Insolation
At the top of the earth’s atmosphere, received insolation  is  expressed  as  the  solar  constant. It receives  at  the  top  of  the  atmospheric surface (thermopause) on  a perpendicular  plane  to the solar beam. The average insolation received at the thermopause i.e.1368Wm2(Watt per squaremetre)energy(solar constant) is in the form of short wave. Thus, it is termed as solar constant for that  mean  distance  from  the  sun. These solar constant  is  varying  over 1 Wm2 by  periodic disturbances and explosions in the solar surface basically related to sun spot. Sun spots are dark and cooler areas visible on the sun’s surface.The  recent  researches  have  shown  that  more and more  energy  is  released  when  the  sunpots  are  in  large  number.The  number  of  sunspots  also increases  or  decreases  on  a  regular  basis,  creating  a  cycle  of  11  years.