It is remarkable that inspite of inconstant sun with large fluctuations in its radiation during solar flares and prominences, etc., the intensity of solar radiation reaching the outer boundary of the earth's atmosphere has remained more or less constant over the last few centuries. The reasons for this may be the following: the large fluctuations that often occur in solar activity mostly affect the extreme ultraviolet and X-ray part of the solar spectrum which contains a very small amount of solar energy as compared to the visible and longwave part which has more than 98% of the energy. Thus, in the mean, the variations are found to be almost negligible and the accepted mean value of the solar constant is about 1368 W m-2.
Angstrom, however, found from some measurements that the solar constant, So, could be connected with the sunspot activity by the formula (see, e.g., Saha and Srivastava, 1931, p 561)
where N is a co-efficient (known as the Wolf and Wolfer's number) characterizing the number and extent of sunspots, and S0 is given in units of Cal/cm2/min.
8.6.2 The Transparency of the Atmosphere - Effects of Clouds and Aerosols
In Sect. 8.4, we pointed out some of the factors which interfere with the passage of the incoming solar radiation through the atmosphere. Amongst these, the presence of the clouds is, perhaps, the most important, since it can cut out almost 70-80% of the incident radiation. Amongst the others involving particulate matter one may, perhaps, mention the periodic release of enormous amount of dust thrown up into the atmosphere by volcanic eruptions around the globe. The erupted material from such explosions remains suspended in the atmosphere sometimes for months and years. However, it is estimated that in the absence of clouds the total loss from the incoming radiation due to factors which cause scattering, reflection and absorption, may not normally exceed 6-7% of the incident radiation.
8.6.3 Distribution of Solar Radiation with Latitude - The Seasonal Cycle
It is well-known that as the earth moves around the sun in an elliptical orbit with the sun at one focus, its obliquity to the sun makes the zenithal position of the sun oscillate between 23.50N and 23.5°S, in a seasonal cycle during the year. It is overhead at the equator at the equinoxes in March and September when days and nights are about equal. The earth is then at its mean distance of about 1.4968 x 108km from the sun and receives solar radiation at a more or less constant rate. It is farthest from the sun in June-July when the sun is overhead at 23.5°N and the days are longer than nights and closest to it in December-January when the sun is overhead at 23.5°S and nights are longer than days.
The angle at which the sun's rays strike the earth's surface varies with time of the day, season and latitude. Since the length of a day or night varies with latitude, the insolation varies with latitude and season only. In the summer hemisphere, the length of the day gets longer and longer with increasing latitude till near the pole, it is almost 24 h of daylight. The reverse is the trend in the winter hemisphere where day gets shorter and shorter with increasing latitude and it is almost 24 h of night near the pole. Thus, the duration of sunlight is the longest over the earth's polar region which receives more radiation from the sun than any other place on earth at the time of the summer solstice, though the distance from the sun is greater and the sun's rays fall at slanting angles. Ignoring any loss in intensity due to passage through the atmosphere and using astronomical data, Milankovitch first computed the rate at which solar energy is received at the earth's surface at different latitudes in different seasons (see Fig. 8.4). In later work, he revised his computations by allowing a constant transmissivity of 70% for the atmosphere at all wavelengths and also allowing the sec 9 effect (vide Eq. 8.5.5). The result of this later computation is shown in Fig. 8.5.
In the first case (Fig. 8.4), Milankovich found that there is little variation in the intensity of the solar radiation over the equatorial belt (the variation being limited to about 100 Ly per day between summer and winter), since the sun always shines overhead between 23.50N and 23.5°S. However, large variations occur in polar latitudes where the intensity changes from nearly zero in winter to about 1100 Ly per day in summer. Thus, a secondary maximum in insolation is to be expected in high latitudes, which arises due to a combination of the effects of increasing duration of sunlight with increasing latitude and increasing radiation intensity with decreasing latitude. It is also observed that the maximum north polar radiation is 72 Ly per day less than the corresponding south polar radiation. This is because the earth is closest to the sun at the time of the southern hemisphere solstice and farthest away at the time of the northern hemisphere solstice.
However, it may be seen from Fig. 8.5 that large changes in computed values of the radiation reaching the earth's surface occur with an atmospheric transmissivity of 70%. For example, near the equator, instead of 790Ly per day, the rate now is about 460 Ly per day. Very significant changes are noticed over the polar-regions, where a net cooling due to longer pathway shifts the maxima over the summer poles to latitudes near 350 at the solstice times.
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Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.