Introduction

The story of this chapter begins where the previous one ended. Chapter 8 showed you that the sun does not heat the earth evenly — some regions get far more energy than others. That uneven heating sets off a chain reaction. Air expands when heated and contracts when cooled, and those changes in volume directly change the pressure the air exerts. Warm air becomes lighter and less dense, cool air becomes heavier and denser. So wherever temperatures differ, pressures differ too.

Pressure differences are the engine of motion. Air always moves from high pressure to low pressure, and that movement is what we call wind — you already know that air in horizontal motion is wind. But pressure does more than push air sideways. It also decides whether air will rise or sink. When air is warmer and lighter than its surroundings, it rises; when it is colder and heavier, it sinks. This vertical movement is just as important as the horizontal.

Why does any of this matter for the planet as a whole? Because wind is nature's redistribution system. It carries heat away from the tropics and moisture from oceans to land, smoothing out the extremes and keeping the planet's overall temperature roughly constant. And when moist air rises, it cools, and that cooling condenses the water vapour into clouds and eventually brings precipitation. So the same pressure-driven motion that creates wind also creates weather.

This chapter is devoted to the full machinery behind all of that. It explains:

• the causes of pressure differences across the earth's surface
• the forces that control atmospheric circulation
• the turbulent, restless pattern of wind
• how air masses form
• what happens when air masses collide — the disturbed, stormy weather that results
• and finally, the violent phenomenon of tropical storms

In short, the chapter takes you from the simple fact that air moves from high to low pressure, all the way to the dramatic weather systems that shape life on earth.