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From: David Straton (no email)
Date: Wed Mar 15 2006 - 16:02:14 EST
Hi cruisers,
I am trying to get my head around meteorology, and as part of it I have
been making notes in the form of a web page. Its going quite well, but
there's a long way to go.
I have struck a problem explaining the Coriolis effect, and I thought
some people on this list might be able to help.
The basic rule of the Coriolis effect is that on a rotating sphere, a
moving object tends to swerve to the right in the northern hemisphere,
and to the left in the southern hemisphere. I can explain this well for
objects moving north, south and east, but my explanation comes up with
the answer that objects moving west should swerve towards the equator,
thus left in the northern hemisphere. Thus my explanation must be wrong,
I think.
If this interests anyone out there, please put me right.
The incomplete website is
http://www.psyberspace.com.au/Cruising/meteorology.htm
The text of it is below.
Cheers
Dave
------------------------------------------------------------------------
Coriolis Effect
One way I have found helpful to think of the Coriolis effect is to
imagine the Earth as consisting of two nine-lane motorways, one in the
northern hemisphere and one in the southern.
Whereas the meridians of longitude are all the same length (approx
40,000km), the parallels of latitude vary.
Since the Earth rotates once every 24 hours, it follows that the
rotational speed in km/h varies with different latitudes.
http://www.psyberspace.com.au/Cruising/Images/DegreLongitude.gif
Latitude
At the north pole, Length of 1° longitude. (Km) = 0. Speed = 0 km/h
At 80°, Length of 1° longitude. (Km) = 19. Speed = 291 km/h
At 70°, Length of 1° longitude. (Km) = 39. Speed = 573 km/h
At 60°, Length of 1° longitude. (Km) = 56. Speed = 837 km/h
At 50°, Length of 1° longitude. (Km) = 72. Speed = 1,075 km/h
At 40°, Length of 1° longitude. (Km) = 85. Speed = 1,281 km/h
At 30°, Length of 1° longitude. (Km) = 96. Speed = 1,447 km/h
At 20°, Length of 1° longitude. (Km) = 105. Speed = 1,570 km/h
At 10°, Length of 1° longitude. (Km) = 110. Speed = 1,644 km/h
At the Equator, Length of 1° longitude. (Km) = 111. Speed = 1,670 km/h
So imagine yourself in the outside lane of this 9-lane motorway, on the
Equator, hooning along in an easterly direction at 1,670 km/h. The car
in the lane inside you, in the northern hemisphere, is going slower, and
the ones inside that, slower still. Now imagine trying to throw a tennis
ball from one car to another. You throw it directly out of the window,
at 90° to your direction of travel. The ball has your easterly velocity,
so tends to pass in front of the car inside you. It appears to swerve to
the right, or to the east.
A ball thrown from a car in one of the slower tracks to a faster car
appears to swerve to the right, or the west, and pass behind the faster
car.
http://www.psyberspace.com.au/Cruising/Images/coriolis1.GIF
The Coriolis effect is a little more difficult to explain for objects
moving in an east-west direction. Imagine throwing your tennis ball
directly east from your moving car on the 40° northern parallel. Your
car continues round the track, a 'small circle' with its centre a point
on the axis of the Earth's rotation, between the equatorial plane and
the Pole. The ball continues its tangent from where you threw it, but
follows a 'great circle' determined by gravity operating towards the
centre of the Earth, on the equatorial plane. To you, in your speeding
car, the ball appears to veer to the right.
Thus, when wind rushes from a high pressure zone towards a lower
pressure, in the northern hemisphere it tends to swerve right, and get
into an anticlockwise spiral.
http://www.psyberspace.com.au/Cruising/Images/coriolis2.GIF
See this anti-clockwise Low Pressure system in the northern hemisphere.
http://www.psyberspace.com.au/Cruising/Images/692px-Low_pressure_system_over_Iceland.jpg
Depression over Iceland
The opposite occurs in the southern hemisphere, with the Coriolis effect
causing wind direction to bend leftwards, and depressions or cyclones to
rotate in clockwise direction.
Southern Hemisphere cyclone
http://www.psyberspace.com.au/Cruising/Images/cyclone_lrg.jpg
Cyclone Fay over Western Australia on March 25, 2004.
Buys-Ballot Rule in the Southern Hemisphere says: 'If you LOOK into the
wind, the LOW is on your LEFT.'
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