concept

Coriolis effect

The apparent sideways deflection of moving air and water caused by a planet's own spin: on a rotating world, a straight flow looks bent to one side. The effect bends winds that would otherwise run straight from equator to pole into east-west streams, and so breaks the circulation into separate loops. The faster the spin, the stronger the effect and the narrower the cells. Note: it only matters at large scales like storms and ocean currents - it does not decide which way a sink drains.