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Atmosphere
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The atmosphere is the gaseous envelope that surrounds the earth and constitutes the transition between its surface and the vacuum of space. The atmosphere is composed primarily of nitrogen (N2) and oxygen (O2) and is made up of many layers of air, each one identified by their thermal characteristics or temperature changes, chemical composition, movement and density. Life on earth is supported by this atmosphere, solar energy, and our planet's magnetic fields.
The first layer is called the troposphere. People, plants, animals, and insects live in the troposphere. It is the layer where all weather occurs. The troposphere begins at ground level and extends 12km (7.5 miles) up into the sky where it meets with the second layer called the stratosphere. The stratosphere begins at the 12km (7.5 mile) point and reaches 50km (21.1 miles) into the sky. An important layer of atmosphere containing ozone is located inside the stratosphere. Ozone (O3) is a special form of oxygen, and the ozone layer is very important to all life on earth. Ozone blocks large amounts of solar ultraviolet radiation from entering the troposphere. Too much solar radiation can harm living things, including people. The third layer is called the mesosphere. The mesosphere begins 50km (21.1miles) above the earth's surface. Temperatures are warmest at the lowest level of the mesosphere and coldest at its highest level. The fourth layer is called the thermosphere. The thermosphere begins 80km (49.7 miles) above the earth. Temperatures in the thermosphere go up when moving farther away from ground level due to the sun's energy. The increase in temperature stops at this height, beyond which lies the exosphere. The exosphere is the highest layer of the atmosphere. The exosphere extends to 40,000 miles above the earth's surface. The thermosphere and the exosphere together make up the upper atmosphere.
The troposphere, stratosphere, mesosphere, and thermosphere act together as a giant safety blanket. They keep the temperature on the earth's surface from dipping to extreme icy cold that would freeze everything solid, or from soaring to blazing heat that would burn up all life. In the study of air pollution control the layers of the air that are most important are the troposphere and the stratosphere.
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