within one common boundary, that is, the lower 48 states of the United States. CONVECTION—Motions  in  a  fluid,  such  as  the atmosphere or water,   which are predominantly vertical, resulting in a vertical transport of mass. CONVECTIVE  CELL—Cumiliform  cloud,  usually with vertical updrafts in the center and sinking downdrafts in the outer region. CONVERGENCE— Atmospheric  flow  approaching the same point from different directions. CORRELATION—A measure of similarity between variables  or  functions. CROSS-SECTION  PRODUCT—A vertical depiction   of   the   atmosphere   produced   by compiling  base  data  vertically,  along  a  user defined  direction. D DDN—Defense  Data  Network. DECIBEL(dB)— A logarithmic expression for the ratio of two quantities. Mathematically, dB = 10 Log (Pl/P2). DEDICATED   COMMUNICATIONS—Communi- cations links that have no other purpose than to transmit  data  between  components  of  the  WSR- 88D. DERIVED  DATA—Data created from the computer processing  of  base  data. DESCENDING  NODE—The period of time when a polar-orbiting  satellite  is  traveling  from  north toward  the  south. DIFFRACTION—The process that causes electromagnetic waves traveling in a straight path to bend around a boundary or obstruction. DISTORTION—An  apparent  warping  and  twisting of an image received from a satellite caused by the characteristics of the satellite lens and the optical alignment. DIVERGENCE—Atmospheric   flow   leaving   the same  point  in  different  directions. DMS—Position   given   in   degrees,   minutes,   and seconds  of  latitude  and  longitude. DMSP—Defense  Meteorological  Satellite  Program. DOPPLER EFFECT—The observed change in the frequency of sound or electromagnetic waves due to  the  relative  motion  of  the  source  and  the observer. DOWNBURST—A    strong    downdraft    from    a thunderstorm  cell  that  induces  an  outburst  of damaging winds on or near the ground. DRYLINE—A   mesoscale   feature   with   its   own associated  vertical  circulation.  It  is  a  narrow, almost   vertical   zone,   across   which   a   sharp moisture  gradient,  but  little  temperature  gradient, occurs at the earth’s surface. DSN—Defense switched network, an upgrade and name  change  from  the  automatic  voice  network (AUTOVON). E EARTH  SYNCHRONOUS—See  geostationary. ECCENTRICITY— The ratio of apogee and perigee. A true circular orbit has an eccentricity of 1. ECHO—Energy backscattered from a target as seen on  the  radar  display. ELECTROMAGNETIC   ENERGY—Energy propagated  through  space  in  the  form  of  an advancing  disturbance  in  electric  and  magnetic fields. Also referred to as radiation. ELECTROMAGNETIC   SPECTRUM—The  total range of the various radiation frequencies and corresponding   wavelengths. ENHANCEMENT— The process of augmenting a black and white satellite image by the addition of either  color  or  alternating  gray  shading. EPHEMERIS  DATA—Temporary data listing the coordinates of a satellite at a number of specific times within a specific period. EPOCH—An instant in time, selected as a point of reference. EQUATORIAL  ORBIT—Satellites that circle the earth over the equator and have an inclination of 0°.  (Geostationary  satellites). EXTERNAL  USER—Anyone other than a Principal User who has access to WSR-88D products, for example,  information  service  companies, broadcast  meteorologists,  aviation  interests,  and universities. AI-2