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! Overview

     Air, or the atmosphere, surrounds Earth out to a distance of 300 miles, although 99% of the atmosphere is only 50 miles thick.  Air gets thinner and thinner until there is no air at all, and here "space" begins.  The atmosphere is composed of 78% nitrogen, 21% oxygen and 1% other gases, and is divided into four zones by temperaturetroposphere, stratosphere, mesosphere and thermosphere.  The temperature of air changes with altitude because air expands when there is less air pressure.  When air expands without more heat being added the temperature falls, and therefore gets colder at higher and higher altitudes.

 Troposphere

     The troposphere is the band of atmosphere closest to Earth's surface and varies from five to ten miles in depth.  In this zone clouds are created and storms occur.  Temperature drops rapidly in this region as the altitude increases.  

 Stratosphere

     The stratosphere is 20 miles thick.  Temperature varies considerably in this layer.  The temperature is fairly constant in the lower portion but increases toward the upper portion.  Airplanes generally fly in the stratosphere so they can stay above harsh weather. 

 Mesosphere

     The mesosphere is 20 miles thick and has the lowest temperatures found in the atmosphere.  In the upper levels of the mesosphere the trails left by meteors can be seen. 

 Thermosphere

     The thermosphere begins about 50 miles above the surface of Earth and extends out into space.  The air in this region is very thin and is heated by the sun to temperatures that can exceed 2700 F.

 Ionosphere

     Centered in the thermosphere is a region of highly charged particles called ions that extend upward for several hundred miles.  Radio waves from Earth's surface bounce off the ionosphere back to ground and thus can be sent around the world.  If it were not for the ionosphere, radio waves would fly straight into space instead of following the curve of Earth. 

 Ozone Layer

     Ozone is formed by a fragile bond of three oxygen atoms (normal oxygen has two).  The third oxygen atom is not as strongly attached as the others, and thus ozone breaks down easily when exposed to human-made pollutants called chlorofluorocarbons (CFC's).  CFC's threaten the ozone layer which is concentrated 15 miles above the Earth.  This layer absorbs ultraviolet radiation streaming down from the sun and protects life on Earth.  However, ozone can be harmful when there is too much of it close to the ground because it combines easily with carbon-based fuels, like car exhaust, to produce smog. 
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    Overview

     Clouds are grouped according to height and appearance. Heights are divided into high, middle and low clouds, while appearances have descriptive labels derived from Latin.  For example, the name for stratus clouds is derived from the term "strato," meaning layer- or sheet-like.  Cumulus clouds come from the term "cumulo," meaning pile or heap.  "Cirro" means curl and describes cirrus clouds.  Nimbus is the name for rain clouds since "nimbo" means rain.  Clouds are often named after a combination of these features and terms.  By looking at the different types of clouds in the sky you can often tell what weather to expect.

 Low Clouds

     Low clouds occur within 6,000 feet of sea level and are either stratus clouds or layered stratocumulus clouds.  Low stratus clouds cover the sky with a thick even blanket that may bring drizzle, rain or snow.  

 Middle Clouds

     Single-layered clouds can be found in the middle region which lies between 6,000 and 20,000 feet above sea level.  Nimbostratus clouds cover the sky with gray and produce rain, whereas altostratus and altocumulus clouds usually indicate a warm, sunny day.  The latter types are formed by many clouds piled on top of each other, although sometimes they do not appear to be connected.

 High Clouds

     High clouds such as cirrus, cirrostratus and cirrocumulus are found above 20,000 feet and are composed solely of ice crystals.  Cirrus clouds are high and wispy and indicate that warm weather is coming.  Cirrostratus clouds are made of one thin layer.  Cirrocumulus clouds form puffy bands that are rarely seen and indicate rain may be coming soon.   

 Cloud Formation

     Heat causes water to evaporate from lakes, oceans or soil and to rise into the air.  When this water vapor rises to a higher, colder altitude, it condenses on little pieces of dust or small salt crystals forming water droplets or ice crystals that we see in the sky as clouds.  Clouds can also form when a mass of warm air is forced to travel up a hill or mountain where it is cooler.  Clouds play an important role in stabilizing climate by reflecting radiation from the sun back into space.  The water and land under a cloud then cools, which slows the evaporation process, forcing the cloud blanket to disappear. 
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  { Overview

     Desertification is a process in which once productive land becomes useless desert.  This process can occur with climatic changes, but more frequently humans are the major contributor. Desertification occurs in dryland areas after the protective layer of vegetation is overgrazed or destroyed.  When that layer is gone, the land can no longer hold moisture, and topsoil is easily eroded with exposure to wind and rain.  
     Deforestation is a similarly destructive process that can occur in rain forests. By 1980, as much as 40% of the world's tropical forests had been wiped out.  Approximately three thousand acres of rain forests are decimated every hour.  However, forest destruction is not limited to tropical rain forests.  At least 85% of old growth forests in the U.S. Pacific Northwest of the United States have been cut down. 
     Several factors contribute to rain forest destruction worldwide.  Often, governments encourage development of rain forest areas, by clearing land for industry, dams, agriculture projects or lucrative logging activities.  Also, poor people, unable to survive on marginal land, clear forests so they can grow crops or raise cattle.  However, the delicate rain forest ecosystem breaks down easily when trees are cleared, and soon the nutrient-depleted soil is useless.  
     Rain forests are home to millions of species of plants, birds and other wildlife.  This rich biological diversity is irreplaceable and benefits people throughout the worldone quarter of the medicinal drugs prescribed in the United States today are derived from natural compounds, many of which are only found in rain forests. 
     Moreover, destruction of rain forest increases carbon dioxide in the atmosphere by the burning of the trees themselves.  Further, the result is fewer trees to convert carbon dioxide to oxygen, thus contributing more to the buildup of carbon dioxide.  Overall, the elevated levels of atmospheric carbon dioxide create a global warming or greenhouse effect, believed to be harmful to all life forms.  A tree can absorb carbon dioxide at a rate of 26 pounds per year. 

 Sahara Desert

     The Sahara Desert is a vast expanse of dry land in north Africa that is about the size of the United States.  Almost 10,000 years ago, during an Ice Age, the Sahara region was much wetter, with several rivers and lakes and more rainfall.  Many animals, including elephants and giraffes lived among the forests and grasslands that covered the region at that time.  Through natural processes the African climate became drier, and the Sahara began to turn into a deserta trend which continues in modern times.   
     In recent decades, however, humans have contributed to a more rapid desertification process.  People have brought about desert spread by allowing animals to overgraze land and by cutting down trees and shrubs along border regions.  Typically, population pressures force overuse of land, with the unfortunate consequence that the land eventually supports no one. 

 Brazilian Rain Forest

     The Amazon rain forest of Brazil is one of Earth's great phenomena, acting as part of the planet's lung system by its continuous exchange of carbon dioxide for oxygen.  This rain forest, which is home to 2-3 million people and over 40,000 varieties of plants, has been the source of growing public concern and innumerable fact-finding missions by politicians, celebrities and environmental groups.  
     Destruction of the Brazilian rain forest began with the arrival and settlement of the Portuguese 500 years ago.  A resurgence of destruction began in the 1960's with incentives offered by the Brazilian government designed to bring "civilization" to the Amazon forest.  Brazil's laws and tax system have made deforestation profitable, prompting big ranchers and land speculators to clear rain forest land.  From 1966 to 1978, 20 million acres of Amazon forest were cut down for cattle pasture.  Cattle raised on cleared rain forest land is often exported to the United States.  Approximately 15% of the Amazon forest has been destroyed overall and at the present rate of deforestation, this forest might disappear altogether in 35 to 100 years. 

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AtmosphereCloudsDeserts and Rain Forests