UPSC Prelims 2010 Q107: Geography – Atmosphere & Climatology
What causes wind to deflect toward left in the Southern hemisphere?
- (a) Temperature
- (b) Magnetic field
- (c) Rotation of the earth
- (d) Pressure
Answer: (c) Rotation of the earth
Explanation
Why wind deflects left in the Southern Hemisphere
This deflection is a classic effect of a specific planetary-scale force.
| Option | Verdict | Reason |
|---|---|---|
| (a) Temperature | Incorrect | Temperature differences drive wind formation but don't cause the directional deflection itself |
| (b) Magnetic field | Incorrect | Earth's magnetic field has no role in deflecting large-scale wind movement |
| (c) Rotation of the earth | Correct | The Coriolis effect, caused by Earth's rotation, deflects moving air to the left in the Southern Hemisphere (and to the right in the Northern Hemisphere) |
| (d) Pressure | Incorrect | Pressure gradients drive wind speed/direction initially, but the leftward deflection specifically is due to Earth's rotation (Coriolis effect), not pressure itself |
Coriolis effect basics:
| Point | Detail |
|---|---|
| Cause | Earth's rotation |
| Northern Hemisphere | Deflects right |
| Southern Hemisphere | Deflects left |
Answer: (c) Rotation of the earth.
Why each option is right or wrong
- (a) Incorrect. Temperature differences drive wind formation but don't cause this specific directional deflection.
- (b) Incorrect. Earth's magnetic field plays no role in large-scale wind deflection.
- (c) CORRECT. The Coriolis effect, caused by Earth's rotation, deflects wind left in the Southern Hemisphere.
- (d) Incorrect. Pressure gradients drive wind movement, but the leftward deflection itself is a rotation (Coriolis) effect.
Key point
Wind deflects left in the Southern Hemisphere due to the CORIOLIS EFFECT, caused by Earth's ROTATION — not temperature, magnetic field, or pressure directly.
Remember
Wind deflects left in the Southern Hemisphere due to the Coriolis effect, caused by Earth's rotation.
Common mistakes
- Confusing the initial driving force of wind (pressure/temperature) with the specific deflecting force (Earth's rotation).
- Not knowing the Coriolis effect's hemisphere-dependent direction (right in North, left in South).
- Attributing wind deflection incorrectly to magnetic field effects.
Exam tip
Coriolis effect = Earth's rotation. Right-deflection in North, left-deflection in South. A foundational, frequently tested geography fact.
Sources
- NCERT Class XI Geography — Fundamentals of Physical Geography — Atmospheric Circulation and Weather Systems
- Savindra Singh — Climatology (Prayag Pustak Bhawan)