Introduction
The Solar energy derived from the Sun is the primary source of energy for all organisms on the Earth. Be it producers such as plants or consumers such as animals, the Sun provides them energy to provide. Although there are other sources of energy apart from solar radiation. These are gravitational energy and energy generated from the endogenetic forces including hydrothermal energy. The insolation or the incoming solar radiation has a variable impact on all the life forms which are present on the earth. These are responsible for various geo-biochemical cycles. Further, the solar energy is also responsible for different climatic events such as driving winds on the earth’s surface, ocean currents, and exogenetic or denudation processes.
Solar Radiation
Solar radiation is the energy which is emitted by the Sun and sent in all directions through space as electromagnetic waves. These are emitted by the surface of the Sun. This energy influences atmospheric and climatological processes on the earth. It is formed due to nuclear fusion which occurs in the Sun’s interior. It leads to the formation of Helium from the hydrogen molecules and finally the release of a large amount of heat and pressure. This heat is transported to outer space through the process of conduction and convection. The formation of heat from the sun’s interior and its transmission to the outer surface or Photosphere is more or less constant.
Fig. Formation of Solar Energy
Solar Constant
It is the amount of solar radiation received per unit area of the earth's surface facing the Sun. Solar Constant is constant because the average distance between the Earth and the Sun is almost constant and the Solar radiation and heat emanating from the Earth are also constant. It’s value has been calculated as 2-gram calories per square centimeter per minute. (2 cal/min/cm2). Its unit is Langley.
Laws for Solar Radiation
The nature of flow of the solar radiation is governed by two laws. These are:
Wien’s Displacement Law
Stefan Boltzmann Law
Wein’s Displacement Law
The Wein’s Displacement law determines the correlation between the wavelength and temperature. According to this law, the wavelength of the radiation is inversely proportional to the absolute temperature of the emitting body.
Stefan Boltzmann Law
Stefan Boltzmann Law states that the total radiant heat power which is emitted from a surface is proportional to the fourth power of its absolute temperature.
Process of emission of Solar Radiation
Dual Nature of the Solar Radiation: The sun’s surface temperature is usually as high as 6000 °C. It heats highly incandescent gases from below which leads to the emission of the array of photons. Photons are also known as light quantum which is essentially a minute energy packet of electromagnetic radiation which has dual property. It means that the photon has the property of light with a wavelength as well as property of a particle.
Short Wave and Long Wave Radiations: The continuous emission of photons from the surface of the Sun causes continuous bands of radiation having certain wavelengths. This radiation is called ‘Short Wave Radiation’. The radiation has been named ‘Short Wave Radiation’ relatively as the outgoing terrestrial radiation which has a longer wavelength, is termed as the ‘Long Wave Radiation. ’Solar energy is radiated in the form of electromagnetic waves called Electromagnetic Radiation.
Insolation
Insolation refers to the ‘Incoming Solar Radiation’. It the radiant energy which is received from the Sun which is analogous to the short waves. The Insolation has wavelengths ranging from 1/250 to 1/6700 mm which travels at a rate of 1,86,000 miles per second. It helps run various geo-biochemical Cycles and in the process of Photosynthesis which is the most important factor for the sustenance of Biospheric communities.
Direct Insolation
Direct Insolation is different from the Insolation. It is the difference between the Solar Constant and the atmospheric losses which is caused by the phenomenon of absorption and scattering of the Sun’s rays when the sun’s rays enter the earth’s atmosphere.
Electromagnetic Spectrum
The sun emits its radiation in the form of an electromagnetic spectrum. It is a combination of different radiations of variable wavelengths. These lights together form the white light which is emitted from the sun in the form of solar radiation.
Fig. Electromagnetic Spectrum
Properties of the electromagnetic spectrum
Speed: The electromagnetic spectrum travels at a speed of 3,00,000 Km per second. It takes 8 minutes and 20 seconds for this light to travel to the surface of the earth. It covers a total distance of 150 million Kilometers.
Direction and Path: The electromagnetic radiation is emitted from the sun’s surface radially and almost in a straight line.
Wavelength and Frequency: This electromagnetic radiation has four spectra of radiation waves with four different wavelengths which have four different frequencies. The human eye can only detect wavelengths from 380 to 700 nanometers. Hence, out of the four spectrums, only the Second Spectrum is visible to the human eye.
Four different Spectrum of Electromagnetic Radiation
First Spectrum
The first spectrum of electromagnetic radiation emitted from the sun’s surface consists of Gamma rays, X-rays and ultraviolet rays (UV Rays). There are two types of X-rays. These are Soft X-rays and Hard X-rays. This Spectrum has a wavelength ranging from 1014 to 109 Angstrong.
Second Spectrum (Visible Light Spectrum)
The Second Spectrum of the electromagnetic spectrum is the Spectrum of white light. It is the only spectrum which is visible to human beings with the naked eye.
Constituents of the White Light: This spectrum consists of seven different monochromatic lights such as Violet, Indigo, Blue, Green, Yellow, Orange and Red in order of increasing wavelength. These lights are together called ‘VIBGYOR’.
Wavelength of the visible light spectrum: The wavelengths of these lights range from 0.4 microns to 0.7 microns. The Red light has the largest wavelength while the Violet light has the least. This spectrum carries 41% of all energy of the electromagnetic spectrum.
Fig. Visible Light Spectrum
Third Spectrum
This spectrum of the electromagnetic spectrum is called the infrared spectrum. It consists of infrared waves and this spectrum has wavelengths ranging from 0.7 microns to 300 microns.
Fourth Spectrum
The fourth spectrum of the electromagnetic spectrum consists of microwaves, radar waves and radio waves. The wavelength of these waves varies from 0.03 cm to 100m. These waves are helpful in the transmission of television and radio communication.
Distribution and Variability of Insolation on the Earth’s surface
The Insolation on the Earth’s surface varies with the latitude, geographical configuration etc. The insolation which is received at the surface varies from about 320 watts/m2 in the tropics to about 70 watts/m2 in the poles. The maximum insolation is received over the subtropical deserts, where the cloud cover is the least. The Equator receives relatively less insolation than the tropics. This is due to the presence of a large amount of clouds over the equator. If the same latitude is considered, the insolation is more over the continent as compared to the oceans. In winter, the middle and higher latitudes receive less radiation than in summer. There are various factors which are associated with the distribution of insolation on the surface of the earth.
Factors Affecting the Distribution of Insolation on the Surface of Earth
There are various factors which affect the distribution of insolation on the surface of the earth. Angle of the Sun’s rays, length of the day and distance between the Sun and the earth are some of the factors which determine the amount of insolation which is received on the surface of the earth. These factors can be summarized as:
The angle of Sun’s Rays
The amount of insolation received at a particular place on the Earth’s surface is dependent on the angle between the Sun’s rays and tangent to the surface of the Earth of that particular place. As the Sun’s rays are more vertical at the equator and more oblique at the poles, the amount of insolation which is received at these two places is different. As the sun angle decreases, light is spread over a larger area and decreases in intensity i.e. energy input per unit area.
Fig: The changes in the insolation with the angle of Sun’s Rays
Fig. The variability in insolation with change in angle
Why is there variability in the insolation with oblique and vertical rays of the Sun? When the Sun’s rays pass through the atmosphere, the energy is lost in scattering, reflection and absorption. These losses are termed as atmospheric losses. When the sun’s rays pass through the atmosphere vertically, it has to travel to a shorter distance and thus passes through the atmosphere with less atmospheric losses and hence the intensity is high. When the oblique rays pass through the atmosphere, they pass through a thicker layer of the atmosphere and hence the atmospheric losses are high leading to less intensity. Hence, there is a variability in the insolation with oblique and the vertical rays of the Sun. |
Length of the Day
Length of the day is one of the determinants of the insolation received at one point on the earth’s surface. The duration of sunlight hours determines the length of the day. It also affects the amount of solar radiation received at the surface. The longer period of sunshine provides a larger supply of radiation which is received by a particular area of the earth. The latitude has the most dominant control throughout sunshine and thereby the length of the day. There is latitudinal and monthly variations in the length of days as illustrated in the Table
Distance between the Earth and the Sun
There is a variation in the amount of insolation with the variation in the distance between the Sun and the Earth. It is because the earth revolves around the sun in an elliptical orbit. When the distance between the sun and the Earth is minimum (Perihelion) the Earth receives the maximum insolation. But when the distance between the Earth and the sun is maximum (Appehelion) the Earth receives minimum insolation. This condition is only true when other factors impacting the insolation such as length of day, effect of atmosphere and impact of Sunspots are kept constant.
Sunspots
Sunspots are those areas on the Sun where the magnetic field can be as much as 2,500 times stronger as compared to the surrounding areas. Some of the sunspots are larger than the size of the earth. These Sunspots lower the temperature of their region relative to their surroundings because the concentrated magnetic field inhibits the flow of hot, new gas from the Sun's interior to the surface.
Cycle of the Sunspots
Sunspots also follow a cyclic pattern. Sunspots increase and decrease through an average cycle of 11 years. Due to these Sunspots, the ultraviolet emission from the sun increases and hence there is a slight increase in the energy output from the sun increasing the insolation.
Effect of Atmosphere
The atmospheric losses are one of the major determinants of the amount of insolation received on the surface of the earth. A large amount of insolation is lost due to reflection, absorption and scattering (together called atmospheric losses) into the atmosphere. There are various factors which impact the absorption, reflection and transmission of the sun’s rays. These are:
Absorption
Out of the total electromagnetic radiation which is produced by the Sun, 14% of the solar radiation is absorbed by atmospheric gases such as Ozone, Oxygen and carbon dioxide. Ozone absorbs maximum insolation which is followed by Oxygen and Carbon-di-Oxide. Different gases which are found in the atmosphere absorb different frequency wavelengths and hence impact the amount of insolation differently.
Scattering
The phenomenon of scattering in the atmosphere is inhibited by the molecules of gases and dust particles which are present in the atmosphere. When sunlight enters the atmosphere of the earth, the atoms and molecules of different gasses present in the air absorb the light. Then these atoms re-emit the light in all directions. This process is known as the Scattering of light. Out of the total incoming insolation, 23% is scattered into the atmosphere by dust particles. Of these total 23% scattered insolation, 6% of the scattered energy goes back to space while the remaining 17% reaches the earth’s surface. Hence, the 6% of the insolation is lost into the atmosphere.
Selective Scattering
A phenomenon of selective scattering is observed in the atmosphere. The phenomenon of scattering only happens when the diameter of the dust particles is shorter than the wavelength of the components of the incoming solar radiation. Thus, in the visible light spectrum, blue light (which has the minimum wavelength) is scattered more as compared to red light (which has the maximum wavelength) making the sky blue.
Heat Budget of the Earth and the Atmosphere
The balance of incoming and outgoing heat on Earth is termed as its heat budget. The earth and the atmosphere exchange heat to maintain the temperature equilibrium of the earth. The earth does not accumulate or lose heat. The heat budget maintains the optimum temperature of the earth which helps in survival of the plants and animal community. Hence, the amount of heat received in the form of insolation is equal to the amount of heat lost by the earth through terrestrial radiation.
What is Global Radiation? The total amount of solar radiation which reaches the Earth's Surface is called Global radiation. It consists of direct shortwave radiation from the sun and diffuse radiation from the earth which is scattered by the atmosphere. Diffuse Radiation: The diffuse radiation is the remaining and the redirected sunlight due to several factors occurring in the atmosphere. |
Calculating the Heat Budget
When the solar radiation passes through the atmosphere and some amount of energy is reflected, scattered and absorbed. There is equilibrium between the total amount of solar radiation which is received by the Earth’s surface and its atmosphere and the heat lost by outgoing long-wave terrestrial radiation and loss of heat from the atmosphere.
Insolation received in the form of Solar Radiation (100%) | Losses by Reflection 35 % of the insolation is lost by the process of reflection
|
Earth’s Heat Budget (Energy Received by Earth ) | Terrestrial Heat Balance (Energy lost by the Earth)
|
51% of the insolation is received by the earth’s surface
| 51% of the heat is lost by the atmosphere.
|
Heat Budget of the Atmosphere | Heat Balance of the Atmosphere |
48% of the insolation is received by the atmosphere
| 48% of the total heat is lost into the atmosphere through process of radiation |
Heat Gained= Heat Lost |
Net Radiation and Latitudinal Heat Balance
Earth's net radiation is also called net flux. It is the balance between incoming and outgoing energy at the top of the atmosphere. It is the total energy that is available to influence the climate. Energy comes into the system when sunlight penetrates the top of the atmosphere. The net Radiation of the whole earth is Zero. However, the net radiation on the surface of the Earth varies.
However, there can be some places on the earth whose net radiation is not zero. There are some places on the earth which are energy surplus while there are some places on the earth which are energy deficient. It depends on different factors which has been discussed above.
Fig. Energy Surplus and Energy Deficient Regions
Energy Surplus areas
These areas which lie between the zones of 20° N and 20° S latitudes are termed as energy surplus regions. The energy gained by the total incoming solar radiation in these areas is higher than energy lost through outgoing long wave terrestrial radiation, making them energy surplus.
Energy Deficit Regions
There is a pattern of decrease in insolation received by the earth’s surface as we move from low latitudes towards mid-latitudes. The net radiation becomes practically zero at 70° in both hemispheres. Afterwards, the insolation keeps on decreasing with increasing latitudes while terrestrial radiation keeps on increasing causing more and more loss of heat. Hence, these areas are called energy deficit regions leading to colder climates.
Fig. Latitudinal Heat Balance
Apart from the ‘Earth’s Surface-Atmosphere heat transfer system’, there exists a separate system which transfers heat from the equator towards the poles. There is a two-way system exits which facilitate heat transfers. It led to the identification of three different zones such as:
A large energy Surplus Region exists between 40° N and 30° S latitudes
A energy deficit region exists in Northern High Latitudes
A energy deficit region exists in Southern High Latitudes
Hence, it is well-accepted that there is a flow of heat from energy-surplus tropical regions to energy-deficit sub-tropical and Polar Regions. This inter-latitudinal flow of heat from the equator-poleward has been termed as ‘Meridional Transport of Heat’.
This meridional circulation is accomplished by the movement of water mass and air mass in the form of Ocean currents and atmospheric circulation. Further, there is also a vertical distribution and heat balance which is accomplished by ascending air. Different processes of the oceans and atmosphere helps in executing this meriodional circulation of the atmosphere.
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