Suppose that an object of unit mass,
initially at latitude φ moving zonally at speed u, relative to the surface of the earth,
is displaced in latitude or in altitude by an impulsive force. As the object is displaced it will conserve
its angular momentum in the absence of a torque in the east–west direction.
Because the distance R
to the axis of rotation changes for a
displacement in latitude or altitude, the absolute angular velocity ( ) must change if the
object is to conserve its absolute angular momentum.
Here is the angular speed
of rotation of the earth. Because is
constant, the relative zonal velocity must change. Thus, the object behaves as though a zonally directed deflection
force were acting on it.