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The Atmosphere (click on layers for details)

layers of the atmosphere

The atmosphere describes all the air surrounding the earth, from the ground all the way up to the edge of deep space. The atmosphere is composed of several layers, each defined because of the various phenomena which occur within the layer. These transitions are gradual, and most heights and measurements mentioned below refer to the average area of transition from one layer to another.



The Troposphere

Oxygen and nitrogen make up the majority of the gases in the earth's atmosphere, even at much higher altitudes. But it is the lowest level of earth's atmosphere where the right mixture works to support life. Here, living things are also free from the radiation showers which flow down through most of the earth's atmosphere.



Compared to the rest of the atmosphere, the troposphere is a tiny layer, extending at most ten miles (16km) up from the earth's surface. Within this small layer almost all of our weather is created—the short term changes in temperature, wind, pressure, and moisture that we experience as part of our daily lives. The lower altitudes are the warmest part of the troposphere, in part because the earth's surface absorbs solar radiation and transfers this heat to the air. Generally, as altitude increases, temperature decreases steadily. But the earth's topography—mountain ranges and plateaus—can cause some lower regions in the troposphere to experience temperature inversions, where temperature actually increases with altitude. Towards the top of the troposphere temperatures fall to an average low of -70deg.F (-57deg.C) and wind speeds increase significantly, making the top of the troposphere an extremely cold and windy place.
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The Stratosphere

The gradual change from the troposphere to the stratosphere begins at approximately 7 miles (11km) high. The temperature in the lower stratosphere is extremely stable and cold at -70deg.F (-57deg.C). Here, strong winds occur as part of defined circulation patterns. High cirrus clouds sometimes form in the lower stratosphere, but for the most part there are no significant weather patterns in the stratosphere.



From the middle of the stratosphere and up, the temperature pattern undergoes a sudden change, sharply increasing with height. Much of this temperature change is due to increasing levels of ozone concentration which absorbs ultraviolet radiation. The temperature can reach a balmy 65deg.F (18deg.C) in the upper stratosphere near an altitude of 25 miles (40km) high.
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The Mesosphere

25 miles (40km) above the earth's surface marks the transition to the mesosphere. In this layer, temperature once again begins to fall as altitude increases, to temperatures as low as -225deg.F (-143deg.C) near its top, 50 miles (81km) above the earth. Such extreme cold allows the formation of so-called noctilucent clouds, thought to be made of ice crystals clinging to dust particles.
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The Thermosphere

The transition from the mesosphere to the final thermosphere layer begins at a height of approximately 50 miles (81km). The thermosphere receives its name from the return to increasing temperature which can reach a staggering 3,600deg.F (1982deg.C). These extreme temperatures are caused by the absorption of the sun's shortwave ultraviolet radiation. This radiation penetrates the upper atmosphere, stripping atoms of their electrons and giving them a positive charge. Electrically charged atoms build up to form a series of layers within the thermosphere. These charged layers are often referred to as the ionosphere, which deflects some radio signals. Before the modern use of satellites, this deflection by the ionosphere was essential for long distance radio communication. Today, radio frequencies which pass through the ionosphere unaffected are chosen for satellite communication.



Beautiful auroras, also known as the Northern and Southern lights, occur in the thermosphere when solar flares from the sun create magnetic storms near the poles. These magnetic storms strip atoms of their electrons. Brilliant green and red light is emitted when the electrons rejoin the atom, returning the atoms to their original state. Even higher—above the auroras and the ionosphere—the gases of this final atmospheric layer begin to dissipate, until finally, several hundred miles above the earth, they fade off into the depths of space.
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