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Which of the following alternatives is the correct one, regarding the surface wind in relation to the air pressure in the Northern hemisphere?
  • A
    The wind blows counter-clockwise around a high and clockwise around a low.
  • B
    The wind over land blows parallel to the isobars.
  • C
    The wind around a high pressure blows clockwise and slants across the isobars towards higher pressure.
  • D
    The wind blows counter-clockwise around a low and slants across the isobars towards lower pressure.

Refer to figure.

THE GENERAL WIND CIRCULATION:
Rotation of air (winds) around a LOW is:

  • Northern Hemisphere = inward to centre and anti-clockwise,
  • Southern Hemisphere = inward to centre and clockwise.
Rotation of air (winds) around a HIGH is:
  • Northern Hemisphere = outward from centre and clockwise,
  • Southern Hemisphere = outward from centre and anti-clockwise.
Near the surface, friction reduces the wind speed, this reduction will reduce the Coriolis force. The weaker Coriolis force no longer balances the pressure gradient force. The latter is now more dominant, and the wind blows across the isobars TOWARDS THE CENTRE OF THE LOWER PRESSURE. This wind is called surface wind.
  • In the Northern Hemisphere, friction causes the surface wind to blow across the isobars at an angle and be deflected to the left compared to the equivalent geostrophic wind (figure b. on the attachment).

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