The troposphere is the layer closest to the earth, and it is the layer in which we live. It is about ten miles deep. Seventy five percent of the mass of all our atmospheric molecules is in the troposphere, and this is where we find water vapor, dust, pollen, and soot particles. Weather happens in the troposphere. This layer is turbulent, with storms and atmospheric mixing.
     In the troposphere, the air cools gradually as it gets further from the earth. At the very top of this layer the air temperature is about 76 degrees below zero on the Fahrenheit scale. This is important, because it changes water vapor into ice, forming the cold trap, a temperature region where water vapor stops going up. If we had no cold trap, water molecules could rise in the atmosphere where they would eventually break down into oxygen and hydrogen. The small, light hydrogen molecules could then escape into space. Earth would loose its water if we had no cold trap.

This diagram shows the different atmospheric layers.

     The stratosphere lies above the troposphere. It is about twenty miles deep. The stratosphere contains about 24% of the mass of all the atmospheric molecules. This layer has the ozone layer in it. The ozone layer protects all life on earth from the harmful and potentially even lethal ultraviolet radiation emitted by the sun. There is not very much ozone -- if it were all together, it would form a layer only three millimeters thick. Three millimeters is not quite this deep: __ .

     The lower part of the stratosphere is cold, but it warms up as it gets farther from the earth -- another effect of the ozone. This layer is peaceful compared to the troposphere. If you have been watching the math, you see that

• The troposphere is 10 miles deep and has 75% of the mass of the atmosphere
• The stratosphere is 20 miles deep and has 24% of the mass of the atmosphere
  

This means that the lowest 30 miles of the atmosphere has 99% of the mass of the molecules! However, the atmosphere goes out to 40,000 miles. It makes sense, then, that the atmospheric molecules get farther and farther apart from each other as they go up the remaining 39,970 miles, and that is just what they do. Although there are some atmospheric molecules up here, they are scattered and occasional, not like our thick ocean of air close to the surface.

     The entire atmosphere of the earth extends out about 40,000 miles above the earth -- and the earth is about 8000 miles across. The moon is a little less than 240,000 miles away.

Sites to visit for more information:

http://www.shodor.org:80/metweb/session1/layers.html

Text about earth's atmosphere with an important graph. Be sure and read this one.

http://wings.ucdavis.edu:80/Book/Atmosphere/instructor/index.html

Adult text description of the atmosphere. OK for the lay person. There is a lot of good information at this attractive site.

http://wings.ucdavis.edu:80/Book/Atmosphere/advanced/index.html

Somewhat less detailed description of the atmosphere than the instructor notes above. Good information. There are easier reading levels containing this information at the same site. Explore!

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