# Earth's rotation related to atmospheric motion

1. Apr 14, 2015

### hagar

How much affect does the rotation of the earth (such as drag friction) have on the motion of the atmosphere (if any).

Respectfully,
Hagar

2. Apr 14, 2015

### Andrew Mason

The rotation of the earth/atmosphere affects air flow due to the coriolis effect. The earth and the atmosphere as a whole rotates together at the same rate. However, the sun causes convective flows within that rotating atmosphere and this causes moving air masses to rotate due to coriolis effects (eg. hurricanes).

AM

3. Apr 14, 2015

### hagar

Thank you for the response .

Does the coriolis effect change direction of the hurricanes rotation depending on whither they are in the northern or southern hemisphere ? I have been told rotating water such as water going down a drain turns in a reverse direction if it is located in the opposite hemisphere and if this is true I wonder if it would be the same with wind rotation.

Respectfully,

Hagar

Last edited: Apr 14, 2015
4. Apr 14, 2015

### D H

Staff Emeritus
Tropical cyclones are called hurricanes, typhoons, or cyclones, based on where they occur. That's just a naming convention. They are all manifestations of the same phenomenon. To answer your question, tropical cyclones rotate rotate counter-clockwise (when viewed from above) in the northern hemisphere, clockwise in the southern hemisphere.

That's a load of malarkey. If you take a trip to a place right on the equator, you will be able to find people who will show you (for a fee, of course) water draining from a pan in one direction north of the equator and in the opposite direction south of the equator. The people who show you this supposed effect are charlatans. They are using pans that are designed to drain in one direction.

The coriolis effect is rather weak, and it is essentially non-existent at or near the equator.

Here's a map of tropical cyclones from 1945 to 2006. Note that there are no tropical cyclones right at the equator. That's because the coriolis effect is very weak near the equator.

5. Apr 14, 2015

### Andrew Mason

The earth rotates from west to east. The speed of the rotating air/earth varies with latitude. So as air moves away from the equator, it begins travelling to the east at a faster rate than the earth below it so it appears to be pushed to the east. For air moving to the northern hemisphere this effect results in counter-clockwise rotation of air being drawn into a low pressure area (such as the eye of a forming hurricane). In the southern hemisphere the effect is clockwise.

The coriolis effect is observed in large movements of air and water. But the effect is not observable in drains.

AM

Last edited: Apr 14, 2015
6. Apr 14, 2015

### D H

Staff Emeritus
That's not a good way to look at the coriolis effect. How do you explain the trade winds (which flow from east to west with respect to the rotating Earth) with that model?

A better way to look at things with regard to cyclones is that the coriolis effect deflects winds to the right in the northern hemisphere, but to the left in the southern hemisphere. Tropical cyclones are synoptic scale (large scale) phenomena, with a region of low pressure at the center. Winds would rush in from all directions to equalize pressure on a non-rotating planet.

On the rotating Earth, winds are deflected to the right (NH) or left (SH) as the move toward the low pressure center. In the northern hemisphere, this means that winds flowing from the east toward the low pressure center are deflected to the north, winds flowing in from the north are deflected to the west, and so on. The result is a system that rotates counter clockwise. The opposite happens in the southern hemisphere.

I said much the same (I said it's a load of malarkey). That is indeed the case for a small pan.

This effect has been observed in draining pans. To see this effect in a draining pan, one needs a very large circular pan with a flat bottom and with a closable drain hole at the very center. Close the drain and fill the pan with water, being careful not to create any rotation in the water. Level the pan and then wait several days for any residual rotation to damp out. Finally, open the drain. Eventually a coriolis-induced rotation will be observed.

The effect won't be observed if the pan is asymmetric or if there is any residual rotation. The specially-prepared pans used by charlatans at the equator to demonstrate the coriolis effect are asymmetric. A rotation does result, but it's because of this asymmetry rather than the coriolis effect. Kitchen sinks, bath tubs, and toilet bowls are too small, asymmetric, and are drained shortly after filling to see the coriolis effect.

7. Apr 14, 2015

### Bandersnatch

The one video I saw of this being shown to a bunch of tourists, had a symmetrical bowl on a stand, with a drain in the centre, which drained to a bucket underneath. The bowl was then carried a few metres farther, 'over the equator', where it was refilled from the bucket and allowed to drain again. The direction of rotation was induced during the refill phase (which direction the bucket was emptied towards), leading to the residual rotation being amplified as the water drained.
This is just to say that it's not necessarily an asymmetry in the apparatus, but it can be also in the set-up of the experiment.

8. Apr 14, 2015

### hagar

Thanks to all for your answers. They resolved my question completely. I had not thought about using a small pan but I was referring to a larger amount of water like a bath tub drain because of course I have seen that occur.

Respectfully,

Hagar

9. Apr 14, 2015

### D H

Staff Emeritus
Water draining out of a bath tub most definitely does develop a nice swirl, but this is not due to the Coriolis effect. There are several things that work against seeing the Coriolis effect set up in a draining bath tub:
• The tub isn't circular.
• The tub likely has a non-slip surface.
• The drain isn't in the center.
• You drain the tub shortly after you get out.
• The drain drains too fast.
What you need to see the Coriolis effect set up in a draining container is a large, circular, smooth, flat-bottomed container that drains from the very center. The drain has to drain straight down and has to be removable from below so as to avoid inducing any circulation while opening the drain. You need to let the water sit for several days, undisturbed, before you open the drain.

10. Apr 15, 2015

### Andrew Mason

I wasn't attempting to explain trade winds. Air movement occurs due to convection currents created by the sun and the effects of heat absorbed by land masses and water and the movement of water in the ocean currents. The direction of those movements is very complicated and requires detailed study. I don't know if the coriolis effect has much to do with the direction of those winds.

I was just saying that if air is drawn for whatever reason from the equator northward, it will tend to curl east and if it is drawn from the north toward the equator it will tend to curl west. This results in counterclockwise rotation of air drawn to a low pressure area situated north of the equator and a clockwise rotation of air drawn into a low pressure area south of the equator.

AM

11. Apr 15, 2015

### hagar

Thanks to all for the help. I now know much more than I would have from only my original question.

Respectfully,

Hagar

12. Apr 16, 2015

### rootone

I once did an 'experiment' with water draining from a bathtub after I got out of the bath.
The vortex at the plug end developed an anticlockwise spin.
I then used my hands to force the spin in the opposite direction and it happily sustained spinning clockwise.

13. Apr 16, 2015

### hagar

Very interesting. It would seem the draining water left to its own devices will always spin anticlockwise, at least for us.

Respectfully,
Hagar

14. Apr 16, 2015

### D H

Staff Emeritus
No! The direction in which water drains from a bathtub has **nothing** to do with which hemisphere the bathtub is in.

15. Apr 16, 2015

### hagar

I understand but that is not what I meant. It would seem that every draining tub I have witnessed does go down anticlockwise and I now know the north or south hemisphere has nothing to do with it. Whatever the cause I only meant that both rootoon and I had observed the same direction. It was meant with tongue in cheek. Also I wanted to acknowledge anyone that had responded to my question. This applies to you as well,thank you for the concern and I apologize for not wording my response in a more understandable manor.

Respectfully,
Hagar

Last edited: Apr 17, 2015
16. Apr 17, 2015

### Dotini

Would you provide a scholarly or peer reviewed reference to substantiate this particular sentence, please?

17. Apr 17, 2015

### rootone

It should be fairly obvious that the atmosphere is a part of the Earth's total mass and that the total mass as a whole rotates as a whole.
There are local variations in atmospheric flow mainly resulting from convection (weather), fluid dynamics (rivers), and gravity (tidal effects of Moon and Sun),
but overall the earth is a single body and the atmosphere is just it's outermost region.

Edit:
Think about this as well - imagine you are onboard the ISS,
Suppose you observe a storm depression in mid-atalantic.
About 2 hrs later the ISS has completed an orbit and is (approximately) in the position where the storm was first observed.
Meanwhile the Earth has rotated and the storm has rotated along with it.
Altough the storm center might have moved, say 30 km in some direction locally, it will also have traveled about 2500km in an Easterly direction along with the rest of the Earth.

Last edited: Apr 17, 2015
18. Apr 17, 2015

### Dotini

19. Apr 17, 2015

### Dotini

With respect to this question, I ran across this:

http://www.ask.com/wiki/Coriolis_effect?o=2800&qsrc=999&ad=doubleDown&an=apn&ap=ask.com [Broken]
In 1908, the Austrian physicist http://www.ask.com/wiki/Ottokar_Tumlirz?qsrc=3044 [Broken] described careful and effective experiments which demonstrated the effect of the rotation of the Earth on the outflow of water through a central aperture.http://www.ask.com/wiki/Coriolis_effect?o=2800&qsrc=999&ad=doubleDown&an=apn&ap=ask.com#cite_note-33 [Broken] The subject was later popularized in a famous article in the journal http://www.ask.com/wiki/Nature_(journal)?qsrc=3044 [Broken], which described an experiment in which all other forces to the system were removed by filling a 6 ft (1.8 m) tank with 300 U.S. gal (1,100 L) of water and allowing it to settle for 24 hours (to allow any movement due to filling the tank to die away), in a room where the temperature had stabilized. The drain plug was then very slowly removed, and tiny pieces of floating wood were used to observe rotation. During the first 12 to 15 minutes, no rotation was observed. Then, a vortex appeared and consistently began to rotate in a counter-clockwise direction (the experiment was performed in http://www.ask.com/wiki/Boston,_Massachusetts?qsrc=3044 [Broken], in the Northern Hemisphere). This was repeated and the results averaged to make sure the effect was real. The report noted that the vortex rotated, "about 30,000 times faster than the effective rotation of the earth in 42° North (the experiment's location)". This shows that the small initial rotation due to the earth is amplified by gravitational draining and conservation of angular momentum to become a rapid vortex and may be observed under carefully controlled http://www.ask.com/wiki/Laboratory?qsrc=3044 [Broken] conditions.http://www.ask.com/wiki/Coriolis_effect?o=2800&qsrc=999&ad=doubleDown&an=apn&ap=ask.com#cite_note-34 [Broken]http://www.ask.com/wiki/Coriolis_effect?o=2800&qsrc=999&ad=doubleDown&an=apn&ap=ask.com#cite_note-35 [Broken]

Last edited by a moderator: May 7, 2017
20. Apr 17, 2015

### D H

Staff Emeritus
The first thing to note is that is not a bathtub. It is a specially designed circular tank with a drain at dead center. The second thing to note is that Tumlirz didn't wait long enough. Various people tried to recreate his experiment and the results were inconclusive. They were conclusive with an even longer wait time than the mere 24 hours that Tumlirz used.

Normal people who take a bath in a bathtub and see a vortex are not following the careful setup and careful procedures performed by Tumlirz and those who followed. Most people who take a bath simply step out of their asymmetric bathtub and immediately open the drain. This violates the very sensitive conditions that allow one to see the Coriolis effect in a draining tub of water.

21. Apr 18, 2015

### Dotini

A few years ago it was asked why the atmosphere spins with the Earth, and the agreed PF answer was friction. https://www.physicsforums.com/threads/why-the-atmosphere-spins-with-the-earth.518100/

Now a new answer is introduced with inertia and conservation of momentum as noted in post #18 above. Internal friction and the moon slow it down, but apparently not much. The role played by surface friction is not quantified.

"The momentum the air has as it travels around the earth is conserved, which means as the air that's over the equator starts moving toward one of the poles, it keeps its eastward motion constant." http://www.srh.noaa.gov/jetstream/global/jet.htm

However, it has been noted that the ionosphere rotates faster than the atmosphere below.

22. Apr 18, 2015

### Andrew Mason

This is the coriolis effect.

AM

23. Apr 18, 2015

### Dotini

Summing what I think I have learned so far:

That the Earth rotates is a vestige of the original angular momentum of the cloud of dust, rocks and gas that coalesced to form the solar system. This is the same reason that the atmosphere rotates with constant eastward motion.

The Coriolis effect is a deflection of moving objects when the motion is described relative to a rotating reference frame.

24. Apr 18, 2015

### FactChecker

Whether the rotation of the atmosphere is from the original rotation during the Earth's formation or not, after this amount of time the forces of friction would have brought it into the current state. Even if the atmosphere and water came later from asteroids impacting the Earth, the results would be the same.

25. Apr 19, 2015

### Andrew Mason

The rotating reference frame is the rotating earth. The apparent "deflection" of moving air moving over the earth surface is due to inertia of the air: the air tending to maintain its west-east velocity as it moves across the earth surface.

AM

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