Atmosphere”  and  are  representative  of  mid-latitude conditions.   The   extension   shown   in   the   insert   is speculative.   These   divisions   are   for   reference   of thermal   structure   (lapse   rates)   or   other   significant features and are not intended to imply that these layers or zones are independent domains. Earth is surrounded by one atmosphere, not by a number of sub-atmospheres. The  layers  and  zones  are  discussed  under  two separate classifications. One is the METEOROLOGICAL classification that defines zones according  to  their  significance  for  the  weather.  The other is the ELECTRICAL classification that defines zones according to electrical characteristics of gases of the atmosphere. Meteorological Classification In  the  meteorological  classification  (commencing with Earth’s surface and proceeding upward) we have the troposphere, tropopause, stratosphere, stratopause, mesosphere, mesopause, thermosphere, and the exosphere. These classifications are based on temperature   characteristics.   (See   fig.   1-9   for   some examples.) TROPOSPHERE.—The troposphere is the layer of   air   enveloping   Earth   immediately   above   Earth’s surface. It is approximately 5 1/2 miles (29,000 ft or 9 kin) thick over the poles, about 7 1/2 miles (40,000 ft or 12.5  kin)  thick  in  the  mid-latitudes,  and  about  11  1/2 miles (61,000 ft or 19 kin) thick over the Equator. The figures  for  thickness  are  average  figures;  they  change somewhat from day to day and from season to season. The troposphere is thicker in summer than in winter and is thicker during the day than during the night. Almost all weather occurs in the troposphere. However, some phenomena such as turbulence, cloudiness (caused by ice  crystals),  and  the  occasional  severe  thunderstorm top occur within the tropopause or stratosphere. The   troposphere   is   composed   of   a   mixture   of several different gases. By volume, the composition of dry  air  in  the  troposphere  is  as  follows:  78  percent nitrogen,  21  percent  oxygen,  nearly  1-percent  argon, and  about  0.03  percent  carbon  dioxide.  In  addition,  it contains minute traces of other gases, such as helium, hydrogen, neon, krypton, and others. The air in the troposphere also contains a variable amount of water vapor. The maximum amount of water vapor that the air can hold depends on the temperature of the air and the pressure. The higher the temperature, the more water vapor it can hold at a given pressure. The air also contains variable amounts of impurities,   such   as   dust,   salt   particles,   soot,   and chemicals.  These  impurities  in  the  air  are  important because  of  their  effect  on  visibility  and  the  part  they play in the condensation of water vapor. If the air were absolutely  pure,  there  would  be  little  condensation. These minute particles act as nuclei for the condensation  of  water  vapor.  Nuclei,  which  have  an affinity  for  water  vapor,  are  called  HYGROSCOPIC NUCLEI. The temperature in the troposphere usually decreases with height, but there may be inversions for relatively thin layers at any level. TROPOPAUSE.—The  tropopause  is  a  transition layer between the troposphere and the stratosphere. It is not uniformly thick, and it is not continuous from the equator to the poles. In each hemisphere the existence of   three   distinct   tropopauses   is   generally   agreed upon—one  in  the  subtropical  latitudes,  one  in  middle latitudes,  and  one  in  subpolar  latitudes.  They  overlap each other where they meet. The  tropopause  is  characterized  by  little  or  no change  in  temperature  with  increasing  altitude.  The composition of gases is about the same as that for the troposphere.  However,  water  vapor  is  found  only  in very minute quantities at the tropopause and above it. STRATOSPHERE.—The    stratosphere    directly overlies the tropopause and extends to about 30 miles (160,000 ft or 48 kilometers). Temperature varies little with height in the stratosphere through the first 30,000 feet (9,000 meters); however, in the upper portion the temperature increases approximately linearly to values nearly  equal  to  surface  temperatures.  This  increase  in temperature   through   this   zone   is   attributed   to   the presence  of  ozone  that  absorbs  incoming  ultraviolet radiation. STRATOPAUSE.—The  stratopause  is  the  top  of the stratosphere. It is the zone marking another reversal with increasing altitude (temperature begins to decrease with height). MESOSPHERE.—The   mesosphere   is   a   layer approximately  20  miles  (100,000  ft  or  32  kilometers) thick directly overlaying the stratopause. The temperature decreases with height. MESOPAUSE.—The    mesopause    is    the    thin boundary   zone   between   the   mesosphere   and   the thermosphere. It is marked by a reversal of temperatures;  i.e.,  temperature  again  increases  with altitude. 1-16