The Atmosphere

The atmosphere of the earth is a thin, layered collection of gases, water vapor and particles. Most living creatures live in the atmosphere. The troposphere is the surface layer of air that absorbs visible sunlight. Heating, cooling, and water evaporation in the troposphere are expressed as weather.

The weight of air around an object exerts pressure. At sea-level the weight of air molecules above each square inch is about 14.7 pounds. Air pressure varies with temperature and weather patterns are described in terms of low pressure fronts interacting with higher pressure fronts.

The air thins as you ascend above the Earth; air pressure and temperature drops. The stratosphere begins at about 12 miles altitude at the equator; about 5 miles at the poles. Solar ultraviolet radiation is absorbed by oxygen and ozone. The "ozone layer" is about 30 miles high; 90% is located within 10 miles above the Earth's surface. The atmosphere thins progressively in the mesosphere, the outer layer that extend to 53 miles altitude. Atmospheric gases eventually disappear into relatively empty, cold space.

Stratospheric ozone absorbs ultraviolet sunlight providing heat and protection for living creatures in the lower atmosphere. At the Earth's surface, ozone from human industry is toxic. In the 20th century, it became obvious that gases created by human activity changed the chemical composition of the troposphere and stratosphere with negative impacts on human survival. Large ozone holes appeared over the Antarctic and the Arctic Polar regions. Smaller areas of ozone depletion were recorded over other, more-populated regions of the Earth.

Increases in surface UV-B radiation have been recorded wherever stratospheric ozone was depleted. A scientific consensus was reached that human-produced chemicals were responsible for ozone depletion. Combinations of volatile chemicals containing chlorine, fluorine, bromine, carbon, and hydrogen were responsible; carbon tetrachloride, and methyl chloroform were used in refrigeration, air conditioning, and as solvents. Halons, which contain carbon, bromine, fluorine, and chlorine were also implicated. An international agreement was achieved so that many governments outlawed CFCs, halons, carbon tetrachloride, and methyl chloroform.

Learn About Aerosols

World Meteorological Organization/United Nations Environment Programme report, Scientific Assessment of Ozone Depletion: 1998 (WMO Global Ozone Research and Monitoring Project-Report No. 44, Geneva, 1999).

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Air and Breathing

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