The region of the the subtropical climates are usually located in between latitudes of 25 and 35 degrees north and south latitude. Many of the world's largest deserts are located here. These deserts include the Sahara desert, the Atacama Desert, the American Southwest (technically this is not a true desert and I will explain why later.), and many others. These deserts usually are the hottest places in the world, mainly because they rarely receive rain or any kind precipitation for that matter. The strange thing about many of the deserts, is that when they do receive rainfall, which is about once every couple of years, it comes in the form of intense downpours, and usually leads to catastrophic flash flooding. Not only are the world's driest places located within this region but also some of the world's wettest places. With this in mind, why is it that some locations within the subtropics are extremely dry and some are extremely wet? Or more importantly why is it that some of these places are extremely hot, but can also be wet too? The next several paragraphs should answer these puzzling questions.
The circulation within the subtropical region is mostly influenced by the Hadley Cell, and the Intertropical Convergence Zone, which means that this region usually sees the most hurricanes of any other region in the world. Because the Intertropical Convergence Zone shifts with the seasons, many places in the Subtropical region see an extremely wet and dry season, especially in Southern Asia, where the Intertropical Convergence Zone shifts the most. As we learned in the last article, the Hadley cell begins when the equator is intensely heated by the sun. Since most areas surrounding the equator, is ocean, the heated air, rises and condenses forming huge, towering thunderstorms, that produce torrential rainfall. As the rising air nears the tropopause, it spreads laterally toward the poles. Due to radiational cooling and convergence of the air, the layer of air causes strong areas of sinking air or subsidence, near latitudes of about 25 to 30 degrees north and south latitude. Since earth's surface is not uniform, these high pressure ridges form not in bands, but in several cells that stretch around the world. These cells also change position and actually move differently due to the differentiation of earth's surface and the seasons. The areas that are generally under the influence of these high pressure ridges, are usually where the deserts are. These strong areas of high pressure cause large areas of subsidence and compression. When the air mass compresses, it dries the air mass out, causing few clouds and clear conditions. The compression also causes a warming of the air mass, which partially explains the heat over the deserts. Clear skies during the day
signal intense surface heating from the high angle sun, causing even hotter conditions and it becomes a cycle of becoming hotter and hotter until the high pressure cell retreats or shifts.These high pressure cells are usually called semi-permanent, because they can be stationary for very long periods of time, causing intense heat waves over the deserts. Areas that are near the oceans, such as the east coast of Saudi Arabia and the Southeastern United States, usually have very high dew points, which combined with the incredible heat, make working outside unbearable. Other areas, such as the west coasts of continents, are usually influenced by cold ocean currents. These cold currents cool the intensely heated surface, causing dense fog and a stratus deck of clouds. This limits the amount of daytime heating along the west coasts of continents. This also causes deserts to form along west coasts that aren't hot at all, but very dry because the stable air mass along with the subtropical highs would cause subsidence, fog, stratus clouds, and no precipitation. Even the driest desert in the world, the Atacama, has daily fog and drizzle, but very little, if any, rainfall. There are some deserts that are really not true deserts because even though they are in the subtropical region, the subtropical high is only part of the reason for these deserts. These deserts are called rainshadow deserts, and are some of the driest in the world. They form very short distances away from extremely wet areas. One such place is the American Southwest, where storm systems may occasionally bring heavy rainfall, mainly during the winter and sometimes summer, but most of the time the storm systems that do make it over the mountains, usually produce little precipitation. This desert in the Southwest forms because as moist air is lifted over a mountain by a strong low pressure system, it causes heavy and intense rainfall for days on the windward side of the mountain. As it starts to descend the mountain, it is now dry because it used very large portion of its moisture on the windward side of the mountain. The air descends and warms by compression at the dry adiabatic rate of 10 degrees celsius for every 1000 meters. Since the air on the windward side was saturated it cools at a lower rate than when it warms on the leeward side of a mountain. This causes extremely hot temperatures in the leeward valley (such as Death Valley), along with dry conditions.
The high pressure cells that cause the heat waves and fog, shift from winter into summer. In the winter, the subtropical highs tend to be along the west coasts of continents, causing the most fog and the least precipitation to be in the winter. In the summer, the subtropical highs are usually located closer to the east coasts of continents but still sitting close to the centers of the oceans. When the subtropical highs sit near the east coasts of continents, they cause warm, humid to be pumped onto the continents, usually the precursor to intense heat waves in the middle-latitudes. Also when the ridges shift north and eastward, the ITCZ also makes a shift northward, which usually means the beginning of hurricane season. When the high is oriented such that the ITCZ is just to the south of it, then expect several hurricanes to form and traipse eastward.
Many people wonder why, if there are so many deserts located in these regions, why are there extremely wet areas around the world that are in the subtropical regions. One reason that there are extremely wet area is because of the ITCZ. In areas around the world where the ITCZ extends farther northward than in other areas, they have a wet and dry season. During the winter in their dry season, these locations usually see very hot temperatures, all the way into late spring. Then as soon as monsoon season hits in late May, these areas see extremely heavy rainfall, sometimes amounting to 200 inches for their average annual precipitation. When the wet monsoon hits, the average temperatures drop where they would normally rise to even hotter levels. This can seen in a climate graph of Calcutta, India. Another reason that some areas in the Subtropics are extremely wet is because when the subtropical highs move toward the east coasts of continents, the high produces an onshore flow of very humid, air which heats intensely from the sun when it moves on land. This causes the air mass to 
become extremely unstable, and because the air mass is very moist, it will rise, condense, and produce towering thunderstorms similar to that of the ITCZ. This effect can be seen in a graph of Orlando, Florida precipitation which usually receives over 55 inches of rain every year. You can also see from their climate graph, that they have wintertime minimum and summertime maximum precipitation. Farther north, in Georgia and Alabama, locations around there are wet all year, mainly because they are hit by numerous low pressure systems during the wintertime, while receiving heavy summertime thunderstorms.In my next article, we will be focusing on the mid-latitude circulations and climates. Middle latitude climates are much more complicated mainly because our weather is controlled by the jet stream. As we all know the jet stream is variable and can change rapidly, causing rapidly changing surface conditions. Even as our understanding of the atmosphere has increased, we still do not fully understand the processes that control the climates of the world.
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