11.1. Pressure Gradient Force#
There are a number of forces (accelerations) that govern atmospheric flows. The most important is the pressure gradient force (PGF). The PGF is the only force that can actually cause the wind to blow. All other forces act upon the already moving air in the atmosphere. Mathematically we can write a pressure gradient (PG) in algebraic form as,
where
By convention, we place the negative sign in front of the PGF so that
the force always points toward low pressure where the units of PGF
are
Looking at this concept graphically we can inspect a 500-hPa plot of geopotential heights (Fig. 11.1). Where lines of geopotential height are closest together indicates where the strongest height gradients are located. In figure 11.1, we can see that the red dashed contours are located where the black geopotential height contours are most closely packed, thus indicating the areas on this particular map where the strongest height gradients are located.

Fig. 11.1 This map shows 500-hPa heights in solid black contours and the magnitude of the height gradient in dashed red contours.#