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You are flying at 2500 ft AGL, with a southerly wind and intend to land at an airport located at sea level directly below your current position. From approximately which direction would you expect the surface wind to blow (mid-latitude, Northern hemisphere)?

  • A

    South.

  • B

    SouthWest.

  • C

    South-SouthWest.

  • D

    South-SouthEast.

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 30° compared to the geostrophic wind and speed is reduced by 50%. (figure a.) - as you descend to the surface due to the friction effect.

=> If at 2500 ft we experience a southerly wind (let’s say 180°) => when landing we will experience a wind from a south-easterly direction (150° - backs 30° from geostrophic wind 180°).

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