Consider the following characteristics of a type of wind : 1. They occur at upper levels in the atmosphere 2. They are subjected to two forces, a pressure gradient force and the Coriolis force 3. When the forces balance, air moves at right angles to the pressure gradient, parallel to the isobars Identify the type of wind on the basis of the given characteristics:

Correct — B, Geostrophic Wind. The three printed characteristics are the definition. A geostrophic wind is an upper-level flow, well above the friction layer that slows air near the ground. The only two forces acting on it are the pressure gradient force, which pushes air from high to low pressure, and the Coriolis force, which deflects moving air to the right in the northern hemisphere and to the left in the southern. When the two come into balance, the air is no longer moving down the pressure gradient at all: it moves at right angles to the gradient, along the isobars, with low pressure on its left in the northern hemisphere. That balance is what the third characteristic describes, and it is the standard idealisation for winds a kilometre or more above the surface.

• (a)Jet Stream — A jet stream is a narrow ribbon of very fast upper-level wind, and it is approximately geostrophic, but the printed characteristics describe the general balance rather than the jet's defining features. A jet stream would be identified by its speed, its narrowness and its position along steep temperature gradients at the tropopause.
• (c)Westerlies — The westerlies are a planetary wind belt between about 30 and 60 degrees in each hemisphere, defined by their location and their direction. They are named for where they blow, not for a force balance.
• (d)Easterlies — The easterlies, or trade winds, blow from the subtropical highs towards the equatorial low, again a belt identified by position and direction. Near the surface friction is a third force, so the two-force balance in the stem does not describe them.

Three forces shape wind. The pressure gradient force drives air from high to low pressure and is the only one that starts motion. The Coriolis force, an effect of the Earth's rotation, deflects moving air to the right in the northern hemisphere and left in the southern; it is zero at the equator and maximum at the poles, and grows with wind speed. Friction acts only near the surface, roughly the lowest kilometre. Remove friction and the first two can balance exactly, giving the geostrophic wind that blows parallel to straight isobars.

This item is answered by noticing that only one option is defined by a force balance while the other three are defined by position or speed. That is the pattern to look for whenever a stem lists physical conditions rather than places. The practical consequence of geostrophic balance is Buys Ballot's law: stand with your back to the wind in the northern hemisphere and low pressure lies to your left. Near the ground friction breaks the balance and the wind blows slightly across the isobars towards the low, which is why surface winds spiral inward into a cyclone.

• A geostrophic wind results from a balance between the pressure gradient force and the Coriolis force.
• It blows parallel to straight isobars, at right angles to the pressure gradient.
• The Coriolis force deflects moving air right in the northern hemisphere and left in the southern; it is zero at the equator.
• Friction acts only in the lowest kilometre or so, which is why geostrophic balance is an upper-level idealisation.
• Buys Ballot's law: with your back to the wind in the northern hemisphere, low pressure lies to your left.

• Choosing the jet stream because it too is an upper-level wind.
• Assuming wind must blow straight from high pressure to low pressure.
• Forgetting that the Coriolis force vanishes at the equator, so geostrophic balance fails there.

An identification item built from three defining characteristics. Only one of the four options is defined by a force balance rather than by position.

• 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 everywhere
  • (d)maximum over oceans only

  Answer(b) zero at the equator and maximum at the poles — it varies as the sine of the latitude.

• practice — not a real PYQ

  Surface winds blow slightly across the isobars rather than along them because of

  • (a)the pressure gradient force
  • (b)the Coriolis force
  • (c)friction
  • (d)gravity

  Answer(c) friction — it slows the air, weakening the Coriolis deflection and breaking the balance.