ABSORPTION.—Earth     and     its     atmosphere absorb  about  64  percent  of  the  insolation.  Land  and water  surfaces  of  Earth  absorb  51  percent  of  this insolation.  Ozone,  carbon  dioxide,  and  water  vapor directly absorb the remaining 13 percent. These gases absorb   the   insolation   at   certain   wavelengths.   For example, ozone absorbs only a small percentage of the insolation. The portion or type the ozone does absorb is critical since it reduces ultraviolet radiation to a level where animal life can safely exist. The most important absorption occurs with carbon dioxide and water vapor, which absorb strongly over a broader wavelength band. Clouds  are  by  far  the  most  important  absorbers  of radiation  at  essentially  all  wavelengths.  In  sunlight clouds reflect a high percentage of the incident solar radiation  and  account  for  most  of  the  brightness  of Earth as seen from space. There  are  regions,  such  as  areas  of  clear  skies, where   carbon   dioxide   and   water   vapor   are   at   a minimum and so is absorption. These areas are called atmospheric  windows   and   allow   insolation   to   pass through the atmosphere relatively unimpeded. Greenhouse Effect The atmosphere conserves the heat energy of Earth because  it  absorbs  radiation  selectively.  Most  of  the solar  radiation  in  clear  skies  is  transmitted  to  Earth’s surface,  but  a  large  part  of  the  outgoing  terrestrial radiation is absorbed and reradiated back to the surface. This  is  called  the  greenhouse  effect.  A  greenhouse permits most of the short-wave solar radiation to pass through the glass roof and sides, and to be absorbed by the   floor,   ground   or   plants   inside.   These   objects reradiate energy at their temperatures of about 300°K, which is a higher temperature than the energy that was initially received. The glass absorbs the energy at these wavelengths   and   sends   part   of   it   back   into   the greenhouse,   causing   the   inside   of   the   structure   to become warmer than the outside. The atmosphere acts similarly, transmitting and absorbing in somewhat the same way as the glass. If the greenhouse effect did not exist,  Earth’s  temperature  would  be  35°C  cooler  than the  15°C  average  temperature  we  now  enjoy,  because the insolation would be reradiated back to space. Of course, the atmosphere is not a contained space like   a   greenhouse   because   there   are   heat   transport mechanisms   such   as   winds,   vertical   currents,   and mixing with surrounding and adjacent cooler air. RADIATION (HEAT) BALANCE IN THE ATMOSPHERE The  Sun  radiates  energy  to  Earth,  Earth  radiates energy  back  to  space,  and  the  atmosphere  radiates energy  also.  As  is  shown  in  figure  1-7,  a  balance  is maintained between incoming and outgoing radiation. This section of the lesson explains the various radiation processes involved in maintaining this critical balance and the effects produced in the atmosphere. We have learned that an object reradiates energy at a higher temperature. Therefore, the more the Sun heats Earth,  the  greater  the  amount  of  heat  energy  Earth reradiates. If this rate of heat loss/gain did not balance, Earth would become continuously colder or warmer. Terrestrial (Earth) Radiation Radiation   emitted   by   Earth   is   almost   entirely long-wave radiation. Most of the terrestrial radiation is absorbed  by  the  water  vapor  in  the  atmosphere  and some   by   other   gases   (about   8   percent   is   radiated directly    to    outer    space).    This    radiant    energy    is reradiated in the atmosphere horizontally and vertically. Horizontal flux (flow or transport) of energy need  not  be  considered  due  to  a  lack  of  horizontal temperature    differences.    The    vertical,    upward    or downward, flux is of extreme significance. Convection and turbulence carry aloft some of this radiation. Water vapor, undergoing the condensation-precipitation-evaporation cycle (hydrological  cycle),  carries  the  remainder  into  the atmosphere. Atmospheric Radiation The  atmosphere  reradiates  to  outer  space  most  of the    terrestrial    radiation    (about    43    percent)    and insolation (about 13 percent) that it has absorbed. Some of this reradiation is emitted earthward and is known as counterradiation. This radiation is of great importance in the greenhouse effect. Heat Balance and Transfer in the Atmosphere Earth does not receive equal radiation at all points as  was  shown  in  figure  1-4.  The  east-west  rotation  of Earth  provides  equal  exposure  to  sunlight  but  latitude and   dispersion   do   affect   the   amount   of   incident radiation  received.  The  poles  receive  far  less  incident radiation than the equator. This uneven heating is called differential insolation. 1-10