From Sun to Earth
The enormous amount of energy continuously emitted by the sun is dispersed into outer space in all directions. Only a small fraction of this energy is intercepted by the earth and other solar planets.
The solar energy reaching the periphery of the earth's atmosphere is considered to be constant for all practical purposes, and is known as the solar constant. Because of the difficulty in achieving accurate measurements, the exact value of the solar constant is not known with certainty but is believed to be between 1,353 and 1,395 W/m2 (approximately 1.4 kW/m2, or 2.0 cal/cm2/min). The solar constant value is estimated on the basis of the solar radiation received on a unit area exposed perpendicularly to the rays of the sun at an average distance between the sun and the earth.
In passing through outer space, which is characterized by vacuum, the different types of solar energy remain intact and are not modified until the radiation reaches the top of the earth's atmosphere. In outer space, therefore, one would expect to encounter the types of radiation listed in Table 1, which are: Gamma ray, X-ray, ultraviolet, and infrared radiations.
Not all of the solar radiation received at the periphery of the atmosphere reaches the surfaces of the earth. This is because the earth's atmosphere plays an important role in selectively controlling the passage towards the earth's surface of the various components of solar radiation.
A considerable portion of solar radiation is reflected back into outer space upon striking the uppermost layers of the atmosphere, and also from the tops of clouds. In the course of penetration through the atmosphere, some of the incoming radiation is either absorbed or scattered in all directions by atmospheric gases, vapours, and dust particles. In fact, there are two processes known to be involved in atmospheric scattering of solar radiation. These are termed selective scattering and non-selective scattering. These two processes are determined by the different sizes of particles in the atmosphere.
Selective scattering is so named because radiations with shorter wavelengths are selectively scattered much more extensively than those with longer wavelengths. It is caused by atmospheric gases or particles that are smaller in dimension than the wavelength of a particular radiation. Such scattering could be caused by gas molecules, smoke, fumes, and haze. Under clear atmospheric conditions, therefore, selective scattering would be much less severe than when the atmosphere is extensively polluted from anthropogenic sources.
Selective atmospheric scattering is, broadly speaking, inversely proportional to the wavelength of radiation and, therefore, decreases in the following order of magnitude: Far UV > near UV > violet > blue > green > yellow > orange > red > infrared. Accordingly, the most severely scattered radiation is that which falls in the ultraviolet, violet, and blue bands of the spectrum. The scattering effect on radiation in these three bands is roughly ten times as great as on the red rays of sunlight.
It is interesting to note that the selective scattering of violet and blue light by the atmosphere causes the blue colour of the sky. When the sun is directly overhead at around noon time, little selective scattering occurs and the sun appears white. This is because sunlight at this time passes through the minimum thickness of atmosphere. At sunrise and sunset, however, sunlight passes obliquely through a much thicker layer of atmosphere. This results in maximum atmospheric scattering of violet and blue light, with only a little effect on the red rays of sunlight. Hence, the sun appears to be red in colour at sunrise and sunset.
Non-selective scattering occurring in the lower atmosphere is caused by dust, fog, and clouds with particle sizes more than ten times the wavelength of the components of solar radiation. Since the amount of scattering is equal for all wavelengths, clouds and fog appear white although their water particles are colourless. Atmospheric gases also absorb solar energy at certain wavelength intervals called absorption bands, in contrast to the wavelength regions characterized by high transmittance of solar radiation called atmospheric transmission bands, or atmospheric windows.
The degree of absorption of solar radiation passing through the outer atmosphere depends upon the component rays of sunlight and their wavelengths. The gamma rays, X-rays, and ultraviolet radiation less than 200 nm in wavelength are absorbed by oxygen and nitrogen. Most of the radiation with a range of wavelengths from 200 to 300 nm is absorbed by the ozone (O3) layer in the upper atmosphere. These absorption phenomena are essential for living things because prolonged exposure to radiation of wavelengths shorter than 300 nm destroys living tissue.
Solar radiation in the red and infrared regions of the spectrum at wavelengths greater than 700 nm is absorbed to some extent by carbon dioxide, ozone, and water present in the atmosphere in the form of vapour and condensed droplets (Table 1). In fact, the water droplets present in clouds not only absorb rays of long wavelengths, but also scatter some of the solar radiation of short wavelengths.
As a result of the atmospheric phenomena involving reflection, scattering, and absorption of radiation, the quantity of solar energy that ultimately reaches the earth's surface is much reduced in intensity as it traverses the atmosphere. The amount of reduction varies with the radiation wavelength, and depends on the length of the atmospheric path through which the solar radiation traverses. The intensity of the direct beams of sunlight thus depends on the altitude of the sun, and also varies with such factors as latitude, season, cloud coverage, and atmospheric pollutants.
The total solar radiation received at ground level includes both direct radiation and indirect (or diffuse) radiation. Diffuse radiation is the component of total radiation caused by atmospheric scattering and reflection of the incident radiation on the ground. Reflection from the ground is primarily visible light with a maximum radiation peak at a wavelength of 555 nm (green light). The relatively small amount of energy radiated from the earth at an average ambient temperature of 17°C at its surface consists of infrared radiation with a peak concentration at 970 nm. This invisible radiation is dominant at night.
During daylight hours, the amount of diffuse radiation may be as much as 10% of the total solar radiation at noon time even when the sky is clear. This value may rise to about 20% in the early morning and late afternoon.
In conclusion, therefore, it is evident that in cloudy weather the total radiation received at ground level is greatly reduced, the amount of reduction being dependent on cloud coverage and cloud thickness. Under extreme cloud conditions a significant proportion of the incident radiation would be in the form of scattered or diffuse light. In addition, lesser solar radiation is expected during the early and late hours of the day. These facts are of practical value for the proper utilization of solar radiation for such purposes as destruction of microorganisms.