The Sun of planet Earth emits radiation at a temperature of about 6000 degrees K. Average radiation emitted by Sun at its surface (Sun’s radiant emittance) is 73×106 W per square meter. Spectrum of this solar radiation shows distinct emission lines indicating that it comprises of radiation of different wavelengths. The intensities and thus the magnitudes of the radiation of different wavelengths are also different. The intensity of far ultra-violet radiation is very low due to absorption of radiation by outer photosphere of the Sun. Further, extreme ultra-violet and X-ray parts of solar radiation are emitted from the chromosphere and corona regions of the Sun. These regions have temperatures as high as 1 million K. The solar radiation falling at the upper boundary of Earth’s atmosphere is termed incident solar radiation. Its average magnitude over the Earth is given by (Solar constant x r2)/4r2, where r = radius of Earth.
The Solar constant is the irradiance on an area at right angle to solar beam and outside the Earth. Its value is 1353 W per square meter. The average incident solar radiation at upper boundary of Earth’s atmosphere is approximately 11 x 109 J/m2 /yr. Important factors that affect this solar radiation received by Earth are:
Spherical shape of Earth: Earth is a sphere and, therefore, the angle at which incoming solar radiation strikes the upper boundary of atmosphere is not same at all points. The radiation strikes Earth at right angle in the center but the angle gradually becomes more acute towards periphery. As a result, the amount of solar radiation reflected back from the upper surface of atmosphere is zero at the center and increases gradually towards periphery. Thus, the amount of radiation penetrating atmosphere and entering Earth-atmosphere system is maximum in the center and gradually decreases towards periphery.
Orbit of Earth: The orbit of Earth around Sun is not perfectly circular but is slightly elliptic. Sun occupies one focus of this elliptic orbit. The mean distance between Sun and Earth is about 150 million kilometers. However, the orbit of Earth is elliptical and so the distance changes during different times in the year. Earth is closest (about 91.5 million miles) to Sun on about January 3, at which time it is said to be in perihelion. It is at greatest distance (about 94.5 million miles) from Sun on about July 4 when it is said to be in aphelion. These differences in distance also cause some difference in amount of solar radiation received by Earth. However, the ellipticity of orbit is not the reason of seasons on Earth. An important fact related to the Earth’s orbit around Sun is that the geometry of Earth’s orbit is not constant. The orbit at present is very slightly elliptical being nearly circular but the shape of Earth’s orbit changes cyclically from almost circular to markedly elliptical and back with a periodicity of 100,000 years. The solar constant is the mean solar irradiance on an area at upper boundary of Earth’s atmosphere perpendicular to incoming solar beam, which is about 1353 W per square meter. In the present state of Earth’s orbit being nearly circular, the difference between perihelion and aphelion is about 3.5% and the difference in solar constant at these two points is about 6.66%. In the state of most elliptic orbit, difference in solar constant at perihelion and aphelion may be as large as 30%.
Inclination of Earth’s axis of rotation: The axis of rotation of Earth is not perpendicular to the plane of ecliptic i.e. the plane in which Earth’s orbit and Sun lie. Earth’s axis of rotation makes an angle of about 66.5 degrees with the plane of ecliptic and is tilted 23.5 degrees from the line perpendicular to plane of ecliptic. The Earth’s axis although always makes an angle of 66.5 degrees with plane of ecliptic, also maintains a fixed orientation with respect to stars. The Earth’s axis continues to point to the same spot in the heavens as it makes its yearly circuit around Sun. This inclination of Earth’s rotational axis alongwith its fixed orientation throughout the whole orbit around Sun causes different seasons on Earth. Between September 23 and March 21, North Pole of Earth’s axis is tilted towards the Sun and South Pole is away from the Sun. During this period, Northern Hemisphere has summers and Southern Hemisphere has winters. In this period, daylength and, therefore solar radiation received increases towards North Pole and decreases towards South Pole. From March 21 to September 23, South Pole is tilted towards Sun and North Pole is tilted away from it, North Hemisphere has winters and South Hemisphere has summers during this period. In this period, daylength and, therefore, solar radiation received increases towards South Pole and decreases towards North Pole. Maximum tilts of North Pole and South Pole towards Sun occur on June 21 and December 22 respectively and these dates are termed summer solstice and winter solstice respectively. Midway between the dates of solstices, twice the Earth’s axis is at right angle to the line drawn from Sun to Earth and neither pole is tilted towards Sun. This condition occurs on March 21 or 22 (vernal equinox) and on September 22 or 23 (autumn equinox). Two important cyclic changes related with inclination of Earth’s axis of rotation have been noted. First is the wobbling of Earth’s axis of rotation with a periodicity of 21,000 years. This causes continuous and cyclic hemispheric variation in the solar constant. Second change is cyclic variation in the angle of inclination of Earth’s axis of rotation within the range of 21.8o and 24.4o (23.45 degrees at present) with a periodicity of about 40,000 years. Therefore, the distribution of solar irradiance at Earth’s two hemispheres varies continuously with this 40,000 years cyclic periodicity.
1TRANSFORMATIONS OF SOLAR RADIATION
The general fate of the incident solar radiation in Earth-atmosphere system is as below:
(a) Absorbed in stratosphere (mainly by Ozone) = 3%
(b) Absorbed in troposphere by:
(i) Carbon dioxide = 1%
(ii) Water vapor = 12%
(iii) Dust = 2%
(iv) Water droplets in clouds = 3%
(c) Reflected from clouds = 21%
(d) Scattering back into atmosphere 6%
(e) Reflected back from Earth’s surface = 4%
(f) Received at Earth’s surface as:
(i) Direct radiation = 27%
(ii) Diffused radiation via clouds or downward scattering = 21%
Solar radiation received by Earth i.e. incident solar radiation undergoes various transformations after entering the uppermost boundary of atmosphere. The solar radiation is absorbed by atmosphere, hydrosphere, lithosphere and biosphere. Some part of solar radiation absorbed in a component provides energy for the dynamic functions of that component. The remaining part of absorbed radiation is re-emitted from the component as long-wave radiation. Two important features in the study of the transformation of solar radiation are albedo and effective radiation, which are discussed below.
The fraction of solar radiation received by a body that is returned back from it forms the albedo of that body. In general, the radiation reflected back from clouds (21%), scattered back into atmosphere (6%) and reflected back from the Earth’s surface (4%) together constitute Earth’s albedo. The average value of albedo of Earth as a whole comes to about 33% or 0.33 (represented as fraction of unity).
The albedo of Earth as a whole has two components:
(a) Albedo of Earth’s surface: It is that fraction of radiation received at the Earth’s surface, which is returned back from the surface. Its value varies depending on the extent of snow cover, vegetation and the soil characteristics. Average albedo values for the snow range from 0.7 to 0.8 and may be as low as 0.4 to 0.5 in case of wet or dirty snow. In the deserts that have light sandy soil and are without vegetation, surface albedo values are typically 0.4 to 0.5. Albedo of damp soil is usually less than that of the corresponding dry soil. In case of damp chernozem soils the albedo values may be as low as 0.05. Albedo of natural Earth surface covered with thick vegetation cover generally ranges from 0.1 to 0.25. Areas covered with coniferous forests have lower albedo than those covered with meadows.
The height of sun in the sky determines the absorption of radiation in the water bodies, mainly the oceans. When sun is relatively high, radiation reaches water surface at high angle. A large part of the incoming radiation penetrates upper layers of water body and is absorbed. When sun is low, the radiation reaches the water surface at low angles and most of it is reflected. Thus it does not penetrate much and the albedo value of water surface increases sharply at low sun. However, in case of diffused radiation, albedo of water surface is much less variable and is about 0.1.
(b) Albedo of Earth-atmosphere system: It is more complex in nature than that of Earth’ surface. Its value is largely determined by the presence-absence, nature and thickness of clouds. In the absence of clouds in the sky, albedo of Earth-atmosphere system depends largely on the albedo of Earth’s surface. If clouds are present, a large portion of solar radiation reaching atmosphere is reflected back from the upper surface of clouds and albedo value of system increases. Albedo of clouds is usually 0.4 to 0.5. In the presence of clouds, albedo of Earth-atmosphere system is usually greater than that of Earth’s surface, except where surface is covered with relatively clean snow.
It is the difference between the amount of Earth’s radiation from Earth’s surface and the amount of long-wave counter-radiation from atmosphere. Most important factor connected with effective radiation is ‘green-house effect’ due to presence of atmosphere. The atmosphere has various gases viz. carbon dioxide, methane, water vapor etc. which selectively absorb long-wave radiation. Due to this Earth’s atmosphere is relatively more transparent to short-wave radiation than to long-wave radiation. Since long-wave radiation from the Earth’s surface is trapped in the atmosphere, average effective radiation from Earth’s surface as a whole is much lower than the short-wave radiation absorbed at the surface.
The effective radiation of Earth’s surface largely depends on the temperature at Earth’s surface, atmospheric humidity and clouds. Experimental data has shown that radiation of Earth’s natural surfaces is generally quite close to the radiation of black body at corresponding temperatures. Further, a significant part of long-wave radiation lost from Earth’s surface is compensated by long-wave counter-radiation from the atmosphere. This counter-radiation mainly depends on the amount of atmospheric water vapor i.e. air humidity and clouds and so these factors affect the effective radiation of Earth’s surface.