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Refer to figure.
Over point N, at 2 000 ft, the wind is 000°/60 kt. The corresponding surface wind at point N is:

  • A

    330°/30 kt

  • B

    330°/42 kt

  • C

    350°/30 kt

  • D

    350°/42 kt

SURFACE WIND OVER LAND AND OVER SEA:

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.

OVER SEA – friction is much less, and the surface winds are closer to geostrophic values.

  • In the Northern Hemisphere, the surface wind backs by 10º from the geostrophic wind and speed reduces to 70%.
  • In the Southern Hemisphere, the surface wind veers by 10º and speed reduces to 70% when compared to geostrophic winds.

⇒ In this case, point N is situated in the Northern Hemisphere over the sea. Consequently, the wind backs by 10º to 350º and amounts to 70% of the geostrophic wind, equaling 42 kt (60 x 0.7).

 DayNight
Directional changePercentage of "2000 ft wind"Directional changePercentage of "2000 ft wind"
Land30°50%40°30%
Sea10°70%10°70%

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