the coriolis effect happens when?

Answer 1

Wikipedia to the rescue:

"The Coriolis effect is an apparent deflection of a moving object in a rotating frame of reference. This effect is sometimes attributed to the fictictious Coriolis force.

The Coriolis effect caused by the rotation of the Earth is responsible for the precession of a Foucault pendulum and for the direction of rotation of cyclones. In general, the effect deflects objects moving along the surface of the Earth to the right in the Northern hemisphere and to the left in the Southern hemisphere. As a consequence, winds around the center of a cyclone rotate counterclockwise on the northern hemisphere and clockwise on the southern hemisphere. However, contrary to popular belief, the Coriolis effect is not a determining factor in the rotation of water in toilets or bathtubs (see the Draining bathtubs/toilets section below).

The effect is named after Gaspard-Gustave Coriolis, a French scientist, who described it in 1835, though the mathematics appeared in the tidal equations of Laplace in 1778."