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Where does the sky start?

RIGHT under your feet (or your surfboard, if you're a beach-lover). That's where the surface of the earth and ocean stops and where the envelope of gases called the atmosphere begins.

Take a deep breath. That was quite a bit of oxygen, wasn't it? Not! You just inhaled about four times as much nitrogen as you did oxygen. Our atmosphere is 78 percent nitrogen, 21 percent oxygen, and 1 percent argon - a gas that also is used to light neon signs. The final fraction of our atmosphere is made from many "trace" gases such as water vapor and ozone.

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Scientists have found that the atmosphere has five main layers, based on temperature changes in each layer. Most of the atmosphere is contained in a layer called the troposphere, which starts at ground level and rises to an average height of 9 miles. (By contrast, commercial airliners cruise at altitudes of about 5-1/2 miles.) This layer is responsible for weather.

Another important layer, the stratosphere, takes over where the troposphere leaves off and rises to an average height of about 31 miles. About 90 percent of Earth's ozone is found here. This ozone reduces ultraviolet rays from the sun to levels that make organic life possible on the planet's surface. By the time you reach an altitude of about 310 miles (about the distance from Milwaukee, Wisc., to Toledo, Ohio) you hit the final layer - the exosphere, which eventually blends with the thinness of space itself. This is space-shuttle height.

These atmospheric gases were part of a big cloud of gas and dust that gave birth to the solar system nearly 5 billion years ago. As Earth cooled, carbon dioxide and water vapor rumbled to the surface through volcanoes. So did smaller amounts of nitrogen. The water vapor near the surface became rain. Carbon became bound into rocks. And sunlight split the hydrogen and oxygen contained in high-altitude water vapor. Hydrogen (which was once used in blimps) is the lightest of all elements, so it escaped into space, leaving the oxygen behind.

Emerging plant life also boosted the proportion of oxygen. And volcanoes added more nitrogen. Over the years, the mix of gases evolved into a recipe we could live and breathe with.

Editor's note: Have you ever wondered why the sky is blue or why balls bounce? Today the Monitor begins a new feature to answer such science-related questions. If you've ever wondered why, write us at: One Norway Street, Boston, MA, 02115; or e-mail to:

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