Ferrel's law, named after American meteorologist W. Ferrel (1817–1891), is the rule that air or water moving horizontally in the Northern Hemisphere is deflected or pushed to the right of its line of motion while air or water moving horizontally in the Southern Hemisphere is deflected to the left of its line of motion. Ferrel's law, which predicts the directions of the large-scale circulations of the earth's atmosphere and oceans, is a restatement in global terms of the action of the Coriolis force.

The Coriolis force is a consequence of the conservation of angular momentum and arises as follows. Consider a spinning disk with an ant perched on its outer edge. The ant's angular momentum (P) is given by its mass (m) times the square of its distance from the center of the disk (r²) times the radial velocity of the disk (ω, how fast it...