UPSC Prelims 2024 Q14: Coriolis Force — Explained
With reference to "Coriolis force", which of the following statements is/are correct?
1. It increases with increase in wind velocity.
2. It is maximum at the poles and is absent at the equator.
Select the answer using the code given below:
- 1 only
- 2 only
- Both 1 and 2
- Neither 1 nor 2
Answer: (c) Both 1 and 2
Statement-wise explanation
- 1: correct — The Coriolis force per unit mass equals 2Ωv·sinφ. Here Ω is Earth's angular velocity, v is the wind speed and φ is the latitude. Because v is in the formula, faster winds are deflected more strongly. The NCERT Class XI text says the deflection is greater when wind velocity is high.
- 2: correct — The force varies with sin φ. That is 1 at the poles (90°) and 0 at the equator (0°). NCERT puts it directly: the Coriolis force is directly proportional to the angle of latitude, is maximum at the poles and is absent at the equator. This is why tropical cyclones almost never form within about 5° of the equator.
How to eliminate
This is a recall question, but the recall is small. Remember one formula: Coriolis force = 2Ωv·sinφ, which grows with both wind speed (v) and latitude (φ). Statement 1 follows from the v term and Statement 2 from the sin φ term. A useful anchor without the formula: cyclones do not form at the equator because there is no Coriolis force there to start rotation. That confirms Statement 2. Neither statement uses an extreme word wrongly. 'Maximum' and 'absent' are the textbook's own words, so do not treat them as traps here.
Know more
Coriolis force is an apparent force. It arises because we measure wind against a rotating Earth. It changes only the direction of moving air or water, not its speed.
Formula: Coriolis force per unit mass = 2Ωv·sinφ (Ω ≈ 7.29 × 10⁻⁵ rad/s)
| Factor | Effect on Coriolis force |
|---|---|
| Wind speed (v) ↑ | Increases |
| Latitude (φ) ↑ | Increases; maximum at 90° |
| Equator (φ = 0°) | Zero |
| Object at rest (v = 0) | Zero |
Direction of deflection: to the right in the Northern Hemisphere and to the left in the Southern Hemisphere (Ferrel's law).
Why it matters:
- Geostrophic wind: In the upper air, friction is negligible. There the pressure gradient force and the Coriolis force balance each other, so wind blows parallel to the isobars. Near the surface, friction slows the wind and the Coriolis effect weakens, so wind crosses the isobars at an angle. At the equator, wind blows perpendicular to the isobars.
- Cyclones: Air flows into a low and is deflected, which sets up rotation: anticlockwise in the north and clockwise in the south. Without Coriolis force there is no spin. So tropical cyclones rarely form between about 5°N and 5°S, and they never cross the equator.
- Ocean currents and gyres: The same deflection bends major currents. Gyres turn clockwise in the Northern Hemisphere and anticlockwise in the Southern Hemisphere.
- Monsoon: South-east trade winds cross the equator and turn into the south-west monsoon because the deflection changes direction across the equator.
Exam traps: Coriolis force does not change wind speed. It does not create wind; the pressure gradient does that. It is not caused by Earth's revolution around the Sun.
NCERT Concepts to Revise
- Coriolis force: Both statements test how this apparent force depends on wind speed and on latitude.
- Pressure gradient force: Pressure gradient force starts the wind moving; Coriolis force only deflects air that is already in motion.
- Geostrophic wind: Geostrophic wind is where Coriolis force balances the pressure gradient, which shows the force grows with wind speed.