From: Ronald Wong (ronwong@inreach.com)
Date: Mon Apr 03 2000 - 16:24:45 PDT
Message-Id: <l03102801b50e25906a62@[209.209.18.21]> Date: Tue, 4 Apr 2000 00:24:45 +0100 From: Ronald Wong <ronwong@inreach.com> Subject: re: westerlies
>From: "Heidi Black" <blackh@exchange.esuhsd.org>
>
>Help-our department is failing to figure the Westerlies out, can't find
>a decent explanation on the web..
As with so many things in the world, the question of the origin of the
westerlies can be answered in a number of ways.
The important thing to keep in mind is that the force involved and the
motion it produces are purely fictitious. They don't exist.
The Coriolis force doesn't really exist because the motion it supposedly
explains is an illusion. The illusion comes about because we are looking at
an event while standing in a frame of reference that is constantly
accelerating. It may not be apparent to most of us, but the earth is said
to be spinning and, if so, we are all going around and around and are
therefore in an accelerated frame of reference.
The simplest way to explain the prevailing winds - the westerlies being one
example - is to start with a simple model. In this case a geocentric
universe with it's non-rotating earth. An earth devoid of land masses and
oceans.
Over the course of a year, most of the energy of the sun falls in the area
of the tropics. As a result, the area around the equator receives an
abundant amount of the sun's energy. Warmed by this energy, the air along
the entire equatorial belt expands and rises up into the atmosphere
creating a low pressure BELT around the earth called the doldrums (an
interesting anecdote comes to mind, but I'm digressing). The rising air
spreads out as it reaches higher elevations and heads towards the poles,
cooling down in the process.
By the time the air mass reaches around 30 degrees north or south, it has
cooled down to the point where, due to it's increased density, it begins to
sink to the surface of the earth. The falling, colder air mass creates a
BELT of high pressure around the earth at this latitude (known in the
northern hemisphere as the horse latitudes - an interesting story but...).
When the air mass reaches the surface of the earth, it spreads out with
some heading back to the equator and the rest heading towards the poles.
The air heading towards the poles from the 30 degree latitude would reach
the poles but for the fact that something has been going on at the poles
while this air has been rising from the equator, moving towards the poles
and falling at the horse latitudes.
Air at the poles is cold and dense and has been moving across the surface
of the earth towards the equator. The air mass that was heading towards the
poles from the 30 degree latitude runs into this cold air mass at the Polar
front (around 60 degrees latitude) and the two mix together with the warmer
air heating the colder. The mixture rises up (producing a low pressure belt
in the process), and spreads out as it cools - with some of it heading back
to the poles and the rest heading back to the 30 degrees latitude where
they will then cool at their respective destinations and drop back to the
surface of the earth.
Here's the important thing to keep in mind from all of the above:
a. From the poles to about 60 degree latitude and from around 30 degree
latitude to the equator, the air is moving across the surface of the
earth towards the equator.
b. Between 30 degrees latitude and 60 degrees latitude, the air is moving
across the earth's surface towards the poles.
In all three cases, the air is moving from high pressure belts to low
pressure belts.
An overly simplified model of this for your class can be had by dividing
your black board into 3 equal areas by drawing a pair of horizontal lines
across your black board. If you use the chalk tray as the equator, then the
lower of the two lines will be 30 degrees North, the higher 60 degrees
North and the top of the board the direction to the North pole.
Draw a series of vertical lines from the top of the board to the bottom.
In the area between the top of the board and 60 degrees North and the area
between 30 degrees North and the equator, draw arrow heads on the lines in
the downward direction. In the mid latitudes draw the arrow heads pointing
up.
This would be the direction of the prevailing winds as viewed by someone
standing on the surface of the earth - if the atmosphere and the earth
weren't rotating.
Now what would it look like if they WERE rotating?
At the equator, a point on the surface of the earth would have a tangential
speed of about 1000 mi/hr (1600 km/hr) due to the earth's rotation from
west to east. A point at 30 degrees latitude would have a tangential speed
LESS than this because it's radial distance to the axis of rotation is less
than that at the equator so it travels a shorter distance over the 24 hours
of a day than a point on the equator. As a result of this, an air mass
heading south along the surface of the earth from 30 degrees north or south
would "fall behind" by a greater and greater amount as it headed towards
the equator while the earth rotated to the east.
Take a different colored piece of chalk and, starting at each point where
one of the vertical lines intersected the 30 degree latitude, move the
chalk downward and towards the left by a correspondingly greater and
greater amount.
What you will have drawn is the famous Northeast trade winds. It's why
Columbus, in sailing west before the winds, ended up in the Bahamas instead
of the east coast of the US.
A similar pattern, for the same reason, applies to the air mass moving down
from the pole (and you can draw them in accordingly on the board). Here the
air mass starts with no tangential velocity at all and, as a result, "falls
behind" as it crosses into latitudes that have increasingly larger and
larger tangential speeds towards the east.
Air masses moving towards the poles will have the complementary conditions
and thus complementary results. Just like points at 0 degrees latitude (the
equator) have a greater tangential speed than points at 30 degrees
latitude, so points at 30 degrees latitude have a greater tangential speed
than points at 60 degrees latitude. The result is that air masses moving
northward over the surface of the earth from 30 degrees north to 60 degrees
north will "move ahead" by greater and greater amounts.
Take your colored chalk and....
Thus the famous westerlies (or the roaring 40's, furious 50's, and
screaming 60's as it's called in the southern hemisphere - spend some time
there at sea and you'll know why.).
This is why Columbus worked his way North and East on his return trips. He
was trying to pick up the prevailing westerlies so that his ships could be
carried home to the old world.
I think it is well worth while to point out to our students here in the bay
area that our latitude is around 37 degrees North. It explains why, more
often than not, the wind along our coastline/beaches is out of the west -
we're in the westerlies.
You might ask them which way the prevailing winds blow on different islands
around the world (you may even have students who can confirm such
speculations based on their own experiences there) and at different times
of the year (the belts of high and low pressure move north and south over
the course of a year). What would it be like in the doldrums where the net
motion of the air is upwards as opposed to the horse latitudes where the
net motion is downwards (maybe they'll enjoy looking up why it is called
the horse latitudes).
The fact that the winds don't always come from the west here in SF creates
the opportunity to discuss the other aspects of meteorology. Specifically,
sea and land breezes, and the high and low pressure CELLS (mentioned by
Patricia-Anne in her post) and how these determine local wind patterns.
In the simple case that I have chosen, the counter-clockwise and clockwise
rotations that Patricia-Anne mentioned do NOT apply to the discussion of
prevailing winds. They are the product of the fictitious Coriolis force
balancing the force due to the pressure gradient between the high and low
pressure CELLS and they play a major role in local weather forecasts.
The thing to remember is that in the Northern hemisphere, the Coriolis
Effect will cause moving air to ALWAYS veer to the right NO MATTER WHICH
DIRECTION it is traveling. This is quite clear when you look at the diagram
that was drawn on the board as far as northerly and southerly directions
are concerned (it also applies to easterly and westerly movements too but
I'm trying to keep the explanation simple and the model is oversimplified).
When you view the motion using the center of a pressure CELL as a frame of
reference the net effect is the directions mentioned in her post (clockwise
about a high pressure cell and counterclockwise about a low pressure cell).
But even in this case, the moving air is still veering to the right as it
moves from the high pressure region to the low. The air only appears to
move clockwise/counterclockwise because we choose to view the motion from
the frame of reference of the center of the pressure cells. It appears
simpler in that frame of reference. In a sense, there is no
clockwise/counterclockwise motion. Only veering to the right in the
Northern hemisphere (and to the left in the Southern hemisphere).
Now if you want to throw continental masses and oceans into the model then
things get a little more "interesting" and Patricia-Anne's pressure cells
start to have an important roll in conjunction with the pressure belts in
the discussion of prevailing winds. But nobody mentioned a course in
meteorology here, so I'm assuming no one is interested in all the details
(I'm not so sure anyone is interested in the ones that have already been
brought up here).
Oh well.
Cheers - ron
This archive was generated by hypermail 2b29 : Thu Oct 19 2000 - 11:10:44 PDT