Troposphere, lowest layer of the earth's atmosphere and site of all weather on the earth. The troposphere is bounded on the top by a layer of air called the tropopause, which separates the troposphere from the stratosphere, and on the bottom by the surface of the earth. The troposphere is wider at the equator (16 km/10 mi) than at the poles (8 km/5 mi).

The temperature of the troposphere is warmest in the tropical (latitude 0º to about 30º north and south) and subtropical (latitude about 30º to about 40º north and south) climatic zones (see climate) and coldest at the polar climatic zones (latitude about 70º to 90º north and south). Observations from weather balloons have shown that temperature decreases with height at an average of 6.5º C per 1000 m (3.6º F per 1000 ft), reaching about -80º C (about -110º F) above the tropical regions and about -50º C (about -60º F) above the polar regions.

The troposphere contains 75 percent of the atmosphere's mass—on an average day the weight of the molecules in air (see Pressure) is 1.03 kg/sq cm (14.7 lb/sq in)—and most of the atmosphere's water vapor. Water vapor concentration varies from trace amounts in polar regions to nearly 4 percent in the tropics. The most prevalent gases are nitrogen (78 percent) and oxygen (21 percent), with the remaining 1 percent consisting of argon (0.9 percent) and traces of hydrogen, ozone (a form of oxygen), methane, and other constituents. Carbon dioxide is present in small amounts, but its concentration has nearly doubled since 1900. Like water vapor, carbon dioxide is a greenhouse gas (see Greenhouse Effect), which traps some of the earth's heat close to the surface and prevents its release into space. Scientists fear that the increasing amounts of carbon dioxide could raise the earth's surface temperature during the next century, bringing significant changes to worldwide weather patterns. Such changes may include a shift in climatic zones and the melting of the polar ice caps, which could raise the level of the world's oceans.

The uneven heating of the regions of the troposphere by the sun (the sun warms the air at the equator more than the air at the poles) causes convection currents (see Heat Transfer), large-scale patterns of winds that move heat and moisture around the globe. In the Northern and Southern hemispheres, air rises along the equator and subpolar (latitude about 50º to about 70º north and south) climatic regions and sinks in the polar and subtropical regions. Air is deflected by the earth's rotation as it moves between the poles and equator, creating belts of surface winds moving from east to west (easterly winds) in tropical and polar regions, and winds moving from west to east (westerly winds) in the middle latitudes. This global circulation is disrupted by the circular wind patterns of migrating high and low air pressure areas, plus locally abrupt changes in wind speed and direction known as turbulence.

A common feature of the troposphere of densely populated areas is smog, which restricts visibility and is irritating to the eyes and throat. Smog is produced when pollutants accumulate close to the surface beneath an inversion layer (a layer of air in which the usual rule that temperature of air decreases with altitude does not apply), and undergo a series of chemical reactions in the presence of sunlight. Inversions suppress convection, or the normal expansion and rise of warm air, and prevent pollutants from escaping into the upper atmosphere. Convection is the mechanism responsible for the vertical transport of heat in the troposphere while horizontal heat transfer is accomplished through advection.

The exchange and movement of water between the earth and atmosphere is called the water cycle. The cycle, which occurs in the troposphere, begins as the sun evaporates large amounts of water from the earth's surface and the moisture is transported to other regions by the wind. As air rises, expands, and cools, water vapor condenses and clouds develop. Clouds cover large portions of the earth at any given time and vary from fair-weather cirrus to towering cumulus clouds (see Cloud). When liquid or solid water particles grow large enough in size, they fall toward the earth as precipitation. The type of precipitation that reaches the ground, be it rain, snow, sleet, or freezing rain, depends upon the temperature of the air through which it falls.

As sunlight enters the atmosphere, a portion is immediately reflected back to space, but the rest penetrates the atmosphere and is absorbed by the earth's surface. This energy is then reemitted by the earth back into the atmosphere as long-wave radiation. Carbon dioxide and water molecules absorb this energy and emit much of it back toward the earth again. This delicate exchange of energy between the earth's surface and atmosphere keeps the average global temperature from changing drastically from year to year.

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