The Coriolis effect is the result of
- (a)Pressure gradient
- (b)Earth’s axis of inclination
- (c)Earth’s rotation
- (d)Earth’s revolution
Correct — C, the Earth's rotation. The Coriolis effect is the apparent deflection of anything moving freely over the Earth's surface, and it arises because we observe that motion from a platform that is itself turning. A parcel of air setting off from one latitude carries the eastward speed of the ground it left, but the ground beneath it moves at a different speed as it travels, so the path appears curved — to the right in the Northern Hemisphere and to the left in the Southern. Its magnitude is proportional to the rate of the Earth's spin and to the sine of the latitude, which is why it is zero at the equator and greatest at the poles, and why it grows with the speed of the moving body.
- (a)Pressure gradient — The pressure gradient is what sets air in motion in the first place, driving it from high pressure to low. The Coriolis effect then acts on that motion and bends it. The two are separate forces, and above the friction layer their balance produces the geostrophic wind that blows parallel to the isobars.
- (b)Earth’s axis of inclination — The tilt of the axis, about 23.5 degrees, is responsible for the seasons and for the shifting of the overhead sun between the tropics. It has nothing to do with the deflection of moving bodies, which would occur on a rotating planet with no tilt at all.
- (d)Earth’s revolution — Revolution is the Earth's yearly orbit around the Sun, and together with the axial tilt it gives us the seasons. The deflection depends on the daily spin about the axis, not on the annual journey.
The Coriolis force is not a real force but an apparent one that appears when motion is described in a rotating frame of reference. On the Earth its magnitude is given by 2 x omega x v x sine of the latitude, where omega is the rate of rotation and v the speed of the moving body. It therefore vanishes at the equator, where the sine is zero, and is largest at the poles. It acts at right angles to the direction of motion, so it changes direction without changing speed.
Almost every consequence students are asked about follows from this one cause. Winds do not blow straight down the pressure gradient but spiral, anticlockwise into a low in the Northern Hemisphere and clockwise into one in the Southern. Ocean gyres turn clockwise north of the equator and anticlockwise south of it. Tropical cyclones cannot form within a few degrees of the equator because there is too little deflection there to set a vortex spinning. Keep the four candidate causes separate: rotation gives deflection and day and night, tilt plus revolution give the seasons, and the pressure gradient gives the initial push.
- The Coriolis effect arises from the Earth's rotation and deflects moving bodies right in the Northern Hemisphere and left in the Southern.
- Its magnitude is 2 x omega x v x sin(latitude), so it is zero at the equator and maximum at the poles.
- It grows with the speed of the moving body and acts perpendicular to the direction of motion.
- Balanced against the pressure gradient force above the friction layer, it produces the geostrophic wind parallel to the isobars.
- The axial tilt of about 23.5 degrees and the annual revolution together cause the seasons, not the deflection.
- Attributing the Coriolis effect to the axial tilt or to the annual revolution.
- Assuming the deflection is largest at the equator, where the Earth's surface moves fastest; it is in fact zero there.
- Treating the Coriolis force as capable of changing a body's speed rather than only its direction.
The Coriolis effect is asked as a cause item like this one, as where it is maximum or minimum, or as a statement pair with the pressure gradient force.
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.
- (a) 1 only
- (b) 2 only
- (c) Both 1 and 2
- (d) Neither 1 nor 2
Answer(c) Both 1 and 2
Takes the same formula apart term by term — the dependence on wind speed and on the sine of the latitude.
Assertion (A): Wind patterns are clockwise in the northern hemisphere and anti-clockwise in the southern hemisphere. Reason (R): The directions of wind patterns in the northern and the southern hemisphere are governed by the Coriolis Effect.
- (a) Both A and R are individually true and R is the correct explanation of A
- (b) Both A and R are individually true but R is not the correct explanation of A
- (c) A is true but R is false
- (d) A is false but R is true
Answer(a) Both A and R are individually true and R is the correct explanation of A
The best-known visible consequence of the effect, and confirmation that rotation is what governs it.
Directions : The following six (6) items consist of two statements, Statement I and Statement II. Examine these two statements carefully and select the correct answer using the code given below. Statement I : Geostrophic wind blows above a height of 600 metres, parallel to the isobars. Statement II : Geostrophic wind is the horizontal wind velocity, in which the Coriolis force balances the horizontal pressure force.
- (a) Both the statements are individually true and Statement II is the correct explanation of Statement I
- (b) Both the statements are individually true but Statement II is not the correct explanation of Statement I
- (c) Statement I is true but Statement II is false
- (d) Statement I is false but Statement II is true
Answer(a) Both the statements are individually true and Statement II is the correct explanation of Statement I
Shows the Coriolis force working against the pressure gradient, which is exactly the distinction option (a) here tries to blur.
- practice — not a real PYQ
The Coriolis force is zero at
- (a)the poles
- (b)the equator
- (c)the Tropic of Cancer
- (d)45 degrees latitude
Answer(b) the equator — the force varies with the sine of the latitude, which is zero there.
- practice — not a real PYQ
In the Southern Hemisphere, air moving freely over the Earth's surface is deflected
- (a)to its right
- (b)to its left
- (c)upward
- (d)not at all
Answer(b) to its left — the mirror image of the deflection in the Northern Hemisphere.