Coriolis Force - (n.)

A force describing the acceleration on particles on a rotating sphere, in the case where the rotating sphere is the frame of reference. In principle, this force works as follows when considering motions on the surface of Earth: any particle at any latitude rotates east around the Earth's axis once in 24 hours. At the equator, this movement is associated with a maximum velocity. Here a particle moves by 1/24 of the circumference of Earth each hour (close to 1000 miles per hour). Places away from the equator have a lower eastward velocity equal to 1/24 of the circumference times the sine of the latitude per hour. Therefore, if a particle moves from the equator to higher latitudes, it will tend to outrun whatever is there already, deflecting it to the east (e.g. the westerlies). Conversely, if a particle moves toward the equator, it will tend to lag behind what is there already, and therefore be deflected to the west, with respect to the Earth's surface (e.g. the trade-winds). Particles that initially move straight east will tend to stay on a great circle, by inertia, and soon find themselves crossing latitudes, which again results in the familiar deflection, and the same is true for particles initially moving straight west. Due to the Coriolis Force, movement in gyres, cyclones and anticylones is the natural way for air and ocean currents to move on the rotating sphere of the Earth.