Skip to main content

Section 5.5 Combining the forces

Buys-Ballots Law is an interesting observation that allows us to identify atmospheric pressure patterns:

  • low pressure lies to the left of the wind (if wind is at your back, the low pressure is to the left)

  • high pressure lies to the right of the wind (if wind is at your back, the high pressure is to the right)

The forces will combine to yield the wind behavior. This will tend to produce a geostrophic wind: in which there is a geostrophic balance between the Coriolis force and the PGF (\(PGF+CF=0\)).

This will cause the wind to tend to move parallel with the isobars. Again, we find that a tighter pressure gradient will yield a stronger geostrophic wind.

If all of the effects are combined, we will see geostrophic balance. Often, however, we will see subgeostrophic flow around low pressure systems and supergeostrophic flow around high pressure systems. This will accomplish gradual movement toward the low pressure center and away from the high pressure center.

Table 5.5.1. Summary of force balances
balance forces expression
hydrostatic vertical PGF, gravity \(dP/dz=-\rho g\)
geostrophic PGF, Coriolis \(dP/dn = -2\Omega \sin \phi v\)
gradient PGF, Coriolis, centrifugal \(\frac{v^2}{r}+2\Omega \sin \phi v+\frac{dP}{dn}=0\)