Surface Wind MET

The reason why surface wind differs from geostrophic wind is because of friction between the moving air and the earth's surface. This effect of this friction exists in a layer called the friction layer, the depth of which is determined by wind speed (faster speed, deeper layer), terrain (rougher terrain, deeper layer) and stability.

The friction slows down the air, meaning the effect of Coriolis is less. This has two major effects. Firstly, the wind is deflected towards the centre of the adjacent low pressure (irrespective of hemisphere). This will be to the left in the northern, and to the right in the southern hemisphere. Secondly, because the friction slows the moving air down the wind speed is less (known as "slacking".) In the northern hemisphere we say the wind "backs and slacks".

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What causes surface wind to deflect to the left of the geostrophic wind in the northern hemisphere?
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