The Atmosphere
Introduction: The earth, as it rotates on its axis and revolves in its elliptical orbit around the sun is constantly surrounded by a gaseous envelope called the atmosphere.

The atmosphere is a mixture of invisible gases extending from the earth's surface to a height of several thousands of miles. At the upper limits of the atmosphere the earth's gravitational attraction cannot keep a gas molecule in orbit. Thus, gas molecules are lost to outer space.

Origin: The development of the earth's atmosphere is closely linked to the formation of the earth. Knowledge of the history of the earth combined with comparisons that can be made with other planetary atmospheres is leading toward an explanation concerning the origin of our atmosphere.

Current theory suggests that the initial formation of the earth occurred about 4.8 billion years ago. The earth was a relatively small, cold object that had little or no gaseous envelope surrounding it.

Geological evidence indicates that about 3.6 billion years ago the earth's interior heated up and with the expulsion of volatile components, a primitive atmosphere was formed.

The primitive atmosphere contained large quantities of ammonia and methane along with water vapor, carbon dioxide, and nitrogen. There was no free oxygen in the earth's primitive atmosphere.

Approximately 2.0 billion years ago free oxygen appeared as an atmospheric element. The major source was the freeing of oxygen through plant photosynthesis. Photosynthesis is a plant process that combines sunlight. carbon dioxide, and water to produce sugar and oxygen.

Composition: Presently the atmosphere consists of three components: dry gases, water vapor, and solid particles.

The four most abundant gases are nitrogen, oxygen, argon and carbon dioxide. Nitrogen and oxygen account for 99.9% by volume of the dry air, while argon and carbon dioxide are less than 1% of the mixture.

The atmosphere contains a variable amount of water vapor. Water vapor, just as the name implies, is water in a gaseous state. The source of atmospheric moisture is through the processes of evaporation from water surfaces and transpiration by plant life.

Suspended throughout the lower atmosphere are minute non-gaseous material collectively called dust. The origin of these materials are from natural sources such as sea salt evaporated from ocean spray, wind blown soil and debris from volcanoes and forest fires. Human activity also contributes to the particle load of the atmosphere. The major sources are through the burning of fossil fuels, man-made forest fires, and agricultural activity. Larger particles settle out of the atmosphere gravitationally, while smaller particles are washed out of the atmosphere in precipitation.

Structure: Based on information obtained from meteorological satellites, balloon and rocket accents revealed differences in physical conditions throughout the atmosphere.

The atmosphere may be generally divided into two vertical layers. The lower layer, called the homosphere, extends from the earth's surface to 50 miles and has a uniform chemical makeup. The upper level of the atmosphere is the heterosphere, which possesses a variable chemical composition and extends upward from an elevation of 50 miles to the edge of outer space.

According to temperature measurements of the atmosphere, a more detailed vertical structure can be identified. The four zones are the troposphere, stratosphere, mesosphere and thermosphere.

The layer closest to the earth's surface is the troposphere. The term is from the Greek tropo meaning to turn, which describes the layers mixing and convective characteristics. The troposphere is a layer where temperature usually decreases with increased elevation. The zone contains 75% of the total mass of the atmosphere. It is of primary interest to the meteorologists, since weather, as we know it, and the elements that influence the weather, take place in this lowest region. The upper boundary of the troposphere, where temperatures stop decreasing with elevation, is called the tropopause. Beyond this zone of transition is the stratosphere.

The term stratosphere comes from the Greek strato meaning horizontally layered. the layer extends 7 to 30 miles above the surface of the earth. The temperature is nearly constant in the lower levels of the stratosphere and increases with height. This results in the reduction of vertical motion and overturning. The stratosphere contains 24% of the mass of the atmosphere. Combined with the troposphere the two layers account for 99% of the mass. Above the stratosphere is the stratopause, a zone of constant temperature with increasing elevation.

Beyond 30 miles from the surface of the earth is the mesosphere. A layer where temperatures once again decrease with elevation, as they did in the troposphere. The relentless bombardment of the thinly scattered air molecules by radiation from the sun causes individual molecules to become electrically charged. The process is known as ionization. Ionized particles concentrated within the layer helps to reflect radio waves sent from the earth's surface.

The last layer 55 miles above the earth is referred to as the thermosphere. Due to the very thin air the region displays very high temperatures. Temperatures can reach as high as 2500F. These temperatures apply only to individual molecules and do not represent the same kind of environmental importance at the earth's surface.

Energy in the Atmosphere: The sun is the major source of heat energy for both the earth's atmosphere and surface. The amount of incoming solar radiation is influenced by two astronomical factors. The factors are the solar constant and distance from the sun.

The solar constant is the amount of incoming solar radiation at the outer boundary of the earth's atmosphere. The amount of solar radiation intercepted is a very small fraction of the total sun's output.

Due to the fluctuations in the orbit of the earth around the sun causes variations in the distance form the sun. Presently, the average distance of the earth to the sun is 93,000,000 miles. Any change would affect the solar constant.

The solar energy received by the earth is redistributed by the atmosphere. Approximately 40% is reflected back to space, 20% is absorbed by the atmosphere, and 20% is scattered around in the atmosphere leaving only 20% to reach the earth's surface.

2,000 years ago the Greeks observed that the difference in heating between the polar and equatorial regions of the earth is the major force for the general circulation of the atmosphere.

Atmosphere in Motion: Air in motion can be observed in a range of scale from the turbulence of smoke from a small fire to swirls of large dimensions revealed in major storms like hurricanes.

In the vertical, air in motion is called a current. Horizontal movement of air is known as wind.

Cycles: The surface layer of the atmosphere interfaces with the ydrosphere, lithosphere and biosphere. Important cycles have developed as a result of the interactions among the spheres. The cycles include the transfer of water, nitrogen, oxygen and carbon dioxide.

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