 The atmosphere is a layer of gases that surround the Earth.  These layers differ in temperature, in density, and in the relative amounts of the different gases that are present.  The two main gases in the atmosphere are nitrogen and oxygen.  Nitrogen makes up about 70% of the atmosphere, Oxygen about 20%, and the rest is a mixture of various other gases such as Carbon Dioxide, Methane, Hydrogen, Neon, etc.

 Layers of the Atmosphere  troposphere  stratosphere  mesosphere  thermosphere

 Troposphere: the lowest layer  Temperature drops as altitude increases  The troposphere is the densest layer  Almost all weather occurs in the troposphere.

 stratosphere: between the troposphere and the mesosphere  temperature increases as altitude increases;  contains the ozone layer  The ozone layer shields living things on Earth’s surface from ultraviolet- radiation damage.

 mesosphere: the coldest layer of the atmosphere,  temperature decreases as altitude increases  Temperatures in the mesosphere decrease to about -80º C.

 thermosphere: the uppermost layer of the atmosphere  temperature increases as altitude increases  Temperatures average about 980 °C (1,796 °F) because the small amount of oxygen absorbs intense solar radiation.

 The ionosphere: the lower thermosphere and upper mesosphere  When solar energy is absorbed, charged ions are formed.  Radio waves are reflected in the ionosphere.  Auroras take place in the ionosphere.

 When Earth began to solidify, about 4.4 billion years ago, volcanic eruptions released a variety of gases. The gases released by volcanoes did not include oxygen.

 Photosynthetic bacteria contribute oxygen to the atmosphere.  Organisms evolved photosynthesis, a method of capturing energy from the sun.  Photosynthesis produces oxygen as a waste product.  Gradually the oxygen content increased to what it is today.

 Animals produce carbon dioxide necessary for photosynthesis.  Oxygen breathing organisms evolved and released carbon dioxide as a waste product.  The oxygen-carbon dioxide cycle maintains a balance of atmospheric gases on Earth.

 Human-made chemicals can deplete the ozone layer.  Ozone forms when the sun’s ultraviolet rays strike molecules of O 2.  Ozone absorbs much of the sun’s ultraviolet radiation.  Without the ozone layer, ultraviolet radiation would damage living cells.  Chlorofluorocarbons, CFCs, are chemicals that destroy the ozone layer, but are now banned in most countries.

 greenhouse effect: the warming of the surface and lower atmosphere of Earth that occurs when carbon dioxide, water vapor, and other gases in the air absorb and reradiate infrared radiation  Greenhouse gases trap the sun’s heat and keep Earth’s surface warm.

 Too much carbon dioxide may cause global warming/climate change  If too much energy is absorbed by the atmosphere, global temperatures will rise.  Global warming could cause problems, such as rising ocean levels, because of melting polar icecaps, and droughts.  Combustion of coal, oil, and gas has added more carbon dioxide to the air.