SPECTRAL CHARACTERISTICS OF SOLAR RADIATION
Solar radiation is radiant energy emitted by a sun as a result of its nuclear fusion reactions. Spectral characteristics of solar radiation, both external to the Earth's atmosphere and at the ground, can be seen in Figure 1. Over 99 % of the energy flux from the sun is in the spectral region of 0.15 to 4 μm, With approximately 50 % in the visible light region of 0.4 to 0.7 μm. The solar spectrum peaks at 0.49 pm, the green portion of visible light. Because of the dominance of visible light in the solar spectrum, it is not surprising that life processes such as photosynthesis and photoperiodism (responses to changing day length) are dependent on specific visible wavelength bands. When ultraviolet radiation is not absorbed by the atmosphere or other protective coating, it can cause a change in human skin pigmentation. Solar radiation is commonly measured with a pyranometer or pyrheliometer.
The sun radiating energy approximately as a blackbody (perfect absorber and emitter of radiation at all wavelengths) at an effective temperature of 6000 K ‘showers’ the atmosphere and the Earth's surfaces with an enormous quantity of electromagnetic energy. The total amount of energy emitted by the sun and received at the extremity of the Earth's atmosphere is constant – 1370 W/m2/sec. That received per unit area of the Earth's surface is 343 W/m2/sec.
Incoming solar radiation is absorbed by atmospheric gases such as O2, O3, CO2, and H2O vapor. UV light at wavelengths of <0.18 μm (180 nm) are strongly absorbed by O2 at altitudes above 100 km (62 mi). Ozone below 60 km (37.2 mi) absorbs most UV between 0.2 and 0.3 μm (200 to 300 nm). Atmospheric absorption above 40 km (24.8 mi) results in attenuation of ~5% of incoming solar radiation. Under clear-sky conditions, another 10 to 15 % is absorbed by the lower atmosphere or is scattered back to space, with 80 to 85 % reaching the ground.
As can be seen in Figure 1, a significant portion of solar radiation incident on the atmosphere is not received at the ground. In addition to reflection by clouds and atmospheric aerosols, considerable attenuation of solar energy results from absorption of different portions of the solar spectrum by atmospheric constituents such as H2O vapor, CO2, O3, and O2. Carbon dioxide and H2O vapor primarily absorb in the infrared region O3 in the UV and portions of the visible region, and O2 in portions of the visible and infrared regions.
The Solar Constant
The Solar constant is the amount of the Sun's incoming electromagnetic radiation (Solar radiation) per unit area, measured on the outer surface of Earth's atmosphere in an aircraft perpendicular to the rays. The Solar constant includes all types of Solar radiation, not just the visible light. It is measured by satellite to be roughly 1,366 watts per square meter (W/m²), though this fluctuates by about 6.9% during a year (from 1,412 W/m² in early January to 1,321 W/m² in early July) due to the earth's varying distance from the Sun, and by a few parts per thousand from day to day. Thus, for the whole Earth (which has a cross section of 127,400,000 km²), the power is 1.740×1017 W, plus or minus 3.5%. The Solar constant does not remain constant over long periods of time (see Solar variation). 1,366 W/m² is equivalent to 1.96 calories per minute per square centimeter, or 1.96 langleys (Ly) per minute.
The Earth receives a total amount of radiation determined by its cross section (π·r²), but as it rotates this energy is distributed across the entire surface area (4·π·r²). Hence the average incoming Solar radiation (sometimes called the Solar irradiance), taking into account the angle at which the rays strike and that at any one moment half the planet does not receive any solar radiation, is one-fourth the Solar constant (approximately 342 W/m²). At any given moment, the amount of Solar radiation received at a location on the Earth's surface depends on the state of the atmosphere and the location's latitude.
The Solar constant includes all wavelengths of Solar electromagnetic radiation, not just the visible light (see Electromagnetic spectrum). It is linked to the apparent magnitude of the Sun, −26.8, in that the Solar constant and the magnitude of the Sun are two methods of describing the apparent brightness of the Sun, though the magnitude only measures the visual output of the Sun.
In 1884, Samuel Pierpont Langley attempted to estimate the Solar constant from Mount Whitney in California. By taking readings at different times of day, he attempted to remove effects due to atmospheric absorption. However, the value he obtained, 2,903 W/m², was still too great. Between 1902 and 1957, measurements by Charles Greeley Abbot and others at various high-altitude sites found values between 1,322 and 1,465 W/m². Abbott proved that one of Langley's corrections was erroneously applied. His results varied between 1.89 and 2.22 calories (1318 to 1548 W/m²), a variation that appeared to be due to the Sun and not the Earth's atmosphere.
The angular diameter of the Earth as seen from the Sun is approximately 1/11,000 radians, (1 rad = 57.2958 degrees) meaning the solid angle of the Earth as seen from the sun is approximately 1/140,000,000 steradians. Thus the Sun emits about two billion times the amount of radiation that is caught by Earth, in other words about 3.86×1026 watts.
* Steradians are used to describe two-dimensional angular spans in three-dimensional space. 1 steradian is the solid angle subtended at the center of a sphere of radius r by a portion of the surface of the sphere having an area r2.
Climate effect of solar radiation
The amount of radiation intercepted by a planetary body varies inversely with the square of the distance between the star and the planet. The Earth's orbit and obliquity change with time (over thousands of years), sometimes forming a nearly perfect circle, and at other times stretching out to an orbital eccentricity of 5% (currently 1.67%). The total insolation (INcident SOLar radiATION) remains almost constant but the seasonal and latitudinal distribution and intensity of solar radiation received at the Earth's surface also varies. For example, at latitudes of 65 degrees the change in solar energy in summer & winter can vary by more than 25% as a result of the Earth's orbital variation. Because changes in winter and summer tend to offset, the change in the annual average insolation at any given location is near zero, but the redistribution of energy between summer and winter does strongly affect the intensity of seasonal cycles. Such changes associated with the redistribution of solar energy are considered a likely cause for the coming and going of recent ice ages.
Insolation is an important consideration when designing a building for a particular climate. It is one of the most important climate variables for human comfort and building energy efficiency.
It can be used in architecture to design buildings that are cool in summer and warm in winter, by providing large vertical windows on the equator-facing side of the building. This maximizes insolation in the winter months when the Sun is low in the sky, and minimizes it in the summer when the noonday Sun is high in the sky. (The Sun's north/south path through the sky spans 47 degrees through the year).
Insolation figures are used to locate solar power systems. The figures can be obtained from an insolation map or by city or region from insolation tables that were generated with historical data over the last 30-50 years.
In the fields of civil engineering and hydrology, numerical models of snowmelt runoff use observations of insolation. This permits estimation of the rate at which water is released from a melting snowpack.
Insolation figures are also used in remote sensing to make allowances for the intensity of radiation received at the sensors.
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