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Solar Radiation

Solar Radiation

Solar radiation is the radiant energy emitted in the interplanetary space of the Sun. This radiation is generated from thermonuclear fusion reactions that occur in the solar nucleus and produce electromagnetic radiation at several frequencies or wavelengths, which it then propagates in space at the typical speeds of these waves. This spread allows you to bring solar energy with them.

The solar constant is the amount of energy received in the form of solar radiation per unit of time and unit of surface, measured in the outer part of the Earth's atmosphere in a plane perpendicular to the sun's rays. The results of its measurement by satellites they yield an average value of 1366 Wm-2.

Solar energy and consequently solar radiation results from the nuclear fusion process that takes place in the Sun. This energy is the main energy source and, therefore, the engine that moves our environment. The solar energy that we receive through solar radiation is directly or indirectly responsible for aspects as important for life as photosynthesis, maintaining a temperature on the planet compatible with life, wind, etc. The solar energy that reaches the Earth's surface is 10,000 times greater than the energy currently consumed by all of humanity.

Definition of radiation

Radiation is the transfer of energy by electromagnetic waves. Radiation occurs directly from the source outward in all directions. These waves do not need a material medium to propagate, they can cross interplanetary space and reach Earth from the Sun.

The wavelength and frequency of electromagnetic waves are important to determine their energy, their visibility and their penetration power. All electromagnetic waves move in a vacuum at a speed of 299,792 km / s.

Characteristics of solar radiation

Solar radiation is not concentrated in a single frequency, but is distributed in a broad spectrum of non-uniform amplitude with the typical shape of a bell, as is typical of the spectrum of a black body with which the solar source is modeled. The maximum radiation is centered in the band of radiation or visible light with a peak at 500 nm outside the Earth's atmosphere according to Wien's law, which corresponds to the cyan green color.

The photosynthetically active radiation band (PAR) oscillates between 400 and 700 nm, corresponds to the visible radiation and is equivalent to 41% of the total radiation. Within the PAR there are sub-bands with radiation:

  • blue-violet (400-490 nm)
  • green (490-560 nm)
  • yellow (560-590 nm)
  • red orange (590-700 nm)

In addition to the visible radiation, an energy-minority component, but nevertheless worthy of mention for its effects is the infrared and, above all, the ultraviolet rays.

Upon crossing the atmosphere, the solar radiation undergoes phenomena of reflection, refraction, absorption and diffusion by the various atmospheric gases to a variable degree as a function of frequency, so that the soil of the solar spectrum is irregular compared to that detected. in external atmosphere thresholds (TOA) with the presence of typical absorption or reflection bands.

Propagation of solar radiation in the atmosphere and on the surface of the earth

Due to the characteristics of the Earth's atmosphere, solar radiation undergoes certain alterations to cross it and reach the surface.

Radiation balance

On average, the Earth receives 1 366 W / m² (solar constant) from the Sun. This is related to the thresholds of the atmosphere and the plane perpendicular to the incoming solar rays: it is therefore necessary to take into account that solar radiation on Earth hits a spherical cap for 1440 minutes each day, decreasing by 75%. The atmosphere in turn filters the Sun's rays to a certain extent, as does each body, causing:

  • a reflection and a back-scattering of the rays, due to their albedo, to the clouds and atmospheric gases themselves;
  • an absorption that causes an increase in temperature, as a result of which it emits radiation in any direction according to Wien's law. However, this absorption is modest in the visible light band, so it is transparent to direct solar radiation.

Approximately half of the solar radiation passes through the atmosphere without alterations, taking the name of net radiation. Half of the net radiation finally contributes to the evaporation of the water masses, therefore, the solar energy available is approximately a quarter of the total energy emitted.

The stratosphere absorbs the ultraviolet rays included in the 200-300 nm band thanks to ozone, the troposphere absorbs and diffuses the infrared thanks to water vapor and CO2. The filtering action of bands in ultraviolet wavelengths, mostly lethal, is essential for the development of life.

Diffused solar radiation

Diffuse radiation is also called indirect radiation. Diffuse solar radiation represents the portion of solar radiation that has hit at least one particle of atmospheric gases by changing the angle of incidence and which, however, reaches the ground because it is directed towards it. Increases in relation to the total in cloudy skies. In particular, Rayleigh scattering of the blue component of solar radiation is responsible for the blue color of the sky. A part of the diffuse radiation is back towards space.

Incidental solar radiation

The incident solar radiation is that radiation that has encountered any obstacle to which it has given all or part of its energy. Energy that does not reach the surface of the earth is said to be extinct and is formed by radiation re-emitted, reflected and backscattered into space.

According to Lambert's law, the amount of radiation hitting the surface unit is proportional to the cosine of the angle of incidence.

The maximum amount of incident solar radiation is obtained with perpendicular incidence, since the angle increases, both the surface affected by the same amount of radiation and the thickness of the atmosphere traversed by these increases. This creates the daily, annual and latitudinal variations in the irradiation.

Reflected solar radiation

The reflected solar radiation is the part of the incident solar radiation reflected from the surface of the earth due to the albedo effect. The albedo is the reflection coefficient c. The values of c are usually between 0 and 1 or are expressed as a percentage. It is given by the relation between the radiant energy reflected from a surface with respect to the incident energy. The Earth has an average value of 40% (c = 0.4). At the altitude of the Earth's albedo, we add the radiations reflected by atmospheric particles to space.

Absorbed solar radiation

After deducting all the losses due to reflection and backscattering of the Earth's atmosphere and surface, the remaining incident solar radiation is absorbed by the Earth's surface and therefore contributes to its warming, in a way that variable depending on latitude and type of surface.

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Last review: April 13, 2017