SUPERSATURATION—The condition existing in a given  portion  of  the  atmosphere  (or  other  space) when  the  relative  humidity  is  greater  than  100 percent, that is, when it contains more water vapor than is needed to produce saturation with respect to a plane surface of pure water or pure ice. SURFACE BOUNDARY LAYER—That thin layer of air adjacent to Earth’s surface, extending up to the so-called  anemometer  level  (the  height  above  the ground   at   which   an   anemometer   is   exposed; usually 10 meters to 100 meters). SURFACE    CHART—(also    called    surface    map, sea-level    chart,    sea-level    pressure    chart)    An analyzed synoptic chart of surface weather observations. It shows the distribution of sea level pressure   (positions   of   highs,   lows,   ridges,   and troughs) and the location and nature of fronts and air  masses.  Often  added  to  this  are  symbols  of occurring weather phenomena, analysis of pressure tendency (isallobars), indications of the movement of pressure systems and fronts, and perhaps others, depending on the use of the chart. SURFACE INVERSION—A  temperature  in-version based  at  Earth’s  surface;  that  is,  an  in-crease  of temperature with height beginning at ground level. SURFACE    OF    DISCONTINUITY—A surface separating   two   fluids   across   which   there   is   a discontinuity   of   some   fluid   property,   such   as density, velocity, etc., or of some derivative of one of  these  properties  in  a  direction  normal  to  the interface.   An   atmospheric   front   is   represented ideally  by  a  surface  of  discontinuity  of  velocity, density,  temperature,  and  pressure  gradient;  the tropopause  is  represented  ideally  by  a  surface  of discontinuity of, for example, the derivatives: lapse rate and wind shear. SYNOPTIC—In general, pertaining to or affording an overall view. In meteorology, this term has become somewhat  specialized  in  referring  to  the  use  of meteorological  data  obtained  simultaneously  over a   wide   area   for   the   purpose   of   presenting   a comprehensive and nearly instantaneous picture of the state of the atmosphere. SYNOPTIC CHART—In  meteorology,  any  chart  or map on which data and analyses are presented that describe  the  state  of  the  atmosphere  over  a  large area at a given moment in time. SYNOPTIC  SCALE—The   scale   of   the   migratory high-    and    low-pressure    systems    (or    cyclonic waves) of the lower troposphere, with wavelengths of 1,000 to 2,500 km. SYNOPTIC SITUATION—The  general  state  of  the atmosphere  as  described  by  the  major  features  of synoptic charts. TEMPERATURE  INVERSION—A  layer  in  which temperature increases with altitude. TERTIARY CIRCULATION—The generally small, localized   atmospheric   circulations. They   are represented by such phenomena as the local winds, thunderstorms, and tornadoes. THERMAL—(1)  Pertaining  to  temperature  or  heat. (2)  A  relatively-small-scale  rising  current  of  air produced  when  the  atmosphere  is  heated  enough locally   by   Earth’s   surface   to   produce   absolute instability in its lower layers. The use of this term is usually reserved to denote those currents either too small and/or too dry to produce convective clouds; thus,  thermals  are  a  common  source  of  low-level clear-air turbulence. THERMAL  GRADIENT—The  rate  of  variation  of temperature either horizontally or vertically. THERMAL    HIGH—An    area    of    high    pressure resulting   from   the   cooling   of   air   by   a   cold underlying surface, and remaining relatively stationary over the cold surface. THERMAL   LOW—An   area   of   low   atmospheric pressure  resulting  from  high  temperatures  caused by intense surface heating. They are stationary with a generally weak and diffuse cyclonic circulation. They are non-frontal. THERMOSPHERE—The atmospheric shell extending from the top of the mesosphere to outer space.   It   is   a   region   of   more   or   less   steadily increasing  temperature  with  height,  starting  at  70 or 80 km. TORNADO—A   violently   rotating   column   of   air, pendant  from  a  cumulonimbus  cloud,  and  nearly always observable as a “funnel cloud” or tuba. TRIPLE POINT—Term commonly used to denote the apex of an occlusion. TROPICAL AIR—A type of air whose characteristics are developed over low latitudes.  Maritime tropical air    (mT)    is    produced    over    the    tropical    and subtropical  seas,  while  continental  tropical  air  is produced over subtropical arid regions. AI-9