Insolation

Insolation is derived  from  the  words "incoming solar radiation". Insolation is specifically applied to radiation which is arriving at earth’s atmosphere first and then earth's surface. The heat is derived from solar energy, normally called solar radiation. Insolation is the solar radiation that reaches the earth's surface. It is measured by the amount of solar energy received per square centimetre per minute. Similarly, solar energy received by the earth is called insolation. It is the amount of incoming solar radiation that is received over a unit area of the earth’s surface. Solar energy received over the planet’s surface varies according to season, latitude, transparency of the atmosphere, and aspect or ground slope.

Insolation affects temperature. The more the insolation, the higher the temperature.

In any given day, the strongest insolation is received at noon.

Factors influencing insolation 
The amount of insolation received on the earth’s surface is not uniform everywhere. It varies according to the place and time. When the tropical regions receive maximum annual insolation, it gradually decreases towards the poles. Insolation happens more in summers and less in winters. The major factors which influence the amount of insolation received are:

 Solar constant
 The angle of incidence of the sun’s rays
 Duration of the day
Earth Distance from Sun
 Transparency of the atmosphere

Solar Constant
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.

The angle of incidence

Since the earth is a geoid resembling a sphere, the sun’s rays strike the surface at different angles at different places. This depends on the latitude of the place.The higher the latitude, the less is the angle they make with the surface of the earth.
The area covered by the vertical rays is always less than the slant rays. If more area is covered, the energy gets distributed and the net energy received per unit area decreases. Moreover, the sun’s rays with small angle traverse more of the atmosphere than rays striking at a large angle.

Duration of The Day 
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.

Earth's Distance from Sun
The earth is  revolving around  the  sun  in  an  elliptical  orbit, resulting  continuous  change  in the distance between sun and the earth on annual basis. It leads to seasonal variation in solar energy received by  the  earth.The  mean  distance  between  the  earth  and  sun  is  about  149,600,000 kilometers(92,900,000  miles).When  earth position is  farthest(152  million  km)  from sun  is known as ‘aphelion’ on  July  4.  It  is  perihelion(147  million  km)occurs on January  3  each  year which  is  the  closest  distance. During  aphelion the  northern  hemisphere  is  facing  the  sun  and therefore  receives  energy  about  7  percent  less  than  the  perihelion  (southern  hemisphere).

Transparency of the Atmosphere 
Atmosphere is not transparent for all the radiation coming from the sun because of different composition and layers.   It is  also  one  of  the  controlling  factors of  insolation to  reach  earth surface. The   atmosphere   is   composed   of   gases,   water   vapour   and particulate   matters.The atmosphere  is  a  mixture  of  gases,such  as nitrogen(N),  oxygen  (O2), Argon, carbon  dioxide, Neon  (Ne),  Helium  (He),  Methane  (CH4),  Krypton  (Kr),  Ozone  (O3),  Nitrous  oxide  (N2O), Hydrogen (H) and Xenon (Xe). The atmosphere also contains water vapour, water in the gaseous state.

The longer the duration of daylight, the more the insolation received per day.

Other websites 
 National Science Digital Library - Insolation 
 Yesterday‘s Australian Solar Radiation Map

Solar energy
Climate