| Atmosphere |
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At sea level, 1 cubic meter of air has a mass of 1.25 kilograms. Because air has mass, gravity acts upon the air giving it weight. The weight of air, in turn, exerts a pressure on any object submerged within the air. This pressure is known as atmospheric pressure. At sea level, the atmospheric pressure is greatest because it is at the bottom of an “ocean of air”. On the mountain, the atmospheric pressure is less.
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Air grows cooler with increasing elevation. At lower elevations, the air is generally warmer. This is because the Earth’s surface radiates much of the heat it absorbs from the sun. As this heat radiates upward, it warms the air—an effect that decreases with increasing distance from the ground. Air grows less dense with increasing elevation. Air molecules close to the Earth’s surface are squeezed together by the greater atmospheric pressure. With increasing elevation, the density of the air gradually decreases because of decreasing atmospheric pressure. More than half of the atmosphere’s mass lies below an altitude of 5.6 kilometers, and about 99 percent lies below an altitude of 30 kilometers. The atmosphere can be divided into layers, each layer distinct in its characteristics. The lowest layer is the troposphere, which contains 90 percent of the atmospheric mass and essentially all of the atmosphere’s water vapor and clouds. This is where weather occurs. The troposphere extends to a height of about 16 kilometers. Its temperature decreases steadily with increasing altitude. At the top of the troposphere, temperatures average about - 500C. Above the troposphere is the stratosphere, which reaches a height of 50 kilometers. In the stratosphere at an altitude of 20 to 30 kilometers lies the ozone layer. Ozone is formed by the action of sunrays on the atmospheric oxygen. Stratospheric ozone acts as a sunscreen, protecting the Earth’s surface from harmful solar ultraviolet radiation. Stratospheric ozone also affects stratospheric temperatures. At the lowest altitudes, the temperature is coolest because of the solar screening effect of ozone; air at this altitude is literally in the shade of ozone. At higher altitudes, less ozone is available for shading and temperature increases all the way to a warm 00C at the top of the stratosphere.
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