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You are on an approach to an airfield, descending through the friction layer. The wind changes from a headwind to a slight right crosswind.
  • A
    This is typical for northern hemisphere.
  • B
    This is typical for southern hemisphere.
  • C
    The airfield is most probably situated near water (on the seaside).
  • D
    This is due to changing wind directions on approach

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.

SURFACE WIND OVER LAND:

  • OVER LAND

In the Northern Hemisphere, the surface wind backs by 30º compared to the geostrophic wind and speed is reduced by 50%.
In the Southern Hemisphere, the surface wind veers by 30º from the 2000ft wind and speed is reduced by 50% compared to the geostrophic wind.

=> If above the friction layer we experience a headwind wind (let’s say 180º) => when landing we will experience a wind from a south-westerly direction (210º - veers 30º from geostrophic wind 180º). Therefore, this is a typical situation on the Southern Hemisphere.

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