Thursday, 31 October 2013

I wish they'd make up their minds

People explain the Coriolis effect using the analogy of a roundabout e.g. Dr Helen Czerski.

So objects or wind moving in a straight line in the northern hemisphere will curve slightly to the right, good stuff, I understand that. Now, if you are constantly turning to the right, then looked at from above you are moving clockwise, are you not?

But we know that hurricanes, tornados and the trade winds move anti-clockwise, so something has got lost in translation here.

What is even more irritating is that the supposed experts cheerfully explain that winds turn to the right and then in the next breath say that this is why hurricanes etc turn anti-clockwise. Do these people just learn stuff parrot-fashion without ever wondering whether it actually makes sense?
UPDATE: Thanks for all your suggestions. View From The Solent emailed me a link to this explanation, which seems reasonably scientific, plausible, and easy to picture:

Storms in the northern hemisphere always circle counterclockwise, or to the left. Storms in the south turn clockwise, to the right. This consistent motion is due to the Coriolis Effect, and yet it if it is, why don't the storms circle to the right?

The mechanics of this are best understood when walked through physically. You'll need some object to mark a spot on the floor, and if possible, a friend. Put a book or a plate or some object down on the floor a few feet away from you. This is a low pressure system. A low pressure system is created when air is heated enough to lift upwards, creating a relative vacuum below. Higher pressure air from around the system rushes in to fill the vacuum. That's you.

We'll pretend that you're in the northern hemisphere. This means the earth turns to the left under you, and your course appears to be diverted to the right. The low pressure system is pulling you in. Take a slow step towards the low pressure system, and have your friend pull on your right sleeve. If you're alone, imagine that someone is turning you sharply to the right. If you've done it correctly, you'll be a little closer to the low pressure system, but you'll have turned to the right, so the system will be on your left.

That low pressure system is still pulling you, so take turn and take another step towards it. Again your friend, imaginary or real, is tugging your right sleeve, turning you to the right. Repeat this step a few times and you'll notice that you're circling the low pressure system in a counterclockwise fashion.

The way that the Coriolis Effect in the northern hemisphere makes objects, including winds, veer to the right of their targets causes them to circle those targets to the left – or counterclockwise. In the southern hemisphere, the way the Coriolis Effect makes objects veer to the left causes them to circle their targets to the right – or clockwise.


Bayard said...

"then looked at from above you are moving clockwise, are you not?"

Well if the Sun rises in the east and sets in the west, the Earth must be moving from west to east. If you look at the Earth from above (the North Pole) then it is turning anti-clockwise. It is the Sun that "turns" clockwise, as one might expect.

DBC Reed said...

Trade winds move clockwise in Northern hemisphere.

Anonymous said...

A nice graphic would have been cheaper than flying her out to paddle in the Gulf of Mexico.
1. Most important, wind blows from high pressure areas into low pressure areas.
2. A good way to think of it is like an aircraft pointing directly to its destination with a cross wind blowing from the left. It gets pushed to the right, and realising this, keeps altering its heading to the left to stay pointing at its destination. So its path is curved to the left (anticlockwise)
3. Key point is the aircraft alters its course, just as the wind is always trying to blow to the centre of low pressure. That is what gives the curvature.

I think she does understand this so takes it for granted. Maybe academics aren't the best teachers?

Dinero said...

You're right the commentary did say to the right over a video of the wind turning to the left.
She did say it was to the right on the northern hemisphere and to the left in the southern.
But why would that be , if you think of the wind blowing over the globe as a cylinder then there's no difference.
The globe rotates counterclockwise whether you are in the north or southern hemispheres.

Mark Wadsworth said...

B, viewed from above, the world turns anti-clockwise, which is why she spins the roundabout anti-clockwise (at about 2 m 30 secs in) to illutrate northern hemisphere.

DBC, possibly, please ignore that bit.

Anon, yes, wind blows from high to low, but it's path is still curved. She explains that - the middle bit of the hurricane is low pressure. Your airplane analogy is unhelpful.

Din, she explains that as well by spinning the roundabout clockwise to illustrate southern hemisphere and the ball appears to curve in the other direction to the left (=anti-clockwise).

Dinero said...
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Dinero said...

I agree with you something is amiss.

Hurricanes rotate anticlockwise as does the earth, which contradicts the counter rotation explanation.

any conclusions yet

Mark Wadsworth said...

Din, the whole thing is a mystery to me.

Everything you google says exactly the same thing, that turning right (which I understand) means going anti-clockwise (which I don't).

And while I disagree that the earth is a cylinder, areas where hurricanes form are close to the equator and that part is as good as being a cylinder, which supports your argument.

You would expect the effect to be stonger nearer the poles.

Dinero said...

I think I've got it now.

if you have wind going south to north and the earth rotating to the right then the bending is to the left which results in the anticlockwise spiral

so explanations referring to "bend to the right" are mistaken.

Mark Wadsworth said...

Din, yes, if we view the earth for convenience as a cylinder and we are standing at the equator, that is quite true for south-to-north winds - they bend to our left.

But a north-to-south wind would bend to our left as well (in other words, the wind bends to the right).

And the "cylinder" theory does not explain why cyclones in the southern hemisphere go clockwise, which is indisputably the case, see e.g. over Australia.

Dinero said...

Leave the cylinder model its not important. I found picturing a globe better.

A north to south wind would also bend to the left as you say , however as it is going south it would then be rotating clockwise

up and to the left is anticlockwise

down and to the left is clockwise

so I'm thinking its the original north south wind direction not the hemispheres that set of the different rotations

I checked the Yasi video - cyclone and anticyclone rings a bell

Bayard said...

"A nice graphic would have been cheaper than flying her out to paddle in the Gulf of Mexico."

surely part of the point of programmes like this is that presenters like her get to paddle in the Gulf of Mexico. I reckon the thinking goes thus "I want a free holiday to Mexico: what can I present a programme on that will plausibly allow me to go there at the BBC's expense. This sort of thing was memorably sent up by Jeremy Clarkson on Top Gear.

BTW, is this the same argument as which way the water turns when it's draining out of a plughole?

Antisthenes said...

It's all down to which hemisphere either south or north. North; winds flow clockwise south; anti-clockwise.

paulc156 said...

Coriolis is just the result of the ground moving at a different rate to objects above it, like air.

High pressure at equator and polar regions so air flows to mid lats where low pressure rules.

Earth rotates fastest at equator and slows as you move toward poles.

Easiest way to get the clockwise/anti-c wind directions of storms is to imagine yourself throwing a ball from the Equator to someone standing at mid latitudes. Because the thrower is moving faster than the the receiver the ball will bend to the right ahead of the receiver, as seen from the throwers perspective.
If you then go the North Pole and throw the ball to the receiver who stays at the mid lat' the ball will again appear to move to the right of the receiver[again from the throwers perspective]this time because the receiver has rotated forward faster than the thrower. That's your anti-clockwise storm winds as seen from directly above the earth. Vice versa for southern hemisphere.

Robin Smith said...

Nope! Stop asking why we are insane. Just accept it and move on.

DBC Reed said...

We appear to have established that the trade winds and hurricanes rotate in opposite directions.So does the Corioli's effect apply in one case and not the other?

Dinero said...

> paulc156

in your example both rotations are clockwise

North and to the east is a clockwise rotation

South and to the west is a clockwise rotation

so I think the answer is that wind flowing north bends to the west resulting in anticlockwise spiral


wind flowing to the south also bends to the west resulting in clockwise spiral.

paulc156 said...

@DBC Reed: "We appear to have established that the trade winds and hurricanes rotate in opposite directions.So does the Corioli's effect apply in one case and not the other?"

The coriolis itself is zero at the equator and rises the higher the latitude one goes. However Trade winds [for north hemis] come from the North and are so directed 'rightwards'/[an easterly trade wind] from the point of view of looking from the north toward the equator. That's the same as the anti clock movement of hurricanes which [once again from the northerly viewpoint] deflects to the right. So the coriolis is still doing its thing due to the fact the trades start off as northerlies.

paulc156 said...

@Dinero "in your example both rotations are clockwise

North and to the east is a clockwise rotation

South and to the west is a clockwise rotation"

Looking at the northern hemi from an aerial viewpoint the coriolis bends the wind that comes from the direction of the equator toward the mid lats to the right [north east] and bends the winds coming from the polar direction down toward the equator to the left [south west]. That's anti clockwise.

Dinero said...

No down and to the left is clockwise

maybe this diagram will come out in blogger comments


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Bayard said...

If they'd printed a graphic instead of sending Dr Czerski to paddle in the Gulf of Mexico, as Anon pointed out, all this confusion could have been avoided. Come on Beeb, get your act together!

paulc156 said...


Down to the left combined with up to the right is anti-c!

This is as described from an aerial position as i said in my last post: "from an aerial viewpoint". Looking if looking at a clock face.

Watch the second hand on a clock. It moves 'clockwise' :) right?

The hand moves from 12 [north] down to the right [past 3 o'clock]. The northern storm moves down from the north [12] to the left, [past 9 0'clock]. Anti clockiwse.

Dinero said...
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Dinero said...

Right I follow that combination of the two directions, makes a full explanation, it seems the direction from north to south , pole to equator, is missing from the video

DBC Reed said...

Trying to make sense of Coriolis, I discovered that the Moon is upside down when viewed from Australia. Very disconcerting.