27 , 2006
Why do tropical cyclones' winds rotate counter-clockwise (clockwise)
in the Northern (Southern) Hemispheres?
A: The reason is that the Earth's rotation sets up an apparent
force (called the Coriolis force) that pulls the winds to the
right in the Northern Hemisphere (and to the left in the Southern
Hemisphere). So when a low pressure starts to form north of the
equator, the surface winds will flow inward trying to fill in
the low and will be deflected to the right and a counter-clockwise
rotation will be initiated. The opposite (a deflection to the
left and a clockwise rotation) will occur south of the equator.
This force is too tiny to effect rotation in, for example, water
that is going down the drains of sinks and toilets. The rotation
in those will be determined by the geometry of the container and
the original motion of the water. Thus one can find both clockwise
and counter-clockwise flowing drains no matter what hemisphere
you are located. If you don't believe this, test it out for yourself.