The combination of all four factors is illustrated in Figure 12.9. The diagram illustrates the following features:
(a) the pressure-gradient force always acts from high to low pressure regions,
(b) the Coriolis force is always at right-angles to the wind's direction, pulling to the left in the southern hemisphere,
(c) the centrifugal force is also always at right-angles to the wind, pulling outwards,
(d) surface friction involves a complex dissipation of energy in generating turbulent eddies, and the resulting drag normally acts in a direction opposite to that of the wind, and generally increases with wind speed.
Note that Figure 12.9 applies only in steady conditions. In practice, forces fluctuate in space and time. For instance, the pressure gradient along a coast may be reversed by daytime heating of the land. In fact, the surface wind near coastlines and mountains is usually very different from the geostrophic or gradient winds, especially when the latter are weak. Large differences also occur near thunderstorms and jet streaks (Section 12.5).
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