Winds power our wind turbines, push our sailboats, cool our houses, and dry our laundry. But winds can also be destructive — in hurricanes, thunderstorms, or mountain downslope windstorms. We design our bridges and skyscrapers to withstand wind gusts. Airplane flights are planned to compensate for headwinds and crosswinds.
Winds are driven by forces acting on air. But these forces can be altered by heat and moisture carried by the air, resulting in a complex interplay we call weather. Newton’s laws of motion describe how forces cause winds — a topic called dynamics.
Many forces such as pressure-gradient, advection, and frictional drag can act in all directions. Inertia creates an apparent centrifugal force, caused when centripetal force (an imbalance of other forces) makes wind change direction. Local gravity acts mostly in the vertical. But a local horizontal component of gravity due to Earth’s non-spherical shape, combined with the contribution to centrifugal force due to Earth’s rotation, results in a net force called Coriolis force.
These different forces are present in different amounts at different places and times, causing large variability in the winds. For example, Fig. 10.1 shows changing wind speed and direction around a lowpressure center. In this chapter we explore forces, winds, and the dynamics that link them.