UKPSC Prelims 2024 General Studies Paper-I · Question 13 of 147

Correct — C, Geostrophic winds.

The stem describes two forces in balance: the pressure gradient force pushing air from high to low pressure, and the Coriolis force deflecting that motion. When they are equal and opposite, no net force acts across the isobars and the air flows parallel to them. That balanced flow is the Geostrophic winds option.

The name carries the idea: geo (earth) plus strophe (turning), a wind turned by the rotating Earth until it runs along the isobars. It is an idealised, frictionless wind of the free atmosphere, above the friction layer that occupies roughly the lowest kilometre, where the ground no longer drags on the air.

The idea to carry away: a wind named for a balance of forces, not for where it comes from. The other three options are named for a source belt or for how often they blow, which is a different kind of definition altogether.

• (a)
  Polar winds — Polar winds are named for where they start, not for a force balance. They flow out of the polar high-pressure cells towards the sub-polar lows and are bent by the Coriolis force into easterlies, which is why they are usually called the polar easterlies.

  The label is the right answer to the planetary winds blowing from the polar highs towards the sub-polar lows.

• (b)
  Prevailing winds — A prevailing wind is a description of frequency: the direction from which the wind blows most often at a place over a period of record. The mid-latitude westerlies are a familiar example. The term says which direction dominates, not which forces are in balance.

  The label is the right answer to the most frequent wind direction observed at a place.

• (d)
  Trade winds — Trade winds are planetary winds flowing from the subtropical highs near 30° latitude towards the equatorial low. The Coriolis force bends them into north-easterlies in the Northern Hemisphere and south-easterlies in the Southern, but bending is not balance: as surface winds they cross the isobars and keep moving equatorward.

  The label is the right answer to the steady easterly winds blowing from the subtropical highs towards the equatorial low.

Air is set moving by the pressure gradient force, which acts from high to low pressure at right angles to the isobars, stronger where they are packed closer. Once air moves, the Earth's rotation deflects it: right in the Northern Hemisphere, left in the Southern. This deflection is the Coriolis force.

The Coriolis force grows with wind speed and with latitude, from zero at the equator to a maximum at the poles. It acts at right angles to the motion, so it changes direction, not speed.

When it grows until it exactly opposes the pressure gradient force, the wind settles parallel to the isobars: the geostrophic wind.

Near the ground, friction slows the air and so weakens the Coriolis force. The pressure gradient force then wins a little, and surface wind crosses the isobars at an angle towards low pressure. Where isobars are curved, a centripetal term joins the balance and the result is called the gradient wind.

• The geostrophic wind blows parallel to straight isobars, with the pressure gradient force and the Coriolis force equal and opposite.
• The pressure gradient force acts from high to low pressure at right angles to the isobars, and is stronger where the isobars are closer together.
• The Coriolis force deflects moving air to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
• The Coriolis force is zero at the equator, greatest at the poles, and increases with wind speed.
• Surface friction weakens the Coriolis force, so surface winds cross the isobars at an angle towards low pressure instead of running parallel to them.
• In the Northern Hemisphere the geostrophic wind keeps low pressure on its left; in the Southern Hemisphere, on its right (Buys Ballot's law).
• When isobars are curved, a centripetal component enters the balance and the resulting flow is called the gradient wind.
• Trade winds blow from the subtropical highs towards the equatorial low, as north-easterlies in the Northern Hemisphere and south-easterlies in the Southern.
• Polar easterlies blow from the polar highs towards the sub-polar lows; the westerlies blow from the subtropical highs towards the same sub-polar lows.
• A prevailing wind is the direction from which the wind blows most frequently at a place over a period of record.

• Reading 'counter balanced' as 'deflected' and reaching for a wind the Coriolis force visibly bends, such as the trades; deflection alone is not balance, and the trades are named for their source belt.
• Applying the geostrophic label to surface winds; friction near the ground breaks the balance, so the concept belongs to the free atmosphere above the friction layer.
• Confusing geostrophic flow (straight isobars, two forces) with gradient flow (curved isobars, a centripetal term added).
• Assuming the Coriolis force slows or speeds the wind; it acts at right angles to the motion and alters direction only.

• practice — not a real PYQ

  Consider the following statements about the geostrophic wind: 1. It blows parallel to the isobars. 2. It results from a balance between the pressure gradient force and the frictional force. 3. It is best developed in the free atmosphere above the friction layer. Which of the statements given above is/are correct?

  • (a)1 only
  • (b)1 and 3 only
  • (c)2 and 3 only
  • (d)1, 2 and 3
  Answerb — Statements 1 and 3 are correct: the balanced wind runs along the isobars and needs the frictionless conditions of the free atmosphere. Statement 2 names the wrong partner: the pressure gradient force is balanced by the Coriolis force, not by friction, which is what breaks the balance.

  So (a) drops a true statement, while (c) and (d) admit a false one.

• practice — not a real PYQ

  In the Northern Hemisphere, a person standing with their back to a geostrophic wind will find the area of low pressure:

  • (a)to the left
  • (b)to the right
  • (c)directly ahead
  • (d)directly behind
  Answera — In the Northern Hemisphere the Coriolis force pushes to the right of the motion, so the pressure gradient force that balances it must point to the left; low pressure therefore lies on the left. This is Buys Ballot's law.

  (b) is the Southern Hemisphere case. (c) and (d) would put the pressure gradient along the wind, which describes flow straight down the gradient, not the balanced isobar-parallel flow.

• practice — not a real PYQ

  Which one of the following statements about the Coriolis force is correct?

  • (a)It changes the speed of the wind but not its direction
  • (b)It is strongest at the equator and vanishes at the poles
  • (c)It acts at right angles to the direction in which the air is moving
  • (d)It pushes air from high pressure towards low pressure
  Answerc — The Coriolis force acts perpendicular to the motion, which is why it can turn the wind without changing its speed.

  (a) has this backwards. (b) reverses the latitude rule: it is zero at the equator and greatest at the poles. (d) describes the pressure gradient force, not the Coriolis force.

• practice — not a real PYQ

  Surface winds cross the isobars at an angle rather than blowing parallel to them mainly because:

  • (a)friction slows the air and so weakens the Coriolis force
  • (b)the pressure gradient force is absent near the surface
  • (c)the Coriolis force is stronger near the surface than aloft
  • (d)the Coriolis force reverses direction near the surface
  Answera — Friction reduces wind speed; a slower wind feels a smaller Coriolis force, so the pressure gradient force is no longer fully balanced and pulls the air across the isobars towards low pressure.

  (b) is wrong because the pressure gradient force is what drives the wind at the surface. (c) is reversed: friction weakens, not strengthens, the Coriolis force near the ground. (d) is wrong because the sense of deflection is fixed by the hemisphere, not by height.