Climatology/Insolation



Solar irradiance (SI) is the power per unit area 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.

Methods of Solar Energy Receipt
The solar radiation received by the earth’s atmosphere and surface by direct radiation or diffuse energy. There are five way of solar radiations. They are:--
 * Transmission
 * Scattering
 * Refraction
 * Absorption
 * Reflection

Transmission
When the solar radiation reaches to the earth surface through a medium is known as transmission of solar  radiation. The dictionary  meaning  of  transmission  is  the  passage  of  shortwave  and longwave energy (electro-magnetic energy) through either the atmosphere or water but the sun’s energy is reaching to the earth without any medium from the space. It is also known as radiation. The atmospheric energy comprises shortwave radiation inputs (ultraviolet light, visible light, and near-infrared wavelengths) and longwave radiation outputs (thermal infrared) that pass through the  atmosphere  by  transmission. In our  atmosphere,  all  the  radiation  is  not  reaching  to the  earth surface  or  in  other  word,  atmosphere  is  not  opaque  for  all  the  radiation. This differential transmission  causes  the  greenhouse  effect  in  the  atmosphere.Due  to  greenhouse  effect  of  the atmosphere, the earth’s atmospheric temperature is  hospitable. Without this  effect,  the  living conditions could not be created.



Scattering
Atmospheric gases and dust particles physically interact with incoming solar radiations through processes of  scattering. A redirection of  energy  through  refraction  and  reflections  called scattering. The solid particles  of  dust,  smoke,  aerosols,  sea  salts  sprays,  pollutants  atmospheric humidity,  smoke  shoots  etc. available in  the  atmosphere  are  responsible  for  scattering  of  sun electromagnetic  energy. It changes the  direction  of  the  light's  movement without  altering  its wavelengths. This phenomenon  is  known  as  scattering  and  represents  7percent of  Earth's reflectivity, or albedo. It is ‘unpredictable’ because of multiple reflections of electromagnetic waves by particles and surfaces. But inreflection, the direction of reflection is predictable. Dust particles, pollutants, ice, cloud droplets,  and water vapor produce  further scattering.



Refraction
Refraction means bending of light. As solar radiation enters the atmosphere, it passes from one medium to  another  of  atmosphere,  from  virtually  empty  space  into  atmospheric  gases. The atmosphere itself  is  made  up  of  different  layers. The different  layers  have  different  density  and change in density of the atmosphere causes the bending of incoming solar radiations at different angles. This refraction  could  also  be  seen  with  water  as  well. Itis known  as  refractionof incoming  radiation. For example,  prism  refracts  light  passing  through  it,  bending  different wavelengths  to different  angles,  separating  the  light  into  different  component  of  colours  to display  the  spectrum. In nature,  rainbow  is  created  when  visible  light  passes  through  many raindrops  and  is  refracted  and  reflected  toward  the  viewer  at  a  precise  angle.

Reflection
The reflection is an ability of the material where a portion of arriving energy strikes. Depending upon the  surface  characteristics,  where  it  strikes,  the  quantum  of  reflectivity  is  determined. It might be  completely bounced  directly  back  into  space  with  or  without  being  absorbed. This returned energy is called reflection especially specular reflectance where angle of incidence and angle of reflectance is same. A mirror is one of the example of reflecting more than 90 per cent of the visible light incidence upon it. The term albedo is used to describe the amount of energy reflected back  in percentage. Albedo is  the  reflective  quality  of  a  surface. It is  an  important control  over  the  amount  of  insolation  that  is  available  for  absorption  by  a  surface. So,the proportion of insolation that is reflected back from the atmosphere, from the tops of the clouds, and earth  surface  including  land  and  water  both,  without  heating  the  receiving  surface,  is  an albedo.

Absorption
In our earth’s atmosphere due to the different composition, act as a barrier to the insolation or it absorbed some  solar  radiation,  may  be  assimilated  by  the  object  is  generally  termed  as atmospheric absorption. The different material on earth has different absorptive capabilities with different wavelength of solar radiation. It is happening in all electromagnetic radiations at certain spectral bands  by  the  composition  present  in  the  atmosphere. The most  efficient  absorbers  of solar  radiation  in  this  regardare  water (H2O),  carbon dioxide (CO2), ozone (O3), Oxygen (O2),nitrous oxide (NO2). The cumulative  effect  of  the  absorption  by  the  various  constituents  can cause the atmosphere to close down completely in certain regions of the spectrum that leads to an increase in temperature. Generally, a good radiator is also a good absorber and a poor radiator is a poor absorber. As colour is concern, dark-colour surface are much more efficient absorbers of radiation in the visible portion of the spectrum than light-colour surface.



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



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  anaverage, 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 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.

Mechanisms of Heating and Cooling of Atmosphere
Heating and cooling of theatmosphere is performed by following processes:--
 * Partial absorption of solar radiation by atmosphere
 * Conduction
 * Terrestrial radiation
 * Convection
 * Advection
 * Latent heat of condensation
 * Expansion and compression of the air