RADIATIONAL COOLING—The cooling of Earth’s   surface   and   adjacent   air,   accomplished (mainly at night) whenever Earth’s surface suffers a net loss of heat due to terrestrial radiation. RADIATION FOG—A  major  type  of  fog,  produced over a land area when radiational cooling reduces the air temperature to or below its dew point. RAINBOW—Any  one  of  a  family  of  circular  arcs consisting  of  concentric  colored  bands,  arranged from red on the inside to blue on the outside, which may  be  seenona‘‘sheet”  of  water  drops  (rain,  fog, or spray). REDUCTION—In   general,   the   transforma-tion   of data  from  a  “raw”  form  to  some  usable  form.  In meteorology, this often refers to the con-version of the observed value of an element to the value that it theoretically   would   have   at   some   selected   or standard  level,  usually  mean  sea  level.   The  most common  reduction  in  observing  is  that  of  station pressure to sea level pressure. REFLECTION—The  process  whereby  a  surface  of discontinuity  turns  back  a  portion  of  the  incident radiation   into   the   medium   through   which   the radiation approached. REFLECTIVITY—A   measure   of   the   fraction   of radiation  reflected  by  a  given  surface;  defined  as the ratio of the radiant energy reflected to the total that is incident upon that surface. The reflectivity of a given surface for a specified broad spectral range, such as the visible spectrum or the solar spectrum, is referred to as albino. REFRACTION—The  process  in  which  the  direction of energy propagation is changed as the result of a change in density within the propagating medium, or   as   the   energy   passes   through   the   interface representing  a  density  discontinuity  between  two media. RESULTANT  WIND—In  climatology,  the  vectorial average  of  all  wind  directions  and  speeds  for  a given level at a given place for a certain period, as a month.   It   is   obtained   by   resolving   each   wind observation into components from north and east, summing   over   the   given   period,   obtaining   the averages, and reconverting the average components into a single vector. SCATTERING—The process by which small particles  suspended  in  a  medium  of  a  different index of refraction diffuse a portion of the incident radiation in all directions. SEA BREEZE—A coastal local wind that blows from sea  to  land,  caused  by  the  temperature  difference when  the  sea  surface  is  colder  than  the  adjacent land. Therefore, it usually blows on relatively calm, sunny,   summer   days;   and   alternates   with   the oppositely   directed,   usually   weaker,   night   land breeze. SEA-BREEZE FRONT—A sea breeze that forms out over the water, moves slowly toward the coast and then moves inland quite suddenly. Often associated with  the  passage  of  this  type  of  sea  breeze  are showers,   a   sharp   wind   shift   from   seaward   to landward,  and  a  sudden  drop  in  temperature.  The leading  edge  of  such  a  sea  breeze  is  sometimes called the sea breeze front. SEA LEVEL—The height or level of the sea surface. SEASON—A  division  of  the  year  according  to  some regularly recurrent phenomena, usually astronomical   or   climatic.   Astronomical   seasons extend from an equinox to the next solstice (or vice versa).    Climatic    seasons    are    often    based    on precipitation (rainy and dry seasons). SECONDARY CIRCULATION—Atmos-pheric circulation features of synoptic scale. SECONDARY FRONT—A front that forms within a baroclinic cold air mass that itself is separated from a warm air mass by a primary frontal system. The most common type is the secondary cold front. SHEAR—The variation (usually the directional derivative) of a vector field along a given direction in space. The most frequent context for this concept is wind shear. SHEAR LINE—A  line  or  narrow  zone  across  which there  is  an  abrupt  change  in  the  horizontal  wind component parallel to this line; a line of maximum horizontal wind shear. SHORT-WAVE RADIATION—A  term  used  loosely to distinguish radiation in the visible and near-visible portions of the electromagnetic spectrum (roughly 0.4 to 1.0 micron in wavelength) from long-wave radiation. SIBERIAN  HIGH—A  cold-core  high--pressure  area that  forms  over  Siberia  in  winter,  and  which  is particularly  apparent  on  mean  charts  of  sea-level pressure. SINGULAR POINT—In a flow field, a point at which the  direction  of  flow  is  not  uniquely  determined, hence a point of zero speed, e.g., a col. AI-7