# I Why do hurricanes rotate counter-clockwise?

Tags:
1. Aug 10, 2018

### warrenchu000

In the northern hemisphere, they rotate counter-clockwise. In the southern hemisphere, they rotate clockwise. This is often attributed to the Coriolis effect, an apparent deflection of moving objects in a rotating frame of reference. While this is true for deflected artillery shells, it is NOT the reason for hurricane rotating COUNTER-CLOCKWISE. In fact, they should rotate CLOCKWISE in the same way artillery shells are deflected in the ABSENCE of the surrounding atmosphere. But it is the DIFFERENTIAL VELOCITIES of the surrounding atmosphere that CARRIES the hurricane winds in their direction, with the atmosphere moving FASTER nearer to the equator and SLOWER nearer to the pole in the west to east direction.
So in the northern hemisphere, as winds blow toward the center of the hurricane, the wind is carried (west-to-east speed sped up) westward when travelling south and carried (west-to-east speed slowed down) eastward when travelling north.
If there was no surrounding atmosphere to carry the winds, the opposite would happen. The hurricane would appear to rotate CLOCKWISE as a result of viewing it in a non-inertial frame and of the pseudo force known as the Coriolis force. Therefore it is incorrect to attribute the rotation of hurricanes to the Coriolis force.

Last edited: Aug 10, 2018
2. Aug 10, 2018

### FactChecker

An artillary shell going South in the Northern hemisphere will drift right (to the West) due to Coreolus. That is the same as the atmosphere being drawn South into a low pressure zone of a hurricane. That gives a counter-clockwise spin to a hurricane. The artillary shell drift and the hurricane spin are in agreement and both are due to Coreolus.

3. Aug 10, 2018

### Bandersnatch

They do both. The wind near the surface flowing towards the centre and the wind at the top flowing away have opposite rotations. Since the Coriolis force on the N hemisphere results in all paths curving to the right, the direction of airflow (towards or away from the centre) results in opposite rotations.
The observation is in agreement with the physics. I'm not sure why you think it isn't.

Shells and bullets don't fly in vacuum, but through the atmosphere, so I don't know what you're talking about here.
You can't attribute 'rotation' to Coriolis deflection of a single shell. They get deflected to the left (S) or to the right (N). You'd need to shoot two shells w/r to some common point of origin or the target, to indicate a rotating pattern, and the direction of shooting towards or away from that point will determine rotation.

Try drawing it all out.

4. Aug 10, 2018

### warrenchu000

The deflection of artillery shells is due to the Coriolis effect. They are deflected to the east when aimed south. This would be like winds in a high pressure center blowing in a counter-clockwise direction - which they do not.

5. Aug 10, 2018

### warrenchu000

No. The artillery shell going south would drift to towards east.

6. Aug 10, 2018

### Mister T

More generally, this is true of all cyclones.

7. Aug 10, 2018

### FactChecker

8. Aug 10, 2018

### Bandersnatch

If you mean on the N hemisphere, then that's wrong. The Earth is rotating towards the east. Deflection is westwards.

9. Aug 10, 2018

### Staff: Mentor

Use math. The coriolis acceleration is $2 v \times \Omega$. For a shell fired horizontally south from 45 degrees north latitude $v$ is due south and horizontal, $\Omega$ is north, but inclined 45 degrees up, so $v \times \Omega$ is horizontal and due west.

10. Aug 10, 2018

### rcgldr

Not mentioned yet is that in the northern hemisphere, low pressure zones rotate counter-clockwise, while high pressure zones rotate clockwise.

11. Aug 10, 2018

### Drakkith

Staff Emeritus
A shell launched straight south had a lower eastward velocity at launch than the points on the Earth's surface south of it. That is, the ground south of the shell's launch location is moving eastward faster than the ground at the launch point. An extreme example is to launch the shell from the north pole, which has zero east velocity. So as the shell flies south, it sees the velocity of the ground underneath it gradually increase from zero. So, since the ground is moving eastward from the shell's point of view, the shell is moving westward from the ground's point of view, and vice-versa when the shell is launched to the north. Thus leading to a rightward curving path, which is westward when moving south and eastward when moving north.

12. Aug 10, 2018

### warrenchu000

My apologies to everyone. I was wrong. The above description is indeed the Coriolis effect, including the atmospheric effect. Thanks to those who made me realize my error.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted