MadSci Network: Other
Query:

Re: Why do high pressure areas rotate clockwise & low pressure, counterclockwis

Date: Sat Apr 4 10:49:56 1998
Posted By: David Winsemius, M.D., BA (physics), MPH
Area of science: Other
ID: 890239453.Ot
Message:

This is a really interesting question, and one that I, too, had trouble 
answering from the Web. The problem I had was that although several weather 
related websites explained it in terms of the Coriolis effect, they weren't 
very complete in explaining the effect. Most of them repeated the mantra: 
"in the northern hemisphere, the Coriolis force is to the right and in the 
south it is to the left" but never derived the the apparent force from 
first principles. Some of them used a merry-go-round explanation, but 
I still have not found an explanation that I feel is complete. The 
best I have found so far comes from Feynman's Lectures in Physics, Volume 
1, chapter 19. I found this in my local Borders bookstore. The Great 
Teacher explained it in terms of conservation of angular momentum on a 
rotating sphere. From another explanation on one of the weather-oriented sites, 
I think it is helpful to remember that the strength of the Coriolis effect is 
related to the relative orientations of the vector of the earth's rotation and 
the vector of the object's (or air masses) motion. See note below.

The Coriolis effect results in moving objects and air masses experiencing 
an apparent force that depends on latitude and velocity. In the Northern 
Hemisphere the apparent force will be to the right of the velocity vector 
and will increase with increasing velocity. It is to the left in the 
Southern Hemisphere. It will be present even if the velocity is in the same 
direction as the Earth's rotation. It is maximal at the Poles and zero at 
the Equator.

As air masses move from high to low pressure zones there are two forces acting 
on them. The first is the pressure gradient force. The second is the Coriolis 
"force" which is really an "effect" or an "apparent force." The Coriolis effect 
increases with increasing speed so that the resultant force vector is no longer 
parallel to the pressure gradient, but almost perpendicular to it. So winds end 
up circling high and low pressure "hills" and "bowls". If they are descending 
into a low pressure "bowl" in the northern hemisphere, they are drawn toward the 
center but at the same time deflected to the right. So they end up circling the 
low pressure zone anti-clockwise.

Conversely, the air sliding down a high pressure "hill" is deflected to the 
right as well and ends up going clockwise down the "hill."

With those "facts" in hand, some of the website explanations begin to make some
sense. 

Take a look at these site's diagrams. Once I understood that the real situation 
was more complex than the 2-dimensional merry-go-round analogy, some of these 
websites were more understandable:
This wasn't a very convincing explanation but it does have some cute movies.
A little better, but does not really explain why the rotation is opposite around lows and highs. Plus its vector diagram of the relation of pressure gradient to Coriolis effect is just plain wrong. Merry-go-rounds are apparently called roundabouts in the UK.
Has a bit of information about orientation of vorticity.
Classroom notes, ahh nostalgia. Lots of vectors and trig.
OK, OK, I understand the Coriolis force, but why the different directions of rotation between H and L?
An experimental apparatus
This was somewhat helpful.
Excerpt from the last site above:
"B. Coriolis Force: 
"Freely moving objects on the surface of the Earth appear to curve to the right 
in the northern hemisphere and to the left in the southern hemisphere due to the 
Earth's rotation. This effect is known as the Coriolis effect and it has a large
influence on atmospheric and ocean circulation. The Coriolis effect arises from 
observing these motions from a moving reference frame: the spinning Earth. The 
object is actually moving in a straight line but the Earth where we are 
observing the motion is moving counterclockwise so the object appears to be 
veering away from us in a clockwise direction or to the right in the northern 
hemisphere and to the left in the southern hemisphere. The magnitude of the 
Coriolis effect increases from zero at the equator where the surface of the 
Earth is parallel to the spin axis to a maximum at the poles where the surface 
of the Earth is perpendicular to the spin axis.

(Administrators note: As best as I can tell, the magnitude of the Coriolis force 
depends on the cross-product of the velocity vector with the angular velocity 
vector of the Earth. Cross-products of parallel vectors will be zero, so there 
will be no Coriolis effect at the Equator. They will be maximal at the Poles 
since the vectors will be perpendicular.)

"The effect of the Coriolis Force is to produce the northeast and southeast 
trade winds in the latitudinal band between the equator and 30 north and south 
respectively. The surface circulation traveling from high pressure in high 
latitudes to low pressure at the equator is turned to the right (west) and left 
(west) by the Coriolis force in the north and south hemispheres respectively. 
The result is a wind that blows from the northeast to the southwest in the 
northern hemisphere (called the northeast trade wind) and from southeast to 
northwest in the southern hemisphere (called the southeast trade winds). This 
pattern is disturbed in the higher latitudes, primarily due to the increase in 
the Coriolis force which produces vortices or circular motions of the atmosphere 
called cyclones when rotation is towards a low pressure center or anticyclone 
when motion is away from a high pressure center. 
Also important is the distribution of land masses which heat up and cool down at 
different rates from the ocean and therefore complicated simple circulation
patterns for a uniform surface." 

This question was a hard one for me. Since you didn't complete the form, I have 
no way of knowing if you are an adult whose background includes vector analysis 
or a high school student for whom this is all Greek. (Coriolis was Greek 
according to one of my sources, but he was actually French-born and published in 
French.) See what you can make of all this anyway.

Respectfully submitted,
David Winsemius, MD
The Other Administrator.













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