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Convergence over a uniform surface is caused by:
  • A
    frictional force together with straight isobars.
  • B
    frictional force only.
  • C
    frictional force together with anti - cyclonic curvature of the isobars.
  • D
    frictional force together with cyclonic curvature of the isobars.

Refer to figure.

Convergence at the surface and divergence in the upper air => describes a LOW PRESSURE/CYCLONIC CURVATURE OF THE ISOBARS.

Winds below 2000ft (Friction Layer) are affected by friction – frictional forces can act to change wind’s direction and slow it down. Coriolis force will be decreased, the PGF is now more dominant. This causes a change in wind direction – wind blows somewhat across the isobars towards the low pressure. As we move higher, surface friction has less effect on the wind until the wind is indeed Geostrophic - and blows parallel to isobars.

=> Frictional forces cause the wind to convergence at the surface around a LOW and diverge around HIGH.

Rotation of air (winds) around a LOW is:

  • Northern Hemisphere = inward and anti-clockwise,
  • Southern Hemisphere = inward 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.

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