Atmospheric motion: principles
For a body to follow a curved path there must be an inward acceleration (c) towards the center of rotation. This is expressed by:
mV2 c = – ––––
where m = the moving mass, V = its velocity and r = the radius of curvature. This effect is sometimes regarded for convenience as a centrifugal ‘force’ operating radially outward (see Note 1). In the case of the earth itself, this is valid. The centrifugal effect due to rotation has in fact resulted in a slight bulging of the earth’s mass in low latitudes and a flattening near the poles. The small decrease in apparent gravity towards the equator (see Note 2) reflects the effect of the centrifugal force working against the gravitational attraction directed towards the earth’s center. It is therefore only necessary to consider the forces involved in the rotation of the air around a local axis of high or low pressure. Here the curved path of the air (parallel to the isobars) is maintained by an inward-acting, or centripetal, acceleration.