Climatology/Insolation and Heat Budget



Solar irradiance (SI) is the power per unit area (watt per square metre, W/m2), received from the Sun in the form of electromagnetic radiation as reported in the wavelength range of the measuring instrument. This integrated solar irradiance is called solar irradiation, solar exposure, solar insolation, or insolation.The sun is the primary source of energy for the earth. The sun radiates its energy in all directions into space in short wavelengths, which is known as solar radiation.
 * The earth’s surface receives only a part of this radiated energy (2 units out of 1,000,000,000 units of energy radiated by the sun).
 * The energy received by the earth’s surface in the form of short waves is termed as Incoming Solar Radiation or Insolation.
 * The amount of insolation received on the earth’s surface is far less than that is radiated from the sun because of the small size of the earth and its distance from the sun.
 * Moreover, water vapour, dust particles, ozone and other gases present in the atmosphere absorb a small amount of solar radiation.
 * The solar radiation received at the top of the atmosphere varies slightly in a year due to the variations in the distance between the earth and the sun.
 * During the earth’s revolution around the sun, the earth is farthest from the sun on 4th July. This position of the earth is called aphelion. On 3rd January, the earth is nearest to the sun. This position is called perihelion.
 * Due to this variation in the distance between the earth and the sun, the annual insolation received by the earth on 3rd January is slightly more than the amount received on 4th July.
 * However, the effect of this variation is masked by some other factors like the distribution of land and sea and the atmospheric circulation. Hence the variation does not have a greater effect on daily weather changes on the surface of the earth.

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 is more in summers and less in winters. The major factors which influence the amount of insolation received are:--


 * 1) Solar output /constant
 * 2) The angle of incidence of the sun’s rays
 * 3) Duration of the day
 * 4) Earth distance from sun
 * 5) Transparency of the atmosphere

Insolation also varies due to the length of the day. The days may be longer or shorter depending on latitudes or the change of season. A longer day means greater insolation because sunlight is available for a longer period. The duration of the day changes from equator towards poles, from season to season and so, inclination also varies accordingly.

Insolation also depends on the transparency of the atmosphere. The amount of cloudiness and its thickness, dust and water vapor which determine the transparency of the atmosphere, affect the reflection, absorption and transmission of solar radiation.

The total amount of insolation is maximum at the tropics. At 45˚latitude it is only 75%,about 58% at the arctic and antarctic circle. And only about 40% at the poles.



Solar output /constant
At the top of the earth’s atmosphere receives insolation is  expressed  as  the  solar  constant.It received  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 square metre)energy (solar constant) 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  Wm2by  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  sunspots  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.The sun’s rays with small angle traverse more of the atmosphere than rays striking at a large angle. The angle  at  which  sun  rays  strike  on  earth surface is  called  angle  of  incidence. Itcontrols the amount of insolation received at the earth’s surface. The amount  of  insolation is  determined  by time of the day (morning, noon and evening), the latitude (equator to poles) and season (summer, autumn,  winter  and spring).When  the  sun  rays strike vertically  or  sun is directly  overhead,  the rays angle of incidence is 90 degree. The beam of rays spread on a smaller area in comparison to oblique/ slanting beam of sun rays.when sun is in a vertical position, the beam of light will spread over one mile but in the oblique position(with 300of angle) of the sun the  same  beam  of  light will  spread  over two miles.It is clear that the larger amount of radiant  energy  is lost in  case  of  slanting  rays  than  in  vertical  rays. Therefore, on  an average, equatorial areas receive approximately 2.4 times more insolation than polar areas.

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 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 AtmosphereOur
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.