Introduction

We live at the bottom of a huge pile of air. You rarely notice it, but it is always there — you feel it only when it moves. That moving air is what we call wind. This chapter is about how that air gets heated and cooled, and how the temperature of the earth’s surface is distributed as a result.

The air around the earth forms a blanket called the atmosphere. It is made up of numerous gases, and these gases support life over the earth’s surface. But the atmosphere does more than just give us something to breathe — it is the medium through which heat moves around the planet.

Almost all of the energy the earth receives comes from the sun. The earth does not keep this energy forever; it radiates the same energy back into space. Because the earth gives back what it receives, it neither warms up nor cools down over a period of time. In other words, the earth stays in a rough balance — it does not keep getting hotter year after year, nor does it keep getting colder.

Now, here is the key point: the amount of heat received by different parts of the earth is not the same. The equator gets more heat than the poles, for instance. This uneven heating causes pressure differences in the atmosphere. Where air is heated more, it expands and rises, creating low pressure; where it is cooler, the air is denser and creates high pressure. These pressure differences set the air in motion, and that motion transfers heat from one region to another. This is how winds carry warmth from hotter areas to cooler ones.

So the whole story of this chapter is the process of heating and cooling of the atmosphere, and the temperature distribution that results over the earth’s surface. Everything else — winds, pressure systems, seasonal changes — follows from this basic idea of uneven heating and the earth’s attempt to balance it out.