APPENDIX I GLOSSARY ABSOLUTE INSTABILITY—The state of a column of    air    in    the    atmosphere    when    it    has    a superadiabatic  lapse  rate  of  temperature.  An  air parcel displaced vertically would be accelerated in the direction of the displacement. ABSOLUTE STABILITY—The state of a column of air   in   the   atmosphere   when   its   lapse   rate   of temperature  is  less  than  the  saturation  adiabatic lapse  rate.  An  air  parcel  will  be  denser  than  its environment  and  tend  to  sink  back  to  its  level  of origin. ADVECTION—The horizontal transport of an atmospheric  property  solely  by  the  mass  motion (velocity field) of the atmosphere. ADVECTION FOG—Fog caused by the advection of moist  air  over  a  cold  surface,  and  the  consequent cooling of that air to below its dew point. AIR   MASS—A   widespread   body   of   air   that   is approximately    homogeneous    in    its    horizontal extent, with reference to temperature and moisture. ANABATIC    WIND—An    upslope    wind;    usually applied only when the wind is blowing up a hill or mountain as the result of surface heating. ANTARCTIC  FRONT—The  semi  permanent,  semi continuous  front  between  the  Antarctic  air  of  the Antarctic   Continent   and   the   polar   air   of   the southern oceans; generally comparable to the arctic front of the Northern Hemisphere. ANTICYCLOGENESIS—The strengthening or development  of  an  anticyclonic  circulation  in  the atmosphere. ANTICYCLOLYSIS—The weakening of an anticyclonic circulation in the atmosphere. ANTICYCLONE—A closed circulation in the atmosphere  that  has  a  clockwise  rotation  in  the Northern   Hemisphere   and   a   counterclockwise rotation in the Southern Hemisphere. Used inter-changeably with high. ANTICYCLONIC—Refers to the rotation pattern of anticyclones. See ANTICYCLONE. ARCTIC    FRONT—The semi permanent, semi continuous front between the deep, cold arctic air and  the  shallower,  basically  less  cold  polar  air  of northern   latitudes;   generally   comparable   to   the Antarctic front of the Southern Hemisphere. AUTOCONVECTIVE LAPSE RATE—The temperature  lapse  rate  in  an  atmosphere  where density is constant with height. BACKING—A    change    in    wind    direction    in    a counterclockwise manner in the Northern Hemisphere    and    a    clockwise    manner    in    the Southern Hemisphere. BLOCKING  HIGH—An  anticyclone  that  re-mains stationary   or   moves   slowly   westward   so   as   to effectively   block   the   movement   of   migratory cyclones across its latitudes. BUYS  BALLOT’S  LAW—The  law  describing  the relationship of horizontal wind direction to pressure:  In  the  Northern  Hemisphere,  with  your back to the wind, the lowest pressure will be to your left;  in  the  Southern  Hemisphere,  the  reverse  is true. CENTER   OF   ACTION—Any   one   of   the   semi permanent high or low-pressure systems. CENTRAL PRESSURE—The atmospheric pressure at the center of a high or low; the highest pressure in a high, the lowest in a low. CHROMOSPHERE—A    thin    layer    of    relatively transparent gases above the photosphere of the Sun. CLOSED HIGH—A high that is completely encircled by an isobar or contour line. CLOSED LOW—A low that is completely encircled by an isobar or contour line. COLD-CORE  HIGH—Any   high   that   is   generally characterized  by  colder  air  near  its  center  than around   its   periphery   at   a   given   level   in   the atmosphere. COLD-CORE   LOW—Any   low   that   is   generally characterized  by  colder  air  near  its  center  than around   its   periphery   at   a   given   level   in   the atmosphere. AI-1