the   realms   of   temperature   and   water   vapor.   The processes of radiation, the exchange of sensible heat, and the evaporation and condensation of water vapor on the sea surface maintain the heat balance of the oceans. The amount of radiant energy absorbed by the sea depends   upon   the   amount   of   energy   reaching   the surface  and  the  amount  of  reflection  by  the  surface. When the Sun is directly overhead, the amount of its energy reflected amounts to only about 3 percent. Even when the Sun is 30° above the horizon, the amount of reflection   is   just   6   percent.   However,   there   is   a reflection of about 25 percent of the energy when the Sun is 10° above the horizon. (See fig. 6-7.) Reflection loss is especially great in the presence of waves when the Sun is low. Much of the insolation is absorbed in the first meter of seawater. This is true of the clearest water as well as of   quite   turbid   (opaque)   water.   In   water   that   is extremely    turbid,    the    absorption    is    in    the    very uppermost layers. Foam and air bubbles are two major causes of a proportionately greater amount of absorption in the uppermost meter of the sea. However, due to vertical mixing, the heat absorbed in the upper layer is carried to great depths of the ocean, which acts as a great heat storage reservoir. There is an exchange of energy between the oceans and  the  atmosphere.  The  surface  of  the  oceans  emits long-wave heat radiation. The sea surface at the same time receives long-wave radiation from the atmosphere. Although  some  of  this  incoming  radiation  from  the atmosphere is reflected from the surface of the oceans, most of it is absorbed in a very thin layer of the water surface. The difference between the incoming long-wave   atmospheric   radiation   and   the   outgoing long-wave  radiation  from  the  sea  surface  is  known  as the    effective   back   radiation.    The    effective    back radiation  depends  primarily  on  the  temperature  of  the sea  surface  and  on  the  water  vapor  content  of  the atmosphere. The time of day and the season have little effect on effective back radiation, since the diurnal and annual variation of the sea-surface temperature and of the  relative  humidity  of  the  air  above  the  oceans  is slight. For  conduction  to  take  place  between  the  oceans and   the   atmosphere   there   must   be   a   temperature difference   between   the   ocean   surface   and   the   air immediately overlying it. On the average, the temperature of the surface of the oceans is higher than that of the overlaying air. It might be expected that all of the   ocean’s   surplus   of   heat   is   either   radiated   or conducted to the atmosphere. This is not the case. Only a   small   percentage   of   the   ocean’s   surplus   heat   is actually conducted to the atmosphere. About 90 percent of the surplus are used for evaporation of ocean water. Due  to  the  processes  of  radiation  and  mixing,  the oceans  act  as  a  thermostat  relative  to  the  atmosphere. The energy stored at one place during one season may be  given  off  at  another  locality  and  during  a  later season.  Hence,  there  seems  to  be  a  constant  effort  by the atmosphere and the oceans to keep their temperatures in balance by an interchange of heat. STABILITY.—The    deciding    factor    of    most weather phenomena is the stability of the atmosphere. Air  masses  may  become  more  stable  or  less  stable  as they   move   over   ocean   surfaces.   The   temperature contrast  between  the  ocean  surface  and  the  lowest layers   of   the   overlying   air   determines   whether   the ocean will promote stability or instability. When  the  air  moving  over  the  ocean  has  a  higher temperature  than  that  of  the  ocean  surface,  the  lower layers  of  the  air  become  stable  in  time.  On  the  other hand, when the air mass is colder than the ocean surface over  which  it  is  moving,  instability  results.  As  the colder air is warmed by the ocean, convective activity eventually   develops.   If   the   warming   is   sufficiently intense, thunderstorms develop. MOISTURE  CONTENT.—The   interchange   of moisture between the atmosphere and the oceans is one of the most important features of the whole meteorological    picture.    Without    this    interchange, 6-18 80^O 60^O 50^O 70^O 40^O 30^O 20^O 10^O 90^O 0^O_40%35%30% 20% 10% 15% 25% 5% 0 PERCENTAGE OF REFLECTION AG5f 0607 Figure 6-7.—Percentage of reflected radiation.