Global Climatology — Page 377, Lesson 346

I want to walk you through the global climatology chapter now. We're looking at how the Earth's orbit and its land masses shape the weather patterns you'll fly through as a professional pilot. Let's start with the sun's apparent movement. On 21 June, the summer solstice in the Northern Hemisphere, the sun is directly overhead at 23½° north — that's the Tropic of Cancer. On 21 December, the winter solstice, it's directly overhead at 23½° south — the Tropic of Capricorn. So from our perspective on Earth, the sun's sub-solar point — the latitude where it's directly overhead — appears to move south from the Tropic of Cancer on 21 June down to the Tropic of Capricorn on 21 December, then it turns around and moves north again, returning to 23½° north by the next June. Now here's an important effect for your navigation and flight planning. Above 66½° north — that's the Arctic Circle — on 21 June the sun stays above the horizon for a full 24 hours. On 21 December, it stays below the horizon for 24 hours. The exact opposite happens in the Antarctic: below 66½° south, the sun is below the horizon for 24 hours on 21 June and above it for 24 hours on 21 December. This seasonal movement of the sun drives the thermal equator — the belt of warmest surface temperature — which shifts into the summer hemisphere. That means tropical rains move north in July and south in January. Between the subtropical high-pressure belts and the equatorial low-pressure trough, you get a transitional region in each hemisphere. In summer, that region gets tropical rain. In winter, it gets dry trade wind weather. Right near the Equator itself, you get rain nearly all the time, but the heaviest rainfall occurs around the equinoxes — March and September — and the least rainfall around the solstices — June and December. Now let's move to temperature and topographical effects. If the world were all ocean, surface temperature would cool evenly as latitude increases, simply because the sun's elevation gets lower. But in the real world, land masses modify this pattern dramatically — especially in the Northern Hemisphere, where Asia and North America are vast continents. One key effect: subtropical anticyclones — those semi-permanent high-pressure belts — sometimes break down in summer because the land heats up, which lowers pressure over the continents. Conversely, continents outside the subtropical high belt can experience wintertime land cooling, which raises pressure. In January, Asia is exceptionally cold. That winter cold air over central Asia forms because of three factors: its distance from the sea, the long nights, and continuous terrestrial radiation — the ground radiating heat away into space all winter long. That cold air is held back from India and Pakistan to the south by the Himalayan mountain range. In North America, the cold is even more enhanced. The Rocky Mountains block warm Pacific air from reaching the interior, and because there's no mountain barrier to the north, Arctic air can move freely southward. Meanwhile, North Atlantic temperatures stay comparatively high because of the warm water current from the Gulf of Mexico — the Gulf Stream. So the prevailing westerly winds blowing from the Atlantic carry that warmth into the UK and Western France, warming those land masses. — actually that figure reference was for the orbital diagram; the temperature maps are separate figures. In the Southern Hemisphere, the isotherms — lines of equal temperature — stay close to the ideal pattern because there's much more ocean area and less land to disturb the temperature distribution. Now in July, the pattern reverses. Central North America is warmed by air that's free to move north from the Gulf of Mexico. The vast area of Asia is warmed by the sun directly. Both continents become warmer than the Gulf Stream-warmed Atlantic Ocean, so the isotherms are reversed compared to January — although the temperature contrasts are less extreme than in winter.