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During an approach to land an aircraft experiences a headwind at 4000 ft followed by an increasing crosswind component from the right in the friction layer. The wind shift:
  • A
    Is typical in the northern hemisphere.
  • B
    Results from a cold front advection in the southern hemisphere.
  • C
    Results from a warm front advection in the southern hemisphere.
  • D
    Is typical in the southern hemisphere.

Refer to figure.

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 lower pressure. This wind is called surface wind.

In the Southern Hemisphere, friction causes the surface wind to blow across the isobars at an angle - towards the LOW - and, therefore, be deflected to the right compared to the equivalent geostrophic wind (ie. Wind at 4000 ft) => hence, the increasing crosswind from the right during approach.

As we move higher, surface friction has less effect on the wind until the wind is indeed Geostrophic.

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