All of these winds and currents on Earth that have to be simulated with
our gridded climate model, are heavily impacted
by the fact that the Earth is rotating.
In fact, you can't even look at a
weather map without seeing the effect of Earth's rotation.
Here, we take into account rotation
using an idea called the Coriolis Acceleration.
Which is basically a fake,
it's a fudged factor which is intended to
account for the fact that the rotation is happening.
To see what's going on imagine
a merry go round with two people on it on opposite sides.
And if we're looking down on the merry go round from a stationary frame.
If we're not rotating but the thing is spinning you'll see say Bob here
throw the ball toward Alice. But by the time the ball
gets to Alice she has moved to there and, and Bob is now here.
And that all makes perfect sense, the ball is going in
a straight line just like Newton's Laws say it should do.
But if we want to spin around with the
merry go round and still make sense of things
we actually see, we don't see Bob or Alice moving anymore.
and what we see is the ball that Bob
throws, it looks like it curves off to the side.
We could sort of hack Newton's Laws of Motion to make
the ball do that by adding this fake force, the Coriolis Acceleration.
It always goes at 90 degrees
to the way the thing is moving, because we want this ball to turn.
If the fake force was pointing forward,
it would make it go faster, and we don't want that.
Or if it was pointing behind it, it would
make it slow down, and we don't want that either.
It has to be at 90 degrees.
It's easy to understand the Coriolis Acceleration on a
merry go round, because everything is spinning at the same rate on the merry go round.
But we're on a planet which is a little more complicated.
And the way to visualize the the rotation of the Earth.
I mean one really cool way to visualize it,
is with a device known as a Foucault's pendulum.
They have these at science museums, usually.
They have a big atrium, because it has to be very tall.
They want this thing to get going in the morning and swing all day long.
And what happens is as you're standing
there, the Earth is actually rotating under the pendulum.
A pendulum that's going in one axis at the beginning of the day
will be going in a slightly different axis later on in the day.
You can set up a little ring of little dominoes or something, and
every ten minutes it'll knock one over as the Earth is rotating under it.
Imagine a Foucault pendulum at the North Pole.
That's totally like the merry go round.
It's no different,
the pendulum is going to want to stay in a fixed plane relative
to the fixed stars, and the Earth will just spin underneath it.
What you see is 360 degrees of rotation
in 24 hours, the actual rotation rate of the Earth.
Now, let's set up a Foucault pendulum at the equator.
And we'll get it going north and south.
That's the easiest way to visualize this,
for me anyway. Hopefully it is for you, too.
We'll get it going North and South, and it
wants to stay in that orientation relative to the fixed stars.
Stars up here, stars down there.
It's going to keep going North South as it rotates around.
Of course it'll always be pointing down
toward the center of the planet as it goes around.
But it's going to be going north/south and
it's not actually going to feel any rotation
at all at the equator.
And then if you have a pendulum somewhere
in between you get an amount of rotation, which is
in between the full rotation of the
planet of the pole, and no rotation at the equator.
Less than 360 degrees in 24 hours, or one full rotation in more than 24 hours.
The way that physicists visualize this
and calculate this is by drawing a line.
A vector, a line that has a direction, which is the
rotation axis, and so that would be like the North Pole.
And then comparing that to a line that points
toward the center of the Earth at any latitude,
which would be like a flagpole.
And the amount of rotation you get is proportional to what
they would call the projection of the flagpole on the rotation axis.
At the North Pole, they're both going in the same direction and so
you get the full rotation, but as you get to the equator they're orthogonal
to each other, and so
you get no rotation at the equator.
And then in the Southern Hemisphere, the rotation works
the same way but in the opposite direction.