SOLAR ENERGY

  The sun is the only significant source of energy to the earth. 
  Received  entirely as  radiation, this energy is subsequently 
  converted, so that at any  given  time and place, an original 
  quantum  may  appear in any one of the several  diverse forms. 
  Thus, with  respect to the  energy  encompassed in the day to 
  day  climate  at  a  given  place  and time, solar energy may 
  appear as the  latent heat of vapor, the advected heat of air 
  masses, radiant  energy from sky and terrestrial objects, and 
  unconverted  solar  radiation  arriving  as  direct beam. The 
  unconverted solar radiation in the form of direct beams  is a 
  very  important  form  that  is  dominant  in  the hydrologic 
  processes of evaporation, transpiration and snow ablation, as 
  well as, in the  growth  and  succession  of  vegetation. The 
  total flux of energy to a site at any given time is difficult 
  to  measure, and  the  flux  of  radiant  energy alone can be 
  measured  readily  at a  point. However, this flux  varies so 
  widely  over  most land surfaces that  point measurements are 
  of  limited  value. The spatial  heterogeneity in the radiant 
  flux is due to the variation in solar beam irradiation caused 
  by  variation in the inclination of the earth's  surface with 
  respect to beam direction. Within a geographic region subject 
  to uniform  atmospheric  conditions, irradiation from the sky 
  is fairly uniform, regardless of surface inclination. However, 
  the total sun and sky irradiation  varies widely with surface 
  orientation and slope. The variation of direct beam radiation 
  varies in time with  atmospheric  condition  and  path length, 
  the combined effect of which can be estimated. 

  Systematic  long-term  measurements  of solar and terrestrial 
  radiation  in   India   using   calibrated   and   maintained 
  instruments as per international standards was started in the 
  year  1957 - 58.  The   radiation  laboratory  of  the  India 
  Meteorological  Department, Pune,  serves  as  the  radiation 
  centre for  India and monitors 25 specifically selected sites,  
  which  form  the  National  Radiation  Network.  Of these  25 
  stations, 13  (principal  stations) are  equipped  to monitor 
  continuous records of global and diffuse solar radiation, and 
  measure  direct  solar  radiation  at  regular  intervals  in 
  addition to recording sunshine.  In the remaining 12 stations 
  (ordinary  stations),  a  minimum  program  is carried out of  
  continuous  recording  of  only  global  solar  radiation  in 
  addition to  recording  sunshine. Apart from these parameters,  
  all   stations   maintain   continuous   records   of   other 
  meteorological  parameters  such  as  temperature ,  relative 
  humidity, rainfall, wind speed and wind direction. 

  The radiation energy of the sun can be captured and converted 
  into  heat  and/or  electricity.  A distinction  must be made 
  between  beam  and diffuse radiation.  The former reaches the 
  receiving  surface in a straight line from the sun, while the 
  latter  is  reflected in the  atmosphere by clouds, dust, etc.  
  This leads to different systems of solar energy conversion.