Insolation
The length  of  the  day  determines  the  duration  of  sunlight  which  affects  the  amount  of solar radiation received by the earth’s surface. The longer  period  of sunshine,  greater  the  quantity  of solar  radiation will be  received by  a  portion  of  earth.For  example,  at  the  equator  the  length  of days  and  nights  is  12  hours in  all  the  months but the  tropics  of  Arctic  and  Antarctic sunshine duration varies between 0 and 24. On the autumn and springe quinoxes (September 23and March 21 respectively),the sun is overhead at the equatorat noon.The night and day all over the earth are equal on these days and maximum amount of insolation is received at the equator, and the amount of insolation decreasing towards  the  poles.It  is  caused  by  vertical  sunshine  at  equator  but  with  increasing  latitudes,  the rays  become  more  and  more  slanting.  Therefore,  poleward,  the  received  energy  keeps  on declining.