Atmosphere  Composition of the Atmosphere  Figure 12-1.  Composition of the atmosphere.  The atmosphere is a blanket of air made up of a mixture of  gases that surrounds the Earth and reaches almost 350 miles  from the surface of the Earth. This mixture is in constant  motion. If the atmosphere were visible, it might look like  an ocean with swirls and eddies, rising and falling air, and  waves that travel for great distances.  Life on Earth is supported by the atmosphere, solar energy,  and the planet’s magnetic fields. The atmosphere absorbs  energy from the sun, recycles water and other chemicals, and  works with the electrical and magnetic forces to provide a  moderate climate. The atmosphere also protects life on Earth  from high energy radiation and the frigid vacuum of space.  In any given volume of air, nitrogen accounts for 78 percent  of the gases that comprise the atmosphere, while oxygen  makes up 21 percent. Argon, carbon dioxide, and traces  of other gases make up the remaining one percent. This  volume of air also contains some water vapor, varying from  zero to about five percent by volume. This small amount of  water vapor is responsible for major changes in the weather.  [Figure 12-1]  The envelope of gases surrounding the Earth changes  from the ground up. Four distinct layers or spheres of the  atmosphere have been identified using thermal characteristics  (temperature changes), chemical composition, movement,  and density.  [Figure 12-2]  The first layer, known as the troposphere, extends from 6  to 20 kilometers (km) (4 to 12 miles) over the northern and  southern poles and up to 48,000 feet (14.5 km) over the  equatorial regions. The vast majority of weather, clouds,  storms, and temperature variances occur within this first  layer of the atmosphere. Inside the troposphere, the average  temperature decreases at a rate of about 2 °Celsius (C) every  Figure 12-2.  Layers of the atmosphere.  12-2  2 1 % %  7 8 % %  NN ii tt rr oo gg ee n n  O x y g e n n  1 %  T r o p o s p h e r e  S t r a t o s p h e r e  M e s o s p h e r e  T h e r m o s p h e r e  2 80 ,00 0 fe et  1 6 0 , 0 00  feet  2 0 , 0 0 0 f eet 