Atmospheric Pressure, Winds & Jet Streams
The global pressure belts, the planetary wind system, the Coriolis force, local winds, and the high-altitude jet streams that steer weather.
Key Takeaways
- There are four major pressure belts, alternating low and high, from the equator to the poles.
- The three planetary winds are the trade winds, the westerlies and the polar easterlies.
- The Coriolis force deflects winds to the right in the Northern Hemisphere and left in the Southern.
Core concept
Uneven heating of the Earth creates differences in atmospheric pressure, and air flows from high to low pressure as wind. On a rotating Earth, the Coriolis force bends these winds, producing the great planetary wind belts that drive ocean currents and weather.
Static foundation — the pressure belts
From the equator to the poles, four belts alternate: Equatorial Low (the 'doldrums'), Subtropical High (~30°, the 'horse latitudes'), Subpolar Low (~60°), and Polar High. These belts shift north and south with the apparent movement of the Sun.
The Planetary (Permanent) Winds
| Wind | Blows between | Direction |
|---|---|---|
| Trade winds | Subtropical high → Equatorial low | NE in the N hemisphere, SE in the S (easterlies) |
| Westerlies | Subtropical high → Subpolar low | SW in the N, NW in the S; the 'Roaring Forties' in the S |
| Polar easterlies | Polar high → Subpolar low | From the poles toward ~60° |
The Coriolis force (Ferrel's Law)
Because the Earth rotates, moving air is deflected to the RIGHT in the Northern Hemisphere and to the LEFT in the Southern Hemisphere (Ferrel's Law). It is zero at the equator and maximum at the poles — which is why cyclones do not form at the equator itself.
Other Winds & Jet Streams
Hot: the Loo (north India), Foehn (Alps), Chinook (Rockies, 'snow-eater'). Cold: the Mistral (France), Bora. They affect local weather dramatically.
Relevance & current linkage
The withdrawal of the subtropical westerly jet from north India is a trigger for the monsoon's burst; a shifting jet stream is also linked to extreme weather (heatwaves, western disturbances). (Add a recent monsoon-forecast or extreme-weather linkage.)
Prelims trap zones
- Trade winds blow FROM the subtropical high TOWARD the equatorial low (not the reverse).
- Coriolis force is zero at the equator, maximum at the poles.
- Chinook and Foehn are warm, dry winds; Mistral is cold.
Prelims Pointers
- Pressure belts: equatorial low, subtropical high, subpolar low and polar high.
- The subtropical high (~30°) is the 'horse latitudes'; the equatorial low is the 'doldrums'.
- The Coriolis force is zero at the equator and maximum at the poles.
- Jet streams are fast, narrow bands of upper-air westerly winds.
Mains Angle
- 'Jet streams play a decisive role in the Indian monsoon.' Discuss.
- Explain the global pattern of pressure belts and planetary winds.
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