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
Correct — A, Both the statements are individually true and Statement II is the correct explanation of Statement I. Statement II is the textbook definition of the geostrophic wind — the horizontal wind in which the Coriolis force exactly balances the horizontal pressure gradient force — and it is correct. Statement I describes what such a wind looks like: it is found above the shallow layer near the ground where friction matters, which standard Indian geography texts put at about 600 metres, and it runs parallel to the isobars. The two are joined by a genuine chain of reasoning. Near the surface friction drags on moving air and turns it across the isobars towards the low. Remove friction and only two horizontal forces are left. The pressure gradient force acts straight across the isobars from high to low; the Coriolis force acts at right angles to the wind. They can cancel only if the wind is running along the isobars, because only then does the Coriolis deflection point straight back up the pressure gradient. So the balance described in Statement II is exactly why the wind of Statement I blows parallel to the isobars, and the code is (a).
- (b)Both the statements are individually true but Statement II is not the correct explanation of Statement I — This would be right if the two statements were unconnected truths. They are not. The two-force balance in Statement II is the mechanism that forces the wind along the isobars, so the explanation test succeeds and the code cannot be (b).
- (c)Statement I is true but Statement II is false — Statement II is not false; it is the standard definition of the geostrophic wind, found in the same form in NCERT and in every meteorology text.
- (d)Statement I is false but Statement II is true — Statement I is not false either. Above the friction layer the wind does blow parallel to the isobars, which is why upper-air charts show the flow running along the height contours.
Three forces act on air moving horizontally. The pressure gradient force pushes air from high pressure towards low, at right angles to the isobars. The Coriolis force, an effect of the Earth's rotation, deflects moving air to the right in the Northern Hemisphere and to the left in the Southern, always at right angles to the motion. Friction opposes the motion and is felt only near the surface. Where friction can be ignored, a steady wind requires the first two to be equal and opposite, and that is possible only along the isobars. That idealised wind is the geostrophic wind.
Do not fight the number. Six hundred metres is the figure the Indian geography textbooks this item draws on give for the top of the friction layer; NCERT puts friction's influence at roughly one to three kilometres, and meteorology treats the geostrophic balance as valid above the atmospheric boundary layer rather than at any sharp height. The examiner is not testing the metre count. He is testing whether you know that taking friction away leaves a two-force balance and that the balance forces the wind along the isobars. Two further points repay carrying. The balance fails at the equator, where the Coriolis force is zero, so surface air there crosses the isobars directly. And where the isobars are curved rather than straight, the balance also has to accommodate the centripetal effect, giving the gradient wind rather than the geostrophic one.
- The geostrophic wind is the idealised wind in which the pressure gradient force is exactly balanced by the Coriolis force.
- Because the Coriolis force acts perpendicular to the motion, the balance can hold only when the wind blows parallel to the isobars.
- The Coriolis force arises from the Earth's rotation, is zero at the equator, is greatest at the poles, and increases with wind speed.
- Near the ground friction slows the wind and swings it across the isobars towards the low, which is why surface winds are not geostrophic.
- Closely spaced isobars mean a steep pressure gradient and therefore a stronger wind.
- Expecting surface winds to blow along the isobars; friction makes them cross the isobars towards the low.
- Forgetting that the Coriolis force vanishes at the equator, so the geostrophic idea does not apply there.
- Treating the Coriolis force as a real push; it is an apparent force arising from the Earth's rotation.
Wind-and-pressure items ask either for the definition of the geostrophic wind or for the behaviour of the Coriolis force with latitude and speed — and often, as here, wrapped in a statement-pair or assertion-reason code.
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 :
- (a) 1 only
- (b) 2 only
- (c) Both 1 and 2
- (d) Neither 1 nor 2
Answer(c) Both 1 and 2
Pins down the force that does half the balancing in Statement II, and explains why the geostrophic idea breaks down at the equator.
The Coriolis effect is the result of
- (a) Pressure gradient
- (b) Earth’s axis of inclination
- (c) Earth’s rotation
- (d) Earth’s revolution
Answer(c) Earth’s rotation
The origin of the force that balances the pressure gradient here, tested a year earlier in the same series.
- practice — not a real PYQ
Surface winds cross the isobars at an angle towards the area of low pressure mainly because of
- (a)the Coriolis force
- (b)frictional force
- (c)the pressure gradient force alone
- (d)the curvature of the isobars
Answer(b) frictional force — friction near the ground slows the wind, weakens the Coriolis deflection and lets the pressure gradient turn the air across the isobars.
- practice — not a real PYQ
The Coriolis force is
- (a)maximum at the equator and zero at the poles
- (b)zero at the equator and maximum at the poles
- (c)the same at every latitude
- (d)maximum at 45° latitude and zero elsewhere
Answer(b) zero at the equator and maximum at the poles — the deflecting effect varies with the sine of the latitude.