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Over open water in the Northern Hemisphere, surface winds typically _____ the gradient wind strength. What completes the sentence?

  • A

    Back approximately 10 degrees and are slowed to one third of.

  • B

    Back approximately 10 degrees and are slowed to two thirds of.

  • C

    Back approximately 30 degrees and are slowed to one third of.

  • D

    Back approximately 30 degrees and are slowed to two thirds of.

Refer to figure.
At altitude, geostrophic wind flows perpendicular to the pressure gradient due to the Coriolis force. However, near the surface, friction slows the wind, reducing the Coriolis effect. As a result, surface wind is not only slower than the free-stream (or "2000 ft wind") but also shifts direction, flowing slightly toward low pressure due to the dominant pressure gradient force.

This means that in the Northern Hemisphere, the surface wind is "backed" from the free stream wind, to go towards the low pressure (and it is "veered" in the Southern Hemisphere). The degree to the speed reduction and the subsequent direction change depends on the surface type (its friction) and the time of day (amount of vertical mixing):

Day Night
Directional change Percentage of "2000 ft wind" Directional change Percentage of "2000 ft wind"
Land 30° 50% 40° 30%
Sea 10° 70% 10° 70%

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