You are likely aware of the devastating impact of hurricanes, including the tornadoes that can "spin off" of these storms. Although less well known, our ocean is home to swirling water systems – known as eddies – that can be spun off larger ocean currents.

What’s a key factor behind these swirling features? Earth’s rotation, specifically the Coriolis effect.

Coriolis deflects moving fluids in our seas (i.e., water) and skies (i.e., air) to the right in the northern hemisphere and to the left in the southern hemisphere. As a result, low-pressure systems, such as hurricanes and typhoons, swirl counterclockwise north of the equator and clockwise south of the equator. Conversely, high-pressure systems spin in the opposite direction in each hemisphere.

What's the underlying cause? The speed at which a point on Earth’s surface rotates depends on its latitude. The rotational speed is fastest at the equator and virtually zero at the poles. As fluids move across latitudes (e.g., north-south), they are deflected due to changes in Earth’s rotational speed.

The Coriolis effect is strongest at the Earth's poles and weakest at the equator. This is why hurricanes strengthen as they move away from the equator… and why they don’t cross it.