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The Coriolis Effect

The coriolis effect is a major influencer on the weather and needs to be understood.


Coriolis is a change in the wind due to the anti-clockwise rotation of the earth on it's North axis. There is a direct relationship between the difference in pressure between two points (called the pressure gradient) and the wind. Wherever there is a pressure gradient there is a wind whose strength is proportional to the gradient. If the earth did not spin then the wind would blow directly from high pressure to low pressure, as you would expect. But the Coriolis force always achieves a balance with the pressure gradient force and makes the wind blow across the pressure gradient (except near the equator).

The wind created by the pressure gradient is the wind you measure in a weather map. Meteorologists, recognising the importance of the Coriolis force call it the Geostrophic (earth turning) wind. This is the wind experienced at about 500m above ground (so as not to be influenced by the ground friction).

If isobars are closely spaced this is a strong pressure gradient and hence a strong wind. If the isobars are far apart then this is a weak pressure gradient and hence a weak wind (latitude comes into the equation as well, since it affects the Coriolis force, the lower the latitude the stronger the wind for the same spacing of the isobars.

An Example

An example of Coriolis effect is to imagine a golf ball driven from say London to Paris. As the ball moves South East the ground beneath it will be moving Eastwards at an increasing speed, since the nearer you get to the equator the more ground there is to spin through in 24 hours. So although the gold ball maintains its initial Eastward ground speed it will be left behind and appear to drift Westwards.

The force of the Coriolis effect is dependent on the latitude, the higher the latitude the stronger it is. Structure of the major wind systems over the oceans is largely a consequence of Coriolis force, an example being the change from steady Trade Winds in low latitudes to disturbed Westerlies in higher latitudes as the force increases. Certain areas of the world experience predominantly high pressure and other areas experience predominantly low pressure.

Most of the weather systems affecting the British Isles travel from west to east. The British Isles is in the dividing line between Warm tropical air from the south and cold polar air from the north.


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Trade winds

The trade winds are a good example of the Coriolis effect. Air moving south towards the equator appears to be deflected West, giving us the North East Trades. Northwards moving air in the Southern hemisphere is also deflected to the West giving us the South East Trades. The Coriolis force disappears near the equator and is ineffective between about 8 degrees North and 8 degrees South. Tropical storms that need the Coriolis force to start them spinning, never form in these lowest latitudes.

The direction of the wind deflection due to the Coriolis effect is best remembered using Buy's Ballot's Law which says: If you stand with your back to the wind in the Northern hemisphere you will have low pressure on you left hand side (the opposite applies to the south).

Global Winds

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Page last updated on October 16, 2007
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