What Is the Coriolis Effect? Why Storms Spin, Winds Curve and Bath Water Does Not Care

Stand at the centre of a spinning roundabout and roll a ball to a friend at the edge. The ball travels in a straight line, but by the time it reaches the edge the friend has moved on, so from the roundabout's point of view the ball curved away. Nothing pushed it; the platform turned beneath it. The Earth is the roundabout, and anything moving across its surface for long enough, air, water, a shell, a plane, appears to curve, to the right in the northern hemisphere and to the left in the southern, because a point on the equator is moving east at 1,670 kilometres an hour with the Earth's spin while a point near the pole is barely moving at all. Air flowing from the equator towards the pole carries its eastward speed with it and outruns the ground beneath, so it drifts east; air flowing towards the equator lags and drifts west. The effect is proportional to the speed of the moving thing and to the sine of the latitude, so it is zero at the equator and strongest at the poles.

The deflection is small over a room and decisive over a continent:

• •Winds: air flowing from high pressure to low is bent until, in the large weather systems, it flows around the pressure centres rather than into them, anticlockwise around a low in the north and clockwise in the south, which is why cyclones and hurricanes spin the way they do and why they do not form within about five degrees of the equator, where there is no spin to start them
• •The trade winds and the westerlies: the global circulation, which would flow straight from equator to pole and back on a still Earth, is bent into the belts of east and west winds that sailing ships planned their voyages around
• •Ocean currents: the great gyres of the Atlantic and Pacific, the Gulf Stream and the Kuroshio, turn clockwise in the north and anticlockwise in the south, and the effect pushes surface water to the right of the wind, which is what makes cold water well up along the coasts of Peru and California
• •Artillery and aircraft: a shell fired 30 kilometres drifts a hundred metres or so, which gunners corrected for from the First World War, and a long flight is planned with it

The claim that water drains clockwise in one hemisphere and anticlockwise in the other, and straight down on the equator, is the most famous thing said about the effect and is false at that scale. The Coriolis acceleration on water moving a few centimetres a second across a basin is some ten million times weaker than the forces from the shape of the basin, the way the tap filled it, the residual swirl from the last person's hand and the tiniest draught, and the direction the plughole vortex turns is decided by those. In 1962 a physicist at MIT filled a two-metre tank, let it settle for a day, and drained it through a tiny hole: it turned anticlockwise, as the effect predicts, and the experiment has been repeated in the southern hemisphere with the opposite result, but it needs a large tank, hours of stillness and a very small drain, none of which a bath has. The roadside demonstrations at the equator, in which a basin drains differently on either side of a painted line a few metres apart, are done by the pouring.

Gaspard-Gustave de Coriolis was a mathematician and engineer at the Ecole Polytechnique who worked on the theory of machines and, in a paper of 1835 on the motion of bodies within a rotating frame such as a water wheel, derived the acceleration that carries his name; he never applied it to the weather and never mentioned the Earth. The deflection of winds by the Earth's rotation had been suggested by George Hadley in 1735 to explain the trade winds and worked out for the atmosphere by William Ferrel in 1856, and the term Coriolis force came into meteorology only around 1900. It is called a fictitious force because it exists only in the rotating frame: from space nothing is deflected, the air goes straight and the Earth turns.

The Coriolis effect is the apparent curving of anything that moves across the rotating Earth, to the right in the northern hemisphere and the left in the southern, because the ground moves faster at the equator than near the poles; it is zero on the equator and strongest at the poles. It bends the winds into the trade winds and the westerlies, spins cyclones and ocean gyres, deflects shells and aircraft, and is far too weak at the scale of a bath to decide which way the water drains.