Everything about Solar Radiation totally explained
Solar radiation is
radiant energy emitted by a
sun as a result of its
nuclear fusion reactions.
The
spectrum of the
Sun's solar radiation is close to that of a
black body with a temperature of about 5,800
K. About half that lies in the
visible short-wave part of the
electromagnetic spectrum and the other half mostly in the near-
infrared part. Some also lies in the
ultraviolet part of the spectrum. When ultraviolet radiation isn't 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.
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 doesn't 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 doesn't 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, 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's caught by Earth, in other words about 3.86×10
26 watts.
Climate effect of solar radiation
On Earth, solar radiation is obvious as daylight when the sun is above the
horizon. This is during daytime, and also in summer near the poles at night, but not at all in winter near the poles. When the direct radiation isn't blocked by clouds, it's experienced as
sunshine, combining the perception of bright white light (sunlight in the strict sense) and warming. The warming on the body and surfaces of other objects is distinguished from the increase in
air temperature.
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 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 (see:
Milankovitch cycles).
Further Information
Get more info on 'Solar Radiation'.
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