Equatorial regions — Page 369, Lesson 345

I want to walk you through the climatic classification system that applies to the equatorial regions and beyond. This is a foundational piece for understanding global weather patterns as a professional pilot. Let's start with Class B: the dry climate. This zone sits between 20° and 35° latitude, and it used to be called the arid subtropical climate. The key characteristic here is that evaporation and transpiration together exceed precipitation — in other words, more water is lost to the atmosphere than falls as rain. As a result, there are no permanent water courses in this zone. The classic example is the Sahara Desert. Next is Class C: the mid-latitude climate, also called the warm temperate climate, and it used to be known as the cool temperate climate. This zone spans from 40° to 70° latitude. It has distinct summer and winter seasons. The defining temperature rule is that the average temperature of the coldest month falls between -3°C and 18°C. Northwest Europe is a typical example. Then we have Class D: the sub-arctic climate, or snowy forest climate, formerly called the boreal climate. This runs from 50°N to 70°N. The temperature conditions are specific: the average temperature of the warmest month is greater than 10°C, while the average temperature of the coldest month is less than -3°C. You find this across northern Eurasia and Canada. Finally, Class E: the snow climate, or polar climate, formerly known simply as the polar climate. This is above 70° latitude. Here, the average temperature of the warmest month is less than 10°C. Examples include northern Greenland and Antarctica. Now, there are also two transitional climatic zones that bridge between the main classes. The first is the tropical transitional climate, from 10° to 20° latitude, formerly called the savannah climate. It has a warm dry winter and a warm wet summer. You see this in northwest Africa, Ghana, and similar regions. The second is the temperate transitional climate, also called the Mediterranean climate, from 35° to 40° latitude, formerly known as the warm temperate climate. This one has a warm dry summer and a cool wet winter — the classic Mediterranean pattern. Let me give you the summary that ties this together. The idealized weather I've just described will be modified by two major factors: local topography — that's the shape of the land — and the proximity of sea areas. The effect of these on temperature, density, and pressure can have a marked effect on local climatology. So as a pilot, you always need to consider the local geography and coastline, not just the broad climate zone. Now, let's talk about the seasonal effect. The model I've been describing, which is shown in Figure 20.1, makes a simplifying assumption: it assumes an all-sea world, and it assumes that the sun's sub point — that's the point on Earth where the sun is directly overhead — encircles the globe along the Equator in all seasons. But in reality, that's not the case. In practice, the Earth's polar axis is inclined at an angle of 23½° to the plane of the path that the Earth travels through space during the year. This path is shown in Figure 20.5. Because of this tilt, the sun's sub point moves north and south of the Equator over the course of the year. It's on the Equator at the equinoxes — specifically on 21 March and 21 September — but it migrates to 23½° north at the June solstice and 23½° south at the December solstice. This seasonal shift is what drives the changing weather patterns across all the climate zones I've just described.