Atmospheric Composition

ALL LIFE ON Earth exists within a thin film of air, water, and soil about 9.3 mi. (15 km.) deep. This spherical shell of life is known as the biosphere. The biosphere can be divided into three layers: the atmosphere (air), the hydrosphere (water), and the lithosphère (rock and soil). The unique attributes of the Earth's atmosphere create a habitable place for humans, animals, and plants. It reaches over 348 mi. (560 km.) from the surface of the Earth. The atmosphere is a mixture of gases and particles that surround the planet. When seen from space, the atmosphere appears as a thin seam of dark blue light on a curved horizon. It is made of layers that surround the Earth like rings. About 99 percent of the Earths atmosphere occurs within 31 mi. (50 km.) above the surface. The remaining one percent extends outward for several hundred km., fading gradually into interplanetary space.

The atmosphere serves several purposes: it provides habitants with the air to breathe, its gases retain the heat that warms the Earth, and its protective layer of ozone shields habitants from harmful ultraviolet rays emitted by the sun. The atmosphere also acts as a reservoir, or storehouse, for natural substances as well as emissions derived from human activities. Within the storehouse, physical and chemical actions and reactions take place. Many of these can affect climate and weather systems. Four distinct layers of the atmosphere have been identified using thermal characteristics (temperature changes), chemical composition, movement, and density. These are the: troposphere, stratosphere, mésosphère, and thermosphère. Beyond these atmospheric layers lies the exosphere.

Approximately 81 percent of Earth's atmosphere occurs in the troposphere, which extends about 3.7–10.5 mi. (6–17 km.) above the Earth's surface and is thickest at the equator. Temperatures in the troposphere generally decrease as altitude increases. Temperature in this layer is highest nearest the earth, in part because gases in the troposphere are warmed by heat radiated from the earth. The stratosphere extends beyond the troposphere to about 31 mi. (50 km.) above the Earth. Gases in the stratosphere are heated mainly by incoming radiation from the sun; temperatures in the stratosphere gradually increase as altitude increases. As a consequence of temperature differences between the troposphere and stratosphere, and the resulting circulation patterns, exchange of air between the two layers is slow.

The stratosphere is also known as the ozone layer. The distribution of ozone is closely linked to the vertical structure of the atmosphere. Approximately 90 percent of all ozone molecules are found in a broad band within the stratosphere. This layer of ozone-rich air acts as an invisible filter to protect life forms from over-exposure to the sun's harmful ultraviolet rays. Human activities have led to a depletion of the ozone layer observed since the mid-1980s.

The mésosphère starts just above the stratosphere and extends to 53 mi. (85 km.) beyond the stratosphere. In this region of atmosphere, temperatures fall as low as minus 135 degrees F (minus 93 degrees C). The thermosphère starts just above the mésosphère and extends 372 mi. (600 km.) beyond mésosphère. The thermosphère has two parts: the ionosphere (the inner part), and the exosphere (the outer part) that gradually merges into space.

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