Global Climatology — Page 369, Lesson 344

I want to walk you through the start of Global Climatology. This is the big-picture view of the world's weather patterns — the kind of knowledge you'll use to anticipate what conditions to expect on a long-haul route before you even look at a specific forecast. Let's begin with the polar front. The polar front is caused by cold air from the polar cell forming a wedge beneath the warmer Ferrel cell. So picture this: the polar cell is the circulation that brings cold air down from the poles, and the Ferrel cell is the mid-latitude circulation that brings warmer air up from the subtropics. Where those two meet, the cold, dense air slides underneath the warmer, less dense air, creating a boundary — that's the polar front. The complex airflow patterns associated with the polar front are responsible for the vagaries — the unpredictable, changeable nature — of the weather in mid latitudes. That's why your mid-latitude weather can swing so dramatically from one day to the next. Now, the pressure distribution we've been discussing gives us four basic climatic zones. Let me list them for you, and you can see them on Figure 20.3. First, a warm, wet equatorial zone. Second, warm, arid subtropical zones. Third, cool, wet temperate zones. And fourth, cold, dry polar zones. To these four, we can add, in the Northern Hemisphere only, a continental area over northern North America and northern Eurasia. That's a distinct zone because large landmasses at high latitudes create their own climate characteristics. Now, here's an important point: the tilt of the earth's axis means that these zones will move north in the Northern Hemisphere summer and south in the Southern Hemisphere summer. This seasonal migration adds two transitional zones, which you can see in Figure 20.4. So the zones aren't fixed in place — they shift with the seasons. These climatic zones follow a classification system devised by an Austrian botanist named Wladimir Koeppens. The system was revised around 1918 in conjunction with his student, Rudolph Geiger. It has since been extended to account for some smaller climatic effects, mainly in North America. While the full classification system now runs to some 30 zones and subdivisions, for your ATPL studies you are only required to know the five basic classes plus the two transitional zones. The knowledge required covers the latitudes of the zones and the typical weather expected in each. Let's start with the first basic class: Class A. This is the tropical rain climate, formerly known as the equatorial climate. It lies between 0° and 10° latitude. The defining criteria are precise: the average temperature of the coldest month is greater than 18°C — so it's warm all year round. And the average monthly rainfall is greater than 60 millimetres, with no dry season. That means it rains consistently throughout the year — there's no month that qualifies as a dry month. That's the equatorial belt: always warm, always wet.