Here at the Clear Science labs we’ve got Hurricane Sandy going over in the next few hours. The motion of cyclones like Hurricane Sandy can be understood by a branch of physics called fluid dynamics. After all, the atmosphere is a fluid stuck to a rigid, spinning sphere (the Earth).

In the map above (taken from this excellent website) you see paths of Atlantic hurricanes, which tend to do two things: move toward the North Pole, and switch direction from west to east around 30 degrees latitude. The math of why these two things happen is a bit in the weeds, but it has to do with 2 kinds of angular momentum a hurricane has:

• It spins around in a circle on the Earth (in other words around an axis normal to the Earth)
• It spins around in space as the Earth spins around, just like everything else on the planet

The angular momentum of a closed system (the hurricane) has to remain constant. Ever done the experiment where you spin in a chair and pull your arms in? It makes you speed up, because your angular momentum has to stay constant. Same kind of thing happens as the hurricane moves north: the earth is less big around up there, so the hurricane has to adjust its velocity. If you’d like to see the math go here and it describes why the laws of physics dictate this characteristic hurricane path.