We have seen that it is warmer in the tropics than at higher latitudes. We will now describe how, through hydrostatic balance, this warmth leads to expansion of tropical air columns relative to polar air columns and hence meridional pressure gradients. It is these pressure gradients that induce fluid accelerations and hence winds.

It is customary in meteorology to use pressure rather than height as the primary vertical coordinate. Some conceptual reasons will become clear in Chapter 6. Since, in hydrostatic balance, pressure is directly related to the overlying mass burden, pressure is actually a mass coordinate. In observations it is simpler to measure pressure in situ than height, so there are practical advantages also.4

With p as a vertical coordinate, height z becomes a dependent variable, so we now speak of z(p)—the height of a pressure surface—rather than p(z). In principle, this is a trivial change: we can easily take a plot of p(z), such as Fig. 3.6, and lay it on its side to give us z(p). From Eq. 3-5 we may then write:

dz dp

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