Jack Kidd, age 13, of Davenport, Iowa, for his question:
HOW FAR UP DOES THE EARTH'S ATMOSPHERE GO?
Atmosphere is the gaseous envelope of air that surrounds the earth. The air extends as far as 620 miles above the surface of the earth.
Scientists divide the earth's air into four layers: the troposphere, the stratosphere, the mesosphere 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 kilometers above the earth's surface, the atmosphere gradually fades into outer 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 goes up from the earth's surface to a height of about 10 kilometers.
Air gets thinner the farther it get from earth. The stratosphere extends from the troposphere to about 30 kilometers 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 about 65 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 mesosphere. It begins about 70 kilometers miles above the earth and extends upward about 90 kilometers. The temperature of the air decreases from about 28 degrees Fahrenheit at the base of the mesosphere 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 mesosphere.
Air in the very highest layer, the upper atmosphere, is the thermosphere and it begins about 120 kilometers above the earth and continues up to about 1,000 miles. The air in this upper layer is extremely thin.
Because the thin air in the thermosphere is fully exposed to the sun's radiation, the temperature rapidly climbs. It reaches over 2,700 degrees Fahrenheit in the thermopause, a layer of the thermosphere that has a uniform temperature.
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.