The atmosphere constitutes a sphere around the Earth consisting of a mixture of gases held in place by gravity. The atmosphere is divided into several layers, based mainly on the vertical temperature gradients that vary significantly with height. The lower 36,000 feet (11 km) of the atmosphere is the troposphere, where the temperature generally decreases gradually, at about 70°F per mile (21°C per km), with increasing height above the surface. This is because the Sun heats the surface that in turn warms the lower part of the troposphere. External energy received from the sun drives processes in the atmosphere.
The atmosphere is always moving, because more of the Sun's heat is received per unit area at the equator than at the poles. The heated air expands and rises to where it spreads out, then cools and sinks, and gradually returns to the equator. This pattern of global air circulation forms Hadley cells that mix air between the equator and midlatitudes. Hadley cells are belts of air that encircle the Earth, rising along the equator, dropping moisture as they rise in the Tropics. As the air moves away from the equator at high elevations, it cools, becomes drier, then descends at 15-30°N and S latitude, where it either returns to the equator or moves toward the poles. The locations of the Hadley Cells move north and south annually in response to the changing apparent seasonal movement of the Sun. High-pressure systems form where the air descends, characterized by stable clear skies and intense evaporation, because the air is so dry. Another pair of major global circulation belts is formed as air cools at the poles and spreads toward the equator. Cold polar fronts form where the polar air mass meets the warmer air that has circulated around the Hadley Cells from the Tropics. In the belts between the polar front and the Hadley Cells, strong westerly winds develop. The position of the polar front and extent of the west-moving wind is controlled by the position of the polar jet stream (formed in the upper troposphere), which is partly fixed in place in the Northern Hemisphere by the high Tibetan Plateau and the Rocky Mountains. Dips and bends in the jet stream path are known as Rossby waves; these partly determine the location of high- and low-pressure systems. Rossby waves tend to be semistable in different seasons and have predictable patterns for summer and winter. If the pattern of Rossby waves in the jet stream changes significantly for a season or longer, it may cause storm systems to track to different locations from normal, causing local droughts or floods. Changes in this global circulation may also change the locations of regional downwelling, cold dry air. This can cause long-term drought and desertification. Such changes may persist for periods of several weeks, months, or years, and may explain several of the severe droughts that have affected Asia, Africa, North America, and elsewhere.
Circulation cells similar to Hadley Cells mix air in middle to high latitudes, and between the poles and high latitudes. The effects of the Earth's rotation modify this simple picture of the atmosphere's circulation. The Coriolis effect describes how any freely moving body in the Northern Hemisphere veers to the right, and toward the left in the Southern Hemisphere. The combination of these effects forms the familiar trade winds, including the easterlies and westerlies, and doldrums.
Like the atmosphere, the ocean is constantly in motion, driven by external energy from the sun. Ocean currents are defined by the movement paths of water in regular courses, controlled by the wind and thermohaline forces across the ocean basins. shallow currents are driven primarily by the wind, but are systematically deflected by the Coriolis force to the right of the atmospheric wind directions in the Northern Hemisphere, and to the left of the prevailing winds in the southern Hemisphere. shallow water currents therefore tend to be oriented about 45° from the predominant wind directions.
Deep-water currents are driven primarily by thermohaline effects, or the movement of water driven by differences in temperature and salinity. The temperature differences are ultimately controlled by different amounts of solar radiation received by different parts of the global oceans. The Atlantic and Pacific Ocean basins both show a general clockwise rotation in the Northern Hemisphere, and a counterclockwise spin in the southern Hemisphere, with the strongest currents in the midlatitude sectors. The pattern in the Indian Ocean is broadly similar but seasonally different and more complex because of the effects of the monsoon. Antarctica is bound on all sides by deep water and has a major clockwise current surrounding it, the Antarctic circumpolar current, lying between 40° and 60° south. This strong current moves at 1.6-5 feet per second (0.5-1.5 m/s), and has a couple of major gyres in it at the Ross Ice Shelf and near the Antarctic Peninsula. The Arctic Ocean has a complex pattern, because it is sometimes ice covered and is nearly completely surrounded by land, with only one major entry and escape route east of Greenland, called Fram Strait. Circulation patterns in the Arctic Ocean are dominated by a slow, 0.4-1.6-inch per second (1-4 cm/s) transpolar drift from Siberia to the Fram Strait, and by a thermohaline-induced anticy-clonic spin known as the Beaufort gyre that causes ice to pile up on the Greenland and Canadian coasts. Together the two effects in the Arctic Ocean bring numerous icebergs into North Atlantic shipping lanes and send much of the cold deep water around Greenland into the North Atlantic Ocean basin.
See also atmosphere; black smoker chimneys; climate; climate change; convection and the Earth's mantle; Earth; earthquakes; geological hazards; hurricanes; hydrosphere; ice ages; mantle plumes; plate tectonics; radioactive decay.
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