13. Atmospheric Force Balances#
One way to describe the wind is to construct force balance diagrams to visualize all of the different forces acting on a “parcel” of air. This goes back to the Newtonian idea of force equaling mass times acceleration:
Where \(\vec{F}\) is the force vector, \(m\) is the mass in kg, and \(\vec{a}\) is the acceleration in \(m\) \(s^{-2}\). A force balance would mean that the sum of forces acting on a particular body sum to zero. Therefore, a body in balance will not be experiencing any acceleration,
There are four main force balances that we will consider plus a ‘balance of balances’. The force balances presented here are: hydrostatic, geostrophic, gradient, Guldberg-Mohn, and the thermal wind balance. Between these balances we can describe most of the flows that occur in the atmosphere, especially the horizontal wind that dominates most atmospheric motion.
Note
When talking about vectors, whether a force vector or a wind vector, it is important to remember that they have both a magnitude and direction. This is especially useful when thinking about explaining how the wind will blow. Any vector can be broken down into its components from which the magnitude (e.g., speed of the wind) and direction can be calculated.