Coriolis explanation for cyclones and anticyclones

[the4thamigo_uk]

This relates to a long standing confusion I have had about the Coriolis explanation for cyclones and anticyclones...

The usual 'turntable' demonstration of the Coriolis 'force' is very intuitive. The demonstration relies on the ball/pen/pendulum not having any friction with the turntable and the inertial observer sitting above only sees motion in a straight line.

However, if I could hover above the north pole and look down at the earth, I would see cyclones appearing to move in curved paths on the surface of the earth, even though I am in a non-rotating frame of reference. If the Coriolis effect is only observed 'from' the point of view of the rotating frame of reference, then how come I see the cyclones moving in circles when I am not rotating?

There is something I am missing here in my understanding. Can anyone explain the key to this?

 

[Drakkith]

Cyclones are not caused by the Coriolis Effect. It is a result of differences in temperature and pressure of air in the area of the cyclone. You can look up more on wikipedia under Cyclone or tornado or anything like that.

 

[the4thamigo_uk]

Cyclones are not caused by the Coriolis Effect. It is a result of differences in temperature and pressure of air in the area of the cyclone. You can look up more on wikipedia under Cyclone or tornado or anything like that.

Yes I read much of what I could understand from it and it says Coriolis force several times as do many other websites... they can't be all wrong, can they?

 

[Drakkith]

From the POV of the north pole you are looking down on a sphere, not a circle. Since the sphere is curved you will see anything moving along it also traveling in a curve. Also, the movement of a cyclone across the Earth is almost entirely due to winds, not the Coriolis Effect. They cyclones are actually being pushed by the wind, they don't just "look" deflected because the Earth is rotating.

Edit: Make sure you aren't confusing the movement of the whole cyclone as a system with the movement of the air that makes up the cyclone. They are 2 different things.

 

[the4thamigo_uk]

From the POV of the north pole you are looking down on a sphere, not a circle. Since the sphere is curved you will see anything moving along it also traveling in a curve.

Yes I am taking the wind movements within a cyclone in the northern hemisphere and projecting them onto a flat disc centered on the north pole and simply tracking the movements on that basis. If I track an individual parcel of air moving around a cyclone and map it onto this disc I would see a slightly distorted view of the trajectory but it will reflect the same essential characteristics, i.e. a continuous moving 'swirl' or 'spiral' around the disc.

Also, the movement of a cyclone across the Earth is almost entirely due to winds, not the Coriolis Effect. They cyclones are actually being pushed by the wind, they don't just "look" deflected because the Earth is rotating.

Edit: Make sure you aren't confusing the movement of the whole cyclone as a system with the movement of the air that makes up the cyclone. They are 2 different things.

Yes it is a complex system, so I would guess there are certainly other effects at work...

Im assuming that once a stable cyclone has been formed that the general rotation of the atmosphere will pull the structure around with it. I am not doubting this for the moment. I am more concerned with how the cyclone initially obtains its circular motion and also how it maintains it.

The seed of a cyclone is an area of low/high pressure, which causes air to move radially inwards/outwards. It is then said that this moving air is 'deflected' by the coriolis effect on its radial journey and that this 'force' causes the circular motion. It is also said that a stable cyclone requires that the coriolis 'force' approximately balances the radial pressure differential across the moving air.

Is it from the Coriolis effect? You say it is not, but many many websites say it is they 'key' effect to produce the circular motion? If it is the Coriolis effect, then this should only to be observed in the rotating frame of reference (edit: i.e the surface of the earth) as I understand it, so then how come I can see it from my inertial viewpoint...

Lets simplify things to a model where we have a perfectly static rotating 'atmosphere' (perhaps even on a flat disc if it makes it easier) with no cross winds or thermal effects, no sun, no other weather etc. If we then create an area of low/high pressure to seed a cyclone what will happen. Will the inertial observer see a rotating cyclone or not?

 

[Drakkith]

Look up the Coriolis Effect. It is a fictitious force that results from a rotating frame and inertia. The air IS circulating, but it is a result of all the effects that start a cyclone.

You can see the air swirling because it IS swirling. It doesn't just look like it is circulating, it actually is. The Coriolis Effect they refer to is merely the inertia of the swirling air. If you were in the frame of the cyclone, rotating inside it with the air, it would "seem" like there was a force that was pushing things in certain directions, but in reality there isn't. It is only because your frame is rotating.

Edit: I don't think I'm explaining it very well, so I apologize for that.

 

[the4thamigo_uk]

Look up the Coriolis Effect. It is a fictitious force that results from a rotating frame and inertia. The air IS circulating, but it is a result of all the effects that start a cyclone.

Yes this is what my question is directed at really. I know the idea of the coriolis effect and I understand it is a fictitious force. So, I think you are simply saying (and this was my suspicion all along) that all these websites are basically propagating yet another myth about the Coriolis effect (in the same way as the water down a plughole)? Or is your point more subtle than this?

 

[the4thamigo_uk]

You can see the air swirling because it IS swirling. It doesn't just look like it is circulating, it actually is. The Coriolis Effect they refer to is merely the inertia of the swirling air. If you were in the frame of the cyclone, rotating inside it with the air, it would "seem" like there was a force that was pushing things in certain directions, but in reality there isn't. It is only because your frame is rotating.

So there are two rotating frames of reference here. (Maybe this is the source of my confusion). There is the frame of reference of an observer on the surface of the Earth (E) and the frame of reference of an observer moving with the rotating air in the cyclone (C).

My understanding of the various descriptions was that it was the Coriolis effect caused/observed from E that is the source of the explanation. That it is the rotation of the Earth that 'causes' the cyclones to begin to swirl via the 'fictitious' coriolis force. I think you are simply saying that they mean the fictitious coriolis force observed from C (in which case it seeks to explain the rotation of air by requiring that the air is already rotating which seems like a 'circular argument' if you forgive the pun !)

So I think your explanation, of a cyclone, is that it is simply a result of the radial force of a low pressure centre and some wind that just happens to be moving (initially) tangentially, and then is pulled inward causing it to spiral in (i.e. in the same basic sense as a satellite in orbit pulled in by gravity). Conversely, a high pressure centre pushes any tangentially moving wind outwards. This makes complete sense to me.

Therefore, invoking the Coriolis effect to 'explain' the rotation of cyclones would seem to be a very poor explanation of 'why' the cyclones rotate. It suggests that it is the mere rotation of the Earth (E) or cyclone (C) that directly causes the swirling, which is wrong. The physics observed from the inertial frame suggests that it is a 'local' effect due to winds and pressures in a small area of the Earth's surface. It is both unnecessary, confusing, and incorrect to use the Coriolis effect to explain this phenomena.

If, as suspected, the coriolis effect has no physical basis for the explanation of the swirling air observed from an inertial frame, then the reason that cyclones have different directions of rotation in the northern and southern hemisphere is as a result of something else? What is this explanation, is it differential latitudinal wind patterns, thermally driven winds or something else?

(Thanks for your responses, but it seems like I am not quite settled in my understanding just yet ) )

 

[Cleonis]

However, if I could hover above the north pole and look down at the earth, I would see cyclones appearing to move in curved paths on the surface of the earth, even though I am in a non-rotating frame of reference.

Let me rephrase your question, I want to make it more focused.

I take it you do not mean the path of the cyclone as a whole. I take it you are referring to the paths of individual parcels of air, as they are part of the cyclonic flow as a whole. Like tracking the path of a weather balloon that is flowing along in the wind.

As you state, that path is curvilinear both with respect to the inertial coordinate system and with respect to the co-rotating coordinate system.

The turntable analogy is insufficient, as the turntable analogy only deals with motion that follows a straight line with respect to the inertial coordinate system.

When there is a low pressure area then it is two factors interacting with each other that give rise to the cyclonic flow: the pressure gradient and the rotation-of-Earth effect that in meteorology is called Coriolis effect.

It may be superfluous to state this, but I'm stating it anyway:
You cannot explain cyclonic flow with just Coriolis effect; it is pressure gradient and Coriolis effect interacting with each other.

The inserted image shows a very schematic representation of the formation of cyclonic flow around a low pressure area.

The red arrows represent the rotation-of-Earth-effect, the blue arrows represent pressure gradient force. Without the pressure gradient force the flow pattern won't form.As mentioned earlier, if you track the motion of something co-moving with the wind, then you will of course find that it's motion with respect to the inertial coordinate system is not a straight line.
The thing is, whatever the path with respect to the inertial coordinate system, the rotation-effect for the path with respect to the co-rotating coordinate system is always that the vector for the Coriolis effect is at right angles to the instantaneous velocity.

See also the discussion (illustrated with animations) of http://www.cleonis.nl/physics/phys256/coriolis_in_meteorology.php" on my own website.

 

[Cleonis]

Therefore, invoking the Coriolis effect to 'explain' the rotation of cyclones would seem to be a very poor explanation of 'why' the cyclones rotate. It suggests that it is the mere rotation of the Earth (E) or cyclone (C) that directly causes the swirling, which is wrong. The physics observed from the inertial frame suggests that it is a 'local' effect due to winds and pressures in a small area of the Earth's surface.

Of course for small atmospheric phenomena (in the order of hundreds of meters) the Earth's rotation is negligable. And even smaller gyrations, such as dust devils, will probably show an even distribution of clockwise and anti-clockwise.

The example from daily life is a sink draining. Depending on what residual rotation is in the water mass it will (near the sink hole) gyrate clockwise or anti-clockwise. And there is always some residual rotation. No involvement of terrestrial Coriolis effect there of course. Gravity is pulling the water towards the sink hole. When a force pulls mass closer to a center of rotation the angular velocity goes up.

Cyclonic flow
But the flow field of cyclonic flow is terrestrial in scale. It's on that terrestrial scale that the Earth rotation does play a significant part.