1.2. Solar radiation: An outline
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Solar radiation that reaches the surface of the Earth as a bundle of parallel rays coming directly from the Sun is called direct solar radiation S. The amount of solar radiation that falls on a horizontal plane S' depends on the position of the Sun over the horizon and is determined by this expression [10]:
| S'= S * sin h | (1.1) |
where h is the elevation of the Sun over the horizon.
Global solar irradiance on a horizontal plane, consisting of direct radiation S' and diffuse radiation D, which reaches the Earth’s surface as a result of scattering of the solar beam, is equal to:
| Q = S' + D | (1.2) |
At the Earth’s surface, solar radiation is redistributed: Some of it is reflected off the surface into the atmosphere (reflected shortwave radiation R), while the rest is absorbed at the surface (absorbed shortwave radiation Bk):
| Bk=Q - R | (1.3) |
The total sum of reflected radiation is contingent on the characteristics of the Earth’s surface (color, content of moisture, surface structure etc.). The value characterizing surface reflectance, or surface albedo, A, is determined by the ratio of surface-reflected radiation to total incident radiation and is usually expressed as a percentage:
| A = Q / R * 100% | (1.4) |
Along with shortwave radiation, atmospheric longwave radiation also reaches the Earth’s surface (counterradiation) Ea, and, in turn, Earth’s surface emits longwave radiation in accordance with its temperature (Earth-emitted radiation) E3. The difference between the radiation of the Earth’s surface and that of the atmosphere is called effective radiation Ee. The algebraic sum of radiation components determines radiation balance B:
| B = S' + D + Ea - R - E3 = Q - R - Eэф | (1.5) |
Depending on the ratio of the incoming and outgoing components, the radiation balance will have a positive value if the Earth’s surface absorbs more radiation than it emits, or a negative value if the surface absorbs less radiation than it emits.
The value of the radiation balance can either be determined as the sum total of all components, each of which has been measured separately, or itself directly measured in an actinometric survey.
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