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The Earth is surrounded by a blanket of air, which we call the atmosphere. Earth has just the perfect mix of air for us to breathe. Air is made up of about 78% nitrogen, 21% oxygen, 0.9% argon, and 0.03% carbon dioxide with very small percentages of other elements and water. Trees take in carbon dioxide and give off oxygen. Humans and animals breathe out carbon dioxide and take in oxygen.

At sea level, the air pressure is about 14.7 pounds per square inch. As altitude increases (for example, if you climb a mountain), the air pressure decreases. At an altitude of 10,000 feet, the air pressure is 10 pound per square inch (and there is less oxygen to breathe).

Life on Earth is supported by the atmosphere, solar energy, and our planet's magnetic fields. The atmosphere absorbs the energy from the Sun, recycles water and other chemicals, and works with the electrical and magnetic forces to provide a moderate climate. The atmosphere also protects us from high-energy radiation and the frigid vacuum of space. Without the atmosphere, life could not exist on Earth. There is no exact place where the atmosphere ends; it just gets thinner and thinner, until it merges with outer space. The altitude of 62 miles is frequently used as the boundary between atmosphere and space.

Atmospheric layers
The atmosphere is divided into four layers, the troposphere, stratosphere, mesosphere and thermosphere. The boundaries between these regions are named the tropopause, stratopause and mesopause.

The troposphere is the lowest region in the Earth's (or any planet's) atmosphere. On the Earth, it goes from ground (or water) level up to about 11 miles (17 kilometers) high. The weather and clouds occur in the troposphere. In the troposphere, the temperature generally decreases from 17 to –52 degrees Celsius as altitude increases. The tropopause separates the troposphere from the next layer. The tropopause and the troposphere are known as the lower atmosphere.

The tropopause is the boundary zone (or transition layer) between the troposphere and the stratosphere. The tropopause is characterized by little or no change in temperature altitude increases.

The stratosphere starts just above the troposphere. Ninety-nine percent of "air" is located in the troposphere and stratosphere. The stratopause separates the stratosphere from the next layer. The temperature in this region increases gradually to -3 degrees Celsius, due to the absorbtion of ultraviolet radiation. The earth's ozone layer is located in the stratosphere. This layer absorbs a lot of ultraviolet solar energy. Only the highest clouds (cirrus, cirrostratus, and cirrocumulus) are in the lower stratosphere. The stratosphere extends between 11 and 31 miles above the earth's surface.

The mesosphere starts just above the stratosphere. The mesosphere is characterized by temperatures that quickly decrease as height increases, falling as low as –93 degrees Celsius. The mesosphere extends from between 31 and 50 miles above the earth's surface. The mesopause separates the mesophere from the thermosphere.

The thermosphere starts just above the mesosphere and extends to 372 miles high. The temperatures go up as you increase in altitude due to the Sun's energy. Temperatures in this region can go as high as 1,727 degrees Celsius. Chemical reactions occur much faster here than on the surface of the Earth. This layer is known as the upper atmosphere.

The exosphere is the outermost layer of the Earth's atmosphere. The exosphere goes from about 400 miles high to about 800 miles.

How much space dust falls to Earth each year?
The U.S. Geological Survey says roughly 1,000 tons of material (mostly dust) enters the atmosphere every year and makes its way to Earth’s surface.

How far does regular dust blow in the wind?
A 1999 study showed that African dust finds its way to Florida. The dust is kicked up by high winds in North Africa and carried as high as 20,000 feet where it's caught up in the trade winds and carried across the sea. Dust from China makes its way to North America, too.

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