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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 nuclear fusion reactions that occur in the solar nucleus. Nuclear radiation produces electromagnetic radiation at various frequencies or wavelengths. Electromagnetic radiation propagates in space at the speed of light.

Why is solar radiation important?

Solar energy is the main energy source and, therefore, the motor that drives our environment. The solar energy that we receive through solar radiation is directly or indirectly responsible for aspects as important to life as:

  • Photosynthesis in plants
  • maintaining a planet temperature compatible with life.
  • of the wind.

The solar energy that reaches the earth's surface is 10,000 times greater than the energy currently consumed by all humanity.

What does radiation mean?

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 traverse interplanetary space and reach Earth from the Sun.

The wavelength and frequency of electromagnetic waves are important in determining their energy, their visibility, and their power of penetration.

All electromagnetic waves move in a vacuum at a speed of 299,792 km / s.

What is the solar constant?

The solar constant is the amount of energy received in the form of solar radiation per unit time and unit area.

The solar constant is measured in the outer part of the Earth's atmosphere in a plane perpendicular to the rays of the Sun. The results of its measurement by satellites give an average value of 1366 W / m2.

Types of solar radiation

The sun's radiation contains three types of rays:

  • 49% are infrared (IR) rays that provide heat.
  • 43% are visible rays (VI) that provide light.
  • 7% are ultraviolet (UV) rays.
  • 1% are other types of lightning.

In turn, ultraviolet (UV) rays are subdivided into three types:

  • Ultraviolet A or UVA: They easily pass through the atmosphere, reaching the entire earth's surface.
  • Ultraviolet B or UVB: Short wavelength. It has greater difficulty to cross the atmosphere. They reach the equatorial zone more easily than at high latitudes.
  • Ultraviolet C or UVC: Short wavelength. They do not pass through the atmosphere. They are absorbed by the ozone layer.

Characteristics of solar radiation

Solar radiation is distributed over a wide 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. Therefore, it is not concentrated on a single frequency.

The radiation maximum is focused on 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 oscillates between 400 and 700 nm, corresponds to the visible radiation and is equivalent to 41% of the total radiation. Within photosynthetically active radiation are subbands with radiation:

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

When crossing the atmosphere the solar radiation is subjected to phenomena of reflection, refraction, absorption and diffusion by the various atmospheric gases to a variable degree depending on the frequency. Consequently, the soil of the solar spectrum is irregular compared to that detected in the external thresholds of the atmosphere with the presence of typical absorption or reflection bands.

How does solar radiation spread 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 the solar radiation in Earth hits a spherical cap for 1,440 minutes each day, decreasing by 75%. The atmosphere in turn filters the rays of the Sun to a certain extent, as each body does, causing:

  • A reflection and a back-scattering of rays, due to their albedo, to 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, making it transparent to direct sunlight.

    Approximately half of the solar radiation passes through the atmosphere without alteration. The radiation that does this is called net radiation. Half of the net radiation finally contributes to the evaporation of the water masses, therefore, the available solar energy is approximately a quarter of the total emitted energy.

    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 CO 2 . The filtering action of the bands in the mostly lethal ultraviolet wavelengths is essential for the development of life.

    Diffuse solar radiation

    Diffuse radiation is also called indirect radiation.

    Diffuse radiation represents the portion of solar radiation that has hit at least one particle of atmospheric gases by changing the angle of incidence and that, however, reaches the ground because it is directed at it.

    It increases relative to the total in cloudy skies. In particular, the 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, the space.

    Incident solar radiation

    Incident solar radiation is that radiation that has encountered any obstacle to which it has delivered all or part of its energy. Energy that does not reach the earth's surface is said to be extinct and is made up of radiation re-emitted, reflected, and scattered back 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 irradiation.

    Reflected solar radiation

    Reflected solar radiation is the part of incident solar radiation reflected from the earth's surface due to the albedo effect.

    The albedo is the reflection coefficient c. Values ​​for c are generally between 0 and 1 or expressed as a percentage. It is given by the relationship between the radiant energy reflected from a surface with respect to the incident energy. Earth has an average value of 40% (c = 0.4). At the altitude of the Earth's albedo, we add the radiation reflected by atmospheric particles into space.

    Absorbed solar radiation

    After deducting all losses due to reflection and backscatter from 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 variable way depending on the latitude and the type of surface.

    How is solar radiation measured?

    To measure solar radiation we distinguish three methods depending on whether it is:

    • Direct solar radiation.
    • Indirect solar radiation.
    • Infrared radiation.

    Measurement of direct solar radiation

    Direct solar radiation is measured by means of pyrheliometers. Using shutters, only radiation from the sun and an annular region of the sky very close to the sun is measured.

    It is required that all the pyrheliometers be mounted on a mechanism that allows a very precise monitoring of the Sun.

    Global and diffuse radiation measurement

    Global radiation is defined as the solar radiation received from a solid angle of 2π steradians on a horizontal surface. Global radiation includes that received directly from the solar disk and also scattered radiation from the sky scattered as it passes through the atmosphere.

    Global radiation is measured by the pyranometer. To measure only the diffuse component of solar radiation, the direct component is covered by means of a screen or shading system. 

    Infrared radiation measurement

    Infrared radiation is measured by pyrgeometers.

    Most of these eliminate short wavelengths using filters that have constant transparency at long wavelengths while being nearly opaque at shorter wavelengths. 

    How does solar radiation affect health?

    Ultraviolet radiation can have various effects on the skin of human beings depending on its intensity and the length of its waves.

    UVA radiation can cause premature skin aging and skin cancer. It can also cause eye and immune system problems. 

    UVB radiation causes sunburn, darkening and thickening of the outer layer of the skin, and melanoma and other types of skin cancer. It can also cause eye and immune system problems.

    The ozone layer prevents most of the UVC radiation from reaching Earth. In medicine, UVC radiation can also come from special lamps or a laser beam and is used to kill germs or to help heal wounds. It is also used to treat certain skin conditions such as psoriasis, vitiligo, and skin nodules that cause cutaneous T-cell lymphoma.

    Summary

    Solar radiation is the energy that comes from the Sun.

    Nuclear reactions occur in the solar nucleus, releasing a large amount of energy. This energy travels to Earth through electromagnetic waves: solar radiation.

    Solar radiation is divided into mainly three types of rays: 

    • Infrared (IR) rays. They provide heat.
    • Visible rays (VI). They provide light.
    • Ultraviolet (UV) rays. They are the ones that have the most influence on human health.

    References

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    Published: May 13, 2015
    Last review: May 4, 2020