Understanding Counter-Clockwise Motion of Low Pressure Systems

[Sailor_Grant]

If the Coriolis effect causes the wind to go right in the northern hemisphere, how do low-pressure systems go counter-clockwise (in the opposite direction of the coriolis effect)?

This is something that has been troubling me and I have not been able to find a good explanation.

[Jim Cate]

the air flow is from high pressure to low pressure, and thus inward towards a low center. When deflected to the right, the flow becomes counter clockwise in the N hemisphere.

Jim

[Dave_S]

My understanding....

Air is pulled with the earth and tries to match the speed. As the speed of the earth becomes less towards its axis it is therefore fastest at the equator and slowest at the axis so as you head south below the equator the air speed decreases.

Imagine the air as two planks of wood with a dowel seperating them. If the top plank (equator) was going faster than the bottom plank (eg. 40° South) which direction would the dowel between them rotate - clockwise.

Below the equator, a low pressure system pulls air towards it. So the air below it is pulled up and the air above it is pulled down. The air below is moving slower so it gets left behind, and the air above is going faster so it pulls ahead of the low. This rotates the air, top ahead, bottom behind = clockwise.

[ChrisJHC]

I assume you’re in the Northern Hemisphere (as you have the low pressure systems going the wrong way), so here’s a US-based explanation:

http://cimss.ssec.wisc.edu/sage/mete.../concepts.html

[capt jgw]

The air flowing into the center of a low pressure system goes UP when it reaches the center. No where else it can go. Visualization of this is in a tornado. The wind spirals inward and up, and debris can be seen traveling upward in the funnel. Sometimes thousands of feet. If you look at a low pressure cell from above like on a map, it's turning to the left and toward you. If you look at it from the ground up, the way it is actually traveling, its turning to the right. The opposite is true of a high pressure cell. It funnels outward and down, turning to the right when viewed from above. Look at from ground level looking up, it turns to the left. All depends on which side you're looking at.

[StuM]

Originally Posted by Sailor_Grant

If the Coriolis effect causes the wind to go right in the northern hemisphere,

It doesn't always

The Coriolis effect results in winds moving towards to equator moving west and winds moving away from the equator moving east with respect to the earth's surface. (i.e. "twisting" to the right when viewed from above)

It has no effect on winds moving east or west.

how do low-pressure systems go counter-clockwise (in the opposite direction of the coriolis effect)?

Air moves towards the centre of a low pressure area.
In the northern hemisphere, winds north of the centre are moving towards the equator so they are "twisted" to the west and winds south of the centre are moving away from the equator and are twisted to the east.
These two opposing "twists" result in an overall counter clockwise rotation.

North Pole
..............|
.........|
.....|
<-- Coriolis effect<<
Low Pressure
(Winds moving towards here)
>> -- Coriolis effect- >
...........|
.......|
...|

Equator

[Singularity]

A sub 3 minute video supplementing the above with an animation:
https://youtu.be/HIyBpi7B-dE

[Dave_S]

Take it a step further.

It seems unlikely to me that the change in the rotational speed of the earth would have much effect across the distance of a sink.

But it seems to....

In the video below, small distances from the equator, where I'd assume the change has its least effect, have enough effect to change the direction of flow to low pressure (a hole).

When I was a kid I assumed that this was a small influence anywhere and that it only favoured one direction slightly over another and the force acting on the water was miniscule. But no, you can force the water to rotate the wrong way by hand and then the force will reverse the direction of kilograms of water travelling at speed in a sink quite quickly just from the imbalance of this force across a very small distance.


https://youtu.be/QTTJJAiQFRc

[Singularity]

Not to get into youtube wars, but the bathtub/sink effect is more caused by local factors (including the guy at the equator taking tips for showing people what they want to see) more than coriolis. This video shows two guys (one with PhD in physics) conducting a kiddy-pool coriolis experiment, one in the Northern hemisphere, the other (PhD guy) in the wrong hemiphere.

https://youtu.be/mXaad0rsV38?t=88

[Dave_S]

I think you are a bit quick to dismiss the smaller vessels, try it and try to reverse it, it's surprising.

[Rigdaddy]

Originally Posted by Singularity

A sub 3 minute video supplementing the above with an animation:
https://youtu.be/HIyBpi7B-dE

Thank you; very good explanation

[SkipMark]

Originally Posted by Jim Cate

the air flow is from high pressure to low pressure, and thus inward towards a low center. When deflected to the right, the flow becomes counter clockwise in the N hemisphere.

Jim

This is the simple and correct answer.

[Davie J.]

For all the geniuses out there. Can any one of you explain why the toilet stool goes down counterclockwise while the only high pressure system is ABOVE the low?

[Singularity]

Originally Posted by Davie J.

Can any one of you explain why the toilet stool goes down counterclockwise while the only high pressure system is ABOVE the low?

As the second video I posted stipulated, toilets drain in the direction that the toilet manufacturer aimed the rinse jets. In other words, for a particular individual toilet, it will swirl in the same direction no matter where on the planet you flush it.

Otherwise, for water "at rest" draining directly down out of an exit in a small vessel the air pressure is not a dominant force at play.

[svfinlandia]

Originally Posted by Singularity

A sub 3 minute video supplementing the above with an animation:
https://youtu.be/HIyBpi7B-dE

Excellent video.
Thanks for the link!!

Al, S/V Finlandia