1.13

Come Rain or Shine

Skip to 0 minutes and 7 secondsSo now we're going to look at a demonstration of the Coriolis force in action. We know that the Coriolis force leads to a deflection on a curved trajectory of anything that's moving over the top of a rotating surface. To simulate that, we're going to use a rotating turntable here to simulate the earth's surface. And this ball bearing is going to simulate a parcel of air moving over the rotating earth's surface. So what I'm going to do is I'm going to start the turntable rotating. I'm going to drop ball bearing onto the turntable, and we're going to see the pattern that it traces.

Skip to 0 minutes and 37 secondsWe going to see that pattern from a camera that's outside the turntable, the one that you're filming me on now. And we're going to see the ball bearing takes one track. We're also going to look at the track of the ball bearing filmed from a camera that's rotating with the turntable. So this camera here is mounted on the turntable, and will rotate with it. And on a separate set of monitors, we're going to look at the apparent track that the ball bearing is taking relative to the rotating turntable. So I'll start the motor running here. It'll take it a little while just to get it to the right speed.

Skip to 1 minute and 13 secondsThat's looking pretty good now. So I'm going to drop the ball bearing down the ramp, and then we're going to watch the path that it takes.

Skip to 1 minute and 25 secondsSo what we can see with the fixed camera is that the ball bearing seems to be pretty much just rolling backwards and forwards across the dish. If we go over to the camera that's mounted on the turntable, what we see is that the ball bearing is actually following a very tightly curved trajectory relative to the motion of the dish itself. So that's like the weather systems above the earth's surface, rotating as they move over the earth's surface. We see that curve trajectory wiggling away like a little snake now. And we can see that the ball bearing's now losing its energy. It's starting to move down into the center of the dish.

Skip to 1 minute and 57 secondsSo what we've seen there is basically the fact that the Coriolis force, the effect of the rotation of the turntable, is making an apparent deflection of the ball bearing's track. So from the camera that was outside the turntable, the ball bearing is pretty much just rolling backwards and forwards. But from the camera that's mounted on the turntable, the ball bearing's taking quite curved, tightly curved, trajectories in a circular path. And that's just like air parcels moving over the earth's surface.

The Coriolis Effect

In this video, Pete uses a turntable and a ball to simulate the effect of the Coriolis force. The ball is deflected on a curved trajectory, across a rotating surface, to help you visualise a parcel of air moving over the Earth’s surface.

As air blows from high to low pressure in the atmosphere, the Coriolis force diverts the air so that it follows the pressure contours. In the Northern Hemisphere, this means that air is blown around low pressure in an anticlockwise direction and around high pressure in a clockwise direction.

Think about a person standing at the Equator. In the course of a day, the planet rotates once, meaning that you travel a colossal 2π x R (the radius of the Earth – 6370km) = 40,000km through space – a speed of about 1700km/ hr. You don’t notice that you are travelling so fast, because the air around you is travelling at the same speed, so there is no wind. On the other hand, if you are standing at a Pole, all you do in the course of a day is turn around on the spot, you have no speed through space and similarly the air around you is stationary.

Now, think about really fast moving, Tropical air which is being pulled towards the poles by a pressure gradient. As it travels polewards, it moves over ground which is rotating more slowly, and so it overtakes the ground, and looks like it is moving from west to east. Similarly, slow moving polar air will be left behind by the rotating Earth and look like it is moving from east to west if it is pulled equatorward by a pressure difference.

In general, moving air in the Northern hemisphere is deflected to the right by the Coriolis Effect.

As the air blows from high to low pressure the Coriolis force acts on it, diverting it, and we end up with air following the pressure contours and blowing around low pressure in an anticlockwise direction and around high pressure in a clockwise direction (both true only for the Northern Hemisphere).

Figure 1:Schematic representation of flow around a low pressure area. Pressure gradient force represented by blue arrows. The Coriolis force, always perpendicular to the velocity, by red arrows. © SVG version, Roland Geider (Ogre), of the original PNG, (Cleontuni)

In this diagram, the black arrows show the direction the air is moving in. The Coriolis force pulls the air to the right (red arrows). As the air is being pulled in to the depression by the pressure gradient (blue arrows), it is continuously deflected by the Coriolis Force. When the air moves in a circle around the depression, the Coriolis force (red arrows) is balanced by the pressure gradient force (blue arrows).

In summary, for the Northern Hemisphere:

• Low pressure is called a cyclone and has anticlockwise winds blowing around it.
• High pressure is called an anticyclone and has clockwise winds blowing around it.
• The wind tends to blow along the pressure contours. For an example, see the BBC website.
• We name winds by the direction they are blowing from.
• Buys Ballot’s Law states that “In the Northern Hemisphere, if you stand with your back to the wind then the lower pressure will be on your left”
• Alternatively, some people find the rule ‘righty tighty, lefty loosey’ a useful reminder of the direction of rotation – high pressure is like tightening a screw (righty tighty) and low pressure like loosening a screw (lefty loosey) (Figure 2).

Figure 2: Air blows around a low pressure in an anticlockwise direction and around a high pressure in a clockwise direction in the Northern Hemisphere © RMetS

What about the Southern Hemisphere?

In the Southern Hemisphere, winds blow around a high pressure in an anticlockwise direction and around a low pressure in a clockwise direction.

The simplest way of visualising why this is the case is to take a ball (or an apple or orange, or anything spherical!). Mark on the poles and the equator, and then mark a spot in the ‘northern hemisphere’ and the ‘southern hemisphere’ of your sphere. Rotate your sphere. Keeping it rotating, tilt your sphere so that you are looking at it from the North Pole – your northern hemisphere spot should be going round in an anticlockwise direction. Now, making sure you keep rotating your sphere in the same direction, tilt it so that you are looking at the ‘south pole’. Your southern hemisphere spot should be rotating in a clockwise direction. This demonstration doesn’t explain the Coriolis effect, but it does show how things can be different in the two hemispheres of the same planet.

This weather map, or synoptic chart, is of 24 January 2009 (Figure 3). The paler black lines show the pressure contours or isobars. They are normally drawn at 4hPa (1 hectoPascal is the same as 1 millibar) intervals – 1016, 1012 etc. There are five low pressure areas on the chart, the lowest pressure is marked with a cross and the pressure at that point is given (L 978). Similarly the centre of any high pressure areas are marked with a cross and the central value given, eg H993.

Note that whether the pressure is High or Low simply depends on whether the pressure around is higher or lower. There is no simple rule, for example, that says ‘everything over 1000hPa is high pressure’. The pressure values in High and Low areas both tend to be higher in the summer than in the winter.

The lowest pressure shown on this chart (Figure 3) is the centre of the Low just off the coast of Iceland (950hPa), but the pressure contours are much closer together around the 963hPa Low in the Bay of Biscay, so it’s that Low which has the faster winds blowing around it.