Curving Wind: The Coriolis Force

Have you ever wondered why hurricanes, low pressure systems and high pressure systems all spin the way they do? Or why they spin the opposite direction in the southern hemisphere? Maybe you’ve heard that toilets flush the other way in the southern hemisphere. Could that be the same reason why the winds switch too? Well, if the toilets actually did flush the opposite direction in the southern hemisphere, it probably would be the same reason for the wind switch: The Coriolis effect.

Though it feels to us like the Earth isn’t moving, it’s actually always spinning and orbiting around the sun. While the orbit is gradual enough to not really have any effect, one full revolution every 24 hours is quick enough to effect the movement of weather and wind. Basically what happens is the wind is trying to blow in a straight line, but the earth is spinning, so for us on the surface of the planet, it appears that it’s curving. Imagine trying to throw a ball from a moving car; if you threw it perpendicular to the direction the car is moving, it would still keep moving forward at the same speed as the car. Someone standing on the side of the road would see the ball moving diagonally, both in the direction you threw it and in the direction the car was going. For the spinning Earth, wind curves to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

So how can we apply this to high and low pressure systems? We need to look at what causes wind. There are three forces that act on air. The pressure gradient force is how much pressure changes over a distance, and pushes the air from high to low pressure. Friction slows down the air, always acting in the opposite direction of the wind. The Coriolis force is the force that we interpret from the Coriolis effect; it’s perpendicular to the wind to the right in the Northern Hemisphere. The size of the Coriolis force depends on the speed of the wind. So if we take a low pressure system, the pressure gradient force will blow directly towards the center of the low. The Coriolis force will deflect it to the right. The trick with the Coriolis though is that it is always perpendicular to the wind, so it will keep trying to turn it until it is balanced by the pressure gradient and friction. This means that the wind ends up blowing almost in a circle around the highs and lows. The pressure gradient is always the starting point, but with deflection to the right, a low ends up with counter clockwise rotation and a high with clockwise rotation.


But what about the flushing toilet? How does the Coriolis force effect that? It doesn’t. Weather systems move slow, much slower than a flushing toilet and move over much larger distances. Toilets are just too small of a scale to be effected by the Earth’s rotation. That one’s all about the jets.

Written By Scott Wolff

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