### 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:
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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
```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

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