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The Atmosphere Pressure and Height The atmosphere is held to the Earth by it's gravity. Most of it is located close to the surface. Fifty percent of the atmosphere (by mass) is closer than 10 km to the Earth; 99% is under 100 km. As a result of the tapering off of gravitational influence and the decreasing density, the pressure of the atmosphere steadily decreases from 1atm (760 mmHg or 100,000 Pa) at sea level to nearly zero at 1000 km and beyond. Atmospheric pressure is measured with barometers. The simplest kind of barometer involves a column of liquid (mercury was first used) contained in an evacuated, sealed tube standing in a reservoir. The pressure of the atmosphere pushes liquid from the reservoir into the tube, thereby forcing the liquid in the tube to move into the vacuum. Basically, the atmosphere pushes down; the liquid has nowhere to go but up. Changes in atmospheric pressure are observed as changes in the height of the column of liquid. Meteorologists often find it more useful to map the atmosphere according to constant pressure regions as opposed to constant altitude as it is the differences in pressure that cause winds and other forms of weather. The resulting maps look much like topographic maps drawn for the surface of the Earth. Temperature and Height Unlike pressure, temperature does not decrease steadily as altitude increases. Instead, it goes through a series of inversions. An inversion is a section of the atmosphere in which the temperature levels out and may even begin to increase. It is these inversions that divide the atmosphere into the many layers shown in the graphic below:
The tropopause is the inversion that ends the troposphere at roughly 10 km. Temperature decreases within the zones as altitude increases. At the tropopause the temperature levels out and then begins to increase in the next layer. The stratopause is the inversion that ends the stratosphere at roughly 50 km. After the temperature levels out here, it begins to decrease. The last of the inversions is the mesopause, which is the transition from the mesosphere into the ionosphere. Here the temperature levels out around -35°C and then begins steadily increasing. The temperature increases until the fringes of the Earth's atmosphere merge with the fringes of the Sun's. The temperature of the atmosphere has been studied since World War II by rockets and satellites. Before 1890, it was thought that the atmosphere got steadily cooler as altitude increased. This early theory was based on the experience of climbing to a snow-covered mountain. Radiation and Insolation Insolation refers to incoming solar radiation, and is the radiant energy from the Sun. The total energy given off from the Sun in all directions in one day is the equivalent on 100 million thunder storms or 1 billion atom bombs. Fortunately for the life on Earth, only a small fraction of this is intercepted by the Earth, and only approximately 47% of that actually reaches the surface. Most of the insolation is reflected or absorbed by the
atmosphere. The ozone and other atmospheric gasses absorb energy while the clouds
and other impurities in the air reflect a significant amount. (Snow reflects 90% of
the energy hitting it, and even fields of crops or grass reflect 20%.) The
percentage of energy that a planet reflects is known as its albedo. Source:
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