Global Climatology — Page 369, Lesson 341

I want to walk you through the start of global climatology. We've spent time looking at individual weather processes — how temperature, pressure, and moisture interact to produce the weather we experience day to day. Now we're zooming way out to look at weather on a planetary scale, and that study is called climatology. Let me define that precisely. Climatology is the study of the weather we can expect in different locations around the world, based on long-term accumulated data. The elements that make up climatology are five specific things: precipitation, temperature, humidity, sunshine, and wind velocity. Those are the building blocks we measure and track over time. Now, these five elements are not uniform across the globe. They are affected differently depending on five key factors: latitude — how far north or south you are; location — whether you're maritime, meaning near an ocean, or continental, meaning inland; the circulation of pressure systems; altitude — how high above sea level you are; and geography — things like mountain ranges, deserts, or forests. Over the years, enough climatological data has been accumulated that weather forecasting on an area basis has become quite accurate. And communications have improved so much that you can easily obtain the weather expected on arrival at your destination, as well as the weather en route. This chapter will deal with climatology on a global basis, including its regional and seasonal variations. Now let's move into the idealized air circulation. The real general air circulation is a very complicated system of air movements. Those movements are based on the fundamental principle of air moving from high pressure to low pressure, combined with the effect of the Earth's rotation. But in reality, they are complicated by three main things. First, the unequal heating of land and sea, together with the disposition — the arrangement — of land and sea masses. Second, variation in land heating caused by different surfaces — for example, a desert heats differently than a forest or an ice sheet. Third, the 23½° inclination of the Earth's axis, which causes movement of the thermal equator — that's the line of highest temperature, which shifts north and south with the seasons. Because these complications make the real circulation hard to understand at first, it's useful to consider an idealized circulation that ignores them. This idealized model makes two assumptions: first, that the Earth's surface is completely covered with sea — no land at all. Second, that the geographic equator and the thermal equator are coincident — meaning they are exactly the same line, not shifted by seasons. In reality, the Southern Hemisphere is largely covered by sea, so climatology there very closely follows this idealized case. So what would the weather look like in this idealized circulation? With a uniform spherical Earth covered entirely by sea, temperature would only vary with latitude. Pressure at any given height over the Equator would then be greater than pressure at the same height over the poles. Because of that pressure difference, air would drift at height from the Equator toward the poles. That upper-level flow would help produce high-latitude anticyclones — those are high-pressure systems near the poles. And that, in turn, would cause a movement on the surface of air from the poles back toward the Equator. So you get a simple circulation cell: air rising at the Equator, moving poleward aloft, sinking at the poles, and returning equatorward at the surface. That's the foundation of the idealized global circulation, and it's the starting point for understanding the real, more complicated patterns we'll build on next.