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Lisa Williams, age 14, of Dillon, S.C., for her question:


Atmosphere is the gaseous envelope of air that surrounds the earth. The air extends as far as 1,000 miles above the surface of the earth.

Scientists divide the earth's air into four layers: the troposphere, the stratosphere, the mesophere and the thermosphere.

Nitrogen makes up 78 percent of the world's atmosphere and oxygen makes up 21 percent. The last one percent consists mainly of argon and small amounts of other gases.

About 1,000 feet up from earth, the atmosphere gradually fades into space.

Water vapor and particles of dust can also be found in the earth's air. Clouds float only in the troposphere, or the lowest part of the atmosphere. It is also here that all of the earth's wind, storms and other features of weather take place. The troposphere extends upward to a height of about 10 miles.

Air gets thinner the farther it gets from earth.

The stratosphere extends from the troposphere to about 30 miles above the earth. It is in this air that many jet airplanes fly. Pilots enjoy flying in the stratosphere since they are above all thunderstorms and snow. Temperatures in this zone of atmosphere range from 67 degrees Fahrenheit at the bottom to about 28 degrees at the top.

The very upper layer of the stratosphere is warmed by the sun's rays as they strike a chemical compound of oxygen, called ozone, in this layer.

Above the stratosphere lies the mesophere. It begins about 30 miles above the earth and extends upward to about 50 miles. The temperature of the air decreases from about 28 degrees Fahrenheit at its base to about minus 135 degrees near the top, which is called the mesopause. Trails left by meteors are seen in the upper levels of the mesophere.

Air in the thermosphere, which begins about 50 miles above the earth, is extremely thin.

Because the thin air in the thermosphere is fully exposed to the sun's radiation, the air temperature rapidly climbs. It reaches over 2700 degrees Fahrenheit in the thermopause, a layer of the thermosphere that has a uniform temperature. This layer lies from 100 to 300 miles high. The aurora borealis, or northern lights, occur in the thermosphere.

The ionosphere is a region of atmosphere centered in the lower thermosphere. Radiation from the sun and outer space causes air particles in the ionosphere to become electrically charged.

When radio waves of certain frequencies strike the ionosphere, they are reflected back to earth and can be received thousands of miles from their source.


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