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Kevin Donnelly, age 12, of Bossier City, La., for his question:

Is it true that the atmosphere is in layers?

The word "layers" suggests sandwiches and birthday cakes, whose flat sections can be separated and the spaces between them stuffed with this and that. There is very little resemblance between these layers and the so called layers of the airy atmosphere. It is true that the quality of the gaseous air changes as we go higher. But usually there is no precise boundary between one layer and the next our planet, as we know, is composed of sturdy solids, fluid liquids and restless gases that refuse to stay put unless imprisoned within walls. The atmosphere is an enormous shell of assorted gases surrounding the en¬tire globe, reaching from the surface up to more than 1000 miles. Within this spacious realm, its airy gases have almost complete freedom to roam. .lost of them are continuously mixing and mingling, flowing and blowing in all directions.

It is hard to imagine how or why our gaseous atmosphere would arrange itself in different layers, or levels one above another. But it does. This happens because gravity and various other outside forces are exerted upon it. For example, the entire atmosphere is held captive by the earth's gravity, and gravity is related to weight. The total weight of our airy atmosphere is estimated to be a stunning 5600 million million tons.

The pull of gravity helps to distribute this enormous weight in differ¬ent layers, because its pull is stronger at the surface of the planet, from whence it diminishes with distance lost of the air is held close to the surface, and about 99 per cent of the atmosphere's total weight is concentrated in the bottom 20 miles. Above this level, the skimpy gases become thinner and still thinner.

From 93 million miles away, the sun also helps to sort the earth's atmosphere into different layers. The dense bottom layer doesn't absorb much heat from solar radiation, but the earth does. The air is heated and  chilled in patches by warm and chilly spots on the earth's surface. The thinner upper levels are exposed to solar radiation. At one upper level, solar energy changes the gaseous molecules into charged ions. Through the higher levels, the temperature rises, and in the upper limits it soars above 2000 degrees F.

The earth and the sun conspire to sort our deep atmosphere into layers, though the boundaries between them are somewhat vague. The dense lower level is the weathery troposphere, which reaches to about ten miles above the Equator and dips to six miles above the poles. Its turbulent gases are stirred up by warm and cool patches on the surface. Above it is the thin cool stratosphere, which reaches upto merge with the thin warm mesosphere. Higher still is the thermosphere, which gets hotter and hotter until its thin, thin topmost gases get lost in space.

The enormous thermosphere has two distinct layers. The lower level is the electrically charged ionosphere. It reaches from about 50 to 300 miles above the surface. Above it, the temperature gradually rises to above 2500 degrees as the outer exosphere reaches more than 1000 miles to the upper limits of the atmosphere.

 

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