Winds are named by where they come from, so N.E. tradewinds are blowing
towards the S.W.
Since the Earth is rotating to the East, air has momentum in that direction. Now
comes the hard part. We'll look at the northern hemisphere first. The southern
just mirrors it. We'll round off numbers here:
Let's assume the earth is 24,000 miles
in circumference at the equator. Thus, the speed of land at the equator is
1,000 miles per hour to the east.
At 30oN it's 20,785 miles around,
and the speed is 866 mph (134 mph less than equator).
At 60oN it's 12,000 miles,
and 500 mph (366 less than 30oN).
At the North Pole, it's 0 miles and 0 mph (you'd be spinning, but not moving)
(500 mph less than 60oN).
Notice that the change in speed increases close to the poles. This results
in the Coriolis effect being strongest at the poles, and weakest at the equator.
Hold your breath:
The eastward speed of air moving towards the equator from the Subtropical
High is 866 mph, but as it moves south the land is moving faster than that. So
the land starts to slip to the east, and the apparent motion of the wind is
to the west. Thus, we have winds blowing toward the southwest, which are then
called N.E. Tradewinds. (Whew!)
The eastward speed of air moving towards the north pole from the Subtropical
High is 866 mph, but as it moves north the land is moving slower than that. So
the land starts to slip to the west, and the apparent motion of the wind
is to the east. Thus, we have winds blowing toward the northeast. In
this case they are just called the westerlies.
And finally, air moving from the polar high towards the equator has no
eastward speed, so the earth moving under it starts to slip east, and
the wind has an apparent west motion. Thus, they are called easterlies.
So, it's the inertia of the winds while the earth spins under them that
generates the appearance of winds moving east or west.
Here is a
nice QuickTime movie showing the Coriolis effect on a playground merry-go-round.
To see the "straight" motion of the ball,
it is best to pause the movie (right
after the narrator says "To an observer above the merry-go-round"), then
use the "step" forward and backward buttons to isolate your thinking on the
ball and not the spinning merry-go-round. You may have to hold a piece of paper
up to the monitor to convince yourself that the ball is actually moving in a
straight line. (The movie is a part of the
TOPEX/Poseidon
project).
Force or Effect?
The Coriolis effect is often referred to as the Coriolis force, but
this is technically inaccurate. A force acts on something to change its motion.
Since objects subject to Coriolis are actually moving in a straight line, no
force has been exerted on them. Thus, the more accurate name is the Coriolis
effect.
Coriolis on TV:
You students may have heard of the Coriolis effect on TV.
The Simpsons had an episode where Lisa explains the Coriolis effect to
Bart, who then calls Australia to confirm it. Lisa's explanation, however,
is not accurate. While the Coriolis effect acts on all bodies, its effect
is so small that its influence on water draining in a sink would be much to
small to overcome other influences such as shape of the basin, currents in the
water due to how it poured in or how someone stirred it.
