PRESENTER: Let's look at four force balances. Let's start which geostrophic balance, which occurs in straight line flow and [INAUDIBLE] troposphere.

In geostrophic flow only the pressure gradient force and the Coriolis force are important. Pressure gradient points to low pressure on the height surface or low height and thus low geopotential on a constant pressure surface is opposed by the Coriolis force which is to the right of the velocity vector in the northern hemisphere and to the left of the velocity vector in the southern hemisphere. Note that we can find the geostrophic velocity if we know the pressure gradient on a constant height surface, or the geopotential or height gradient on a constant pressure surface.

For inertial balance the Coriolis force is balanced by the horizontal centrifugal force with the Coriolis force to the right of the velocity vector in the northern hemisphere and to the left in the southern hemisphere. This balance is rarely seen in the atmosphere. Because there's almost always a pressure gradient force of the same magnitude as the centrifugal force and the Coriolis force.

In cyclostrophic balance the pressure gradient force is balanced by the centrifugal force. In this case, the velocity vector can be either to the right or to the left of the centrifugal force in both hemispheres. And the Coriolis force is much smaller. This balance is seen in tornadoes and other small vortices. In the northern hemisphere tornadoes are mostly cyclonic with only a few percent anticyclonic. While smaller vortices are about as often anticyclonic as they are cyclonic.

For gradient wind balance the pressure gradient force, Coriolis force, and horizontal centrifugal force are all about equal. To two physical cases are shown for the northern hemisphere in the figure along with the geostrophic balance. With a cyclonic gradient-- that is curvature around the low pressure center-- the PGF points to the low and is constant as long as the pressure gradient is constant. In this case, the PGF is opposed by both the Coriolis force, which depends on the velocity, and the centrifugal force, which depends on the velocity squared. Since the PGF is constant then the sum of the centrifugal and Coriolis force must be equal to it.

And since they both depend on velocity the velocity must be less than in the geostrophic case in order for them to be in force balance. This velocity is called subgeostrophic because it is less than the geostrophic velocity. For the anticyclonic gradient-- which is flow around a high-- the PGF points away from the high and is joined by the centrifugal force, which means that the Coriolis force must be stronger than in the geostrophic case because it must balance both the PGF-- which is the same in the geostrophic case-- and the centrifugal force.

The Coriolis force can only be greater if the velocity is greater. Thus this velocity is called super geostrophic. Because it is greater than the geostrophic velocity.