If you’ve ever noticed all low pressure systems, systems that bring rain and thunderstorms, always spin counter-clockwise. Meteorologists call this motion cyclonic.

This cyclonic rotation in the Northern Hemisphere is due to a product of rotational physics, called the Coriolis Effect. This is an effect where a mass moving in a rotating system experiences a force acting perpendicular to the direction of motion and to the axis of rotation.

In other words, when an object is in a rotating system, like Earth, it will deflect to the right (East) when moving to the axis of rotation (to the N. Pole), and will still deflect to the right but this time moving West when traveling away from the axis of rotation (to the Equator). The graphics below should show how this works a bit.

Objects deflect due to newton’s first law, “An object at rest remains at rest, and an object in motion remains in motion at constant speed and in a straight line unless acted on by an unbalanced force”.

In our case on Earth, depending on how far North or South you are determines how fast you move on the surface. So if you were standing on the equator you’d be moving with the Earth at 100% of its rotational speed, which is around 1037 MPH to the East.

As you travel North and move towards the north pole the rotational speed slows down. If you were standing on the pole you’d be moving 0 MPH. With that in mind here’s the approximated speeds at each latitude on Earth. This can be calculated by multiplying the equatorial speed by the cosine of the latitude in degrees. (i.e. Speed = 1037mph x Cosine(42º) = 770.64 MPH)

0º (Equator) = 1037 MPH
10º = 1022 MPH
20º = 975 MPH
30º = 898 MPH
40º = 795 MPH
50º = 667 MPH
60º = 519 MPH
70º = 355 MPH
80º = 180 MPH
90º (Poles) = 0 MPH

Due to the rotational speed on Earth and keeping Newton’s law of conserved motion in mind an object thrown from the equator northward would keep its rotational speed the entire time (ignoring air resistance). As this object travels North the ground below it is moving slower, keep in mind the object is not attached to the ground. So, from the perspective of a viewer on the ground the object will appear to travel out ahead. From the perspective of the thrower the object would appear to hook to the right!

Inversely, when moving away from the pole (now traveling South) the objects rotational speed is slower than the ground below. From the perspective of a viewer on the ground, the object would appear to lag behind, and from the thrower the object will still hook to the right!

With all of that in mind, lets apply the Coriolis Force to our winds and weather systems!

You can see the effects for yourself with a simple experiment! National Geographic has a wonderful video showing the experiment on a small merry-go-round and how the Coriolis Force works on a smaller scale!
Coriolis Effect Video