Global Climatology — Page 387, Lesson 355

I want to walk you through the "Other winds" and the weather patterns that follow in the global climatology picture. We've covered the main global wind belts, and now we're looking at the local and regional variations that are just as important for a pilot to understand. First, "Other winds" are those that exist outside the main planetary currents. They fall into two categories. The first is winds that are applicable to the local pressure system prevailing at the time. In other words, wherever a high or low pressure system sits, the wind around it follows the rules we've already learned—it doesn't just follow the global belt pattern. The book gives an example in Figure 20.19, which shows a cool temperate winter situation. From that figure, we get two specific observations. Near the South Pole, we have strong easterlies. This is the outflow from the South Polar high turning left due to the Coriolis effect in the Southern Hemisphere. Near the North Pole, we generally have strong easterlies as well, but there's an important seasonal exception: in summer, over the North Atlantic and North Pacific seas, the winds actually become westerlies. So the polar region isn't a simple easterly belt year-round—the local sea surface and pressure patterns flip the wind direction in summer over those ocean areas. Figure 20.20 is labelled "Polar Easterlies," which is the name for that general belt of winds near the poles. The second category of "other winds" is sea breezes. The book notes that these can be dominant in lower latitudes. A sea breeze is a local wind that blows from the sea onto the land during the day, caused by the land heating faster than the sea. In the tropics and subtropics, this effect can be strong enough to override the background trade wind flow. Now let's move into the weather section. We start with temperate latitude depressions. These are the frontal depressions—the low pressure systems with warm and cold fronts—that we associate with mid-latitude weather. They breed along the polar front, which is the boundary between cold polar air and warmer subtropical air. This happens where the polar front lies over wide ocean areas. In the Northern Hemisphere, this occurs between 35°N and 65°N across the Atlantic, between North America and Europe. A similar pattern exists across the North Pacific, affecting the west coast of North America. In the Southern Hemisphere, polar front depressions centre around 50°S in all seasons, with fronts affecting the west coast of South America, as well as New Zealand and the south coast of Australia. Figure 20.22 shows the alignment of the polar front in winter and summer. Next we have polar air outbreaks. These are found generally in wintertime. They are depressions affecting Central and North China, as well as Central and Southern United States. The key feature is what happens behind the cold front: fresh outbreaks of very cold continental polar air greatly reduce mean temperatures. These winter mean temperatures are considerably below those of equivalent latitudes elsewhere. So a city in central China or the central US gets much colder than a city at the same latitude on a coast, because the cold air pours straight down from the continent. Figure 20.21 shows a sea breeze at Darwin in the winter, blowing against the flow—a nice example of a local wind overpowering the larger-scale pattern. Now we come to the Equatorial Trough, also known as the ITCZ—the Intertropical Convergence Zone. Let me define this carefully. The trough is centred on the thermal equator. High temperatures cause low pressure, particularly over land, with widespread lifting of air from the Trade Winds which converge below at the surface. That area of convergence is what we call the ITCZ. The main feature of the ITCZ is extensive cumulus, cumulonimbus, and thunderstorms. When stable air exists, there will be extensive sheets of altostratus and nimbostratus cloud and more continuous-type rain—so it's not always just thunderstorms; it can be steady rain too. The ITCZ can vary from 25 nautical miles to 300 nautical miles in width. There is no well-defined frontal surface. This is a critical point: the cloud is not caused by air mass temperature differences, as at the polar front. Instead, it is caused by convergence of the northeast and southeast trade winds, which are normally the same temperature. So you're just piling warm, moist air together and forcing it upward. The cloud tops are sometimes as low as 20,000 feet, but more frequently they reach 50,000 feet or more. That's significant for aircraft—you cannot overfly these storms in most airliners. Turbulence is usually severe, as is icing, which can be from 16,000 feet upwards. So the ITCZ is a region to treat with great respect. The book mentions vigorous and quiet ITCZ cross-sections, shown in Figures 20.24 and 20.25. A vigorous ITCZ has deep, tall cumulonimbus towers; a quiet ITCZ has more layered cloud and less intense convection. Finally, we have monsoons. The definition is precise: when trade winds blow to continental low pressure or from continental high pressure, the associated weather is known as a monsoon. So it's not just "rainy season"—it's a seasonal reversal of wind direction driven by the temperature contrast between a continent and the ocean. There are three monsoon flows: the northeast, the northwest, and the southwest. These are the directions the wind blows from during different seasons in the affected regions.