Friction of the Ground

The drag exerted by the Earth's roughness is a fourth factor affecting winds within the lowest kilometre of the atmosphere. Friction reduces the wind's speed, so lessening the Coriolis effect, causing the pressure-gradient force to exceed the Coriolis force and therefore the air to flow slightly towards the lower pressure. This explains why surface winds do not precisely circle high-pressure regions but spiral out to the right of them instead, while winds around lows spiral inwards, again to the right of the isobars in the southern hemisphere.

This deflection to the right and the reduction of speed due to friction are most at levels closest to the ground, and so winds at various levels may be represented by an Ekman spiral like that near the surface of an ocean (Section 11.4). In a fully developed Ekman spiral, the surface wind blows from a direction 45° clockwise from the isobars. (We say that the surface wind is made to 'veer' by the friction in the southern hemisphere, whereas an anticlockwise change of direction is called 'backing'.) However, a full spiral rarely develops in reality, certainly not on warm days, when the planetary boundary layer becomes unstable and there is vertical mixing to disturb the distinct layers implicit in the Ekman effect. Variations of wind speed and direction in the lowest kilometre are greatest when the atmosphere is stable, and then the depth occupied by the spiral (called the 'Ekman layer') is the same as the PBL.

Friction is less over the oceans because they are flat, so winds at sea tend to be stronger than inland and blow at only 10-20 degrees from the direction of the gradient wind. The angle is larger over a rough surface like a forest or city. In the Australian desert, sand-dunes tend to be blown to lie 20-30° clockwise of the prevailing gradient wind.

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