tamil Nadu).
Temperate cyclones (depressions) form along polar front; bring winter rain to northwest India.
Chapter Summary
- Pressure differences drive global wind systems and circulation cells
- Trade winds, westerlies, and polar easterlies form the planetary pattern
- Cyclones (low) and anticyclones (high) shape weather
- Jet streams influence monsoon and storm tracks
- India affected by tropical cyclones, western disturbances, and local winds
Exercises (NCERT)
- Exercise 1 — pressure belts and planetary wind systems
- Exercise 2 — Coriolis effect and wind deflection
- Exercise 3 — tropical vs temperate cyclones; Indian examples