What is Solar Radiation?
Solar radiation radiant energy emit in interplanetary space by the sun. This radiation generates from nuclear fusion reactions that occur in the solar core.
Nuclear radiation produces electromagnetic radiation at various frequencies or wavelengths. Electromagnetic radiation spreads in space at the speed of light.
Why is Solar Radiation Important?
Solar energy is the primary energy source and, therefore, the engine that drives our environment. The solar energy that we receive through solar radiation is directly or indirectly responsible for aspects as essential to life as:
- The photosynthesis in plants
- maintaining a planet’s temperature compatible with life.
- And also, of the wind.
- The solar energy that reaches the Earth’s surface is 10,000 times greater than the energy currently consumed by humanity.
Types of Solar Radiation
Radiation from the sun 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.
- And also, 1% are other types of rays.
In turn, ultraviolet (UV) rays subdivide into three types:
- Ultraviolet A or UVA: They quickly pass through the atmosphere, reaching the entire Earth’s surface.
- Ultraviolet B or UVB: Short-wavelength. It has more incredible difficulty passing through the atmosphere. They reach the equatorial zone more quickly than at high latitudes.
- And also, ultraviolet C or UVC: Short-wavelength. They do not pass through the atmosphere. The ozone layer absorbs them.
Characteristics of Solar Radiation
Solar radiation distributes in a broad spectrum of non-uniform amplitude with the typical shape of a bell, standard of the range of a black body with which the solar source model. Therefore, it does not focus on a single frequency.
According to Wien’s law, the radiation maximum is centred in the band of radiation or visible light at 500 nm outside the Earth’s atmosphere, which corresponds to the colour cyan green.
The photosynthetically active radiation band oscillates between 400 and 700 nm, corresponds to visible radiation, and is equivalent to 41% of the total radiation. And also, within photosynthetically active radiation, there are subbands with radiation:
- blue-violet (400-490 nm)
- green (490-560 nm)
- yellow (560-590 nm)
- And also, orange-red (590-700 nm)
- When crossing the atmosphere, solar radiation is subjected to reflection, refraction, absorption, and diffusion by the various atmospheric gases to a variable degree as a function of frequency.
- Consequently, the ground 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.
- Due to the characteristics of the Earth’s atmosphere, solar radiation undergoes specific alterations to cross it and reach the surface.
1. Radiation Balance
On average, the Earth receives 1 366 W / m² (solar constant) from the Sun. This relates to the thresholds of the atmosphere and the plane perpendicular to the inward solar rays.
It is necessary to consider that the solar radiation in Earth hits a spherical cap for 1440 minutes each day, decreasing by 75%. And also, the atmosphere, in turn, filters the Sun’s rays to a certain extent, as does everybody, causing:
- A reflection and a back-scattering of the rays, due to their albedo, to the clouds and atmospheric gases themselves.
- According to Wien’s law, an absorption that causes an increase in temperature results from which it emits radiation in any direction.
- However, this absorption is modest in the visible light band, making it transparent to direct solar radiation.
- About half of it passes through the atmosphere unaltered. The radiation that does this is called net radiation. Half of the net radiation ultimately contributes to the evaporation of water masses.
- Therefore the available solar energy is approximately a quarter of the total energy emitted.
- And also, the stratosphere absorbs ultraviolet rays included in the 200-300 nm band thanks to ozone.
- The troposphere absorbs and diffuses infrared thanks to water vapour and CO 2. The filtering action of the bands in the ultraviolet wavelengths, mostly lethal, is essential for developing life.
2. Diffuse Solar Radiation
- It 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. Nevertheless, it reaches the ground because it direct towards it.
- Increases relative to total in cloudy skies. In specific, the Rayleigh scattering of the blue constituent is responsible for the blue colour of the sky. And also, a part of the diffuse radiation is back towards space.
3. Incident Solar Radiation
- Incident solar radiation is that radiation that meets any obstacle to which it has delivered all or part of its energy.
- The energy that does not reach the Earth’s surface is said to be extinct, and the makeup of radiation re-emitted, reflected, and backscattered into space.
- According to Lambert’s law, the amount of radiation that hits the unit area is proportional to the cosine of the angle of incidence.
- And also, the maximum amount of incident obtains with perpendicular incidence.
- Since the angle increases, the surface effects by the same amount of radiation and thickness of the atmosphere cross by these increases. This creates the daily, annual, and latitudinal variations in irradiance.
4. Reflected Solar Radiation
- Reflected solar radiation is the part of incident it reflected from the Earth’s surface due to the albedo effect.
- The albedo is the reflection coefficient c. Values of care generally between 0 and 1 or express as a percentage.
- And also, it gives by the relationship between the radiant energy reflected from a surface for the incident energy.
- And also, the Earth has an average value of 40% (c = 0.4). At the albedo altitude of the Earth, we add the radiations reflected by atmospheric particles into space.
5. Absorbed Solar Radiation
- After deducting all the losses due to reflection and backscattering from the atmosphere and the Earth’s surface.
- The remaining incident it absorbs by the Earth’s surface and variably contributes to its warming. Depending on the latitude and the type of surface.
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How is Solar Radiation Measured?
To measure, we distinguish three methods depending on whether it is:
- Direct solar radiation.
- Indirect solar radiation.
- And also, Infrared radiation.
1. Measurement of direct Solar Radiation
- Direct solar radiation measure utilizing pyrheliometers. And also, use of shutters, only radiation from the sun and an annular region of the sky are very close to the sun.
- All pyrheliometers require to mount on a mechanism that allows accurate monitoring of the Sun.
2. Measurement of Global and Diffuse Radiation
- Global radiation defines from a solid angle of 2π steradians on a horizontal surface. And also, global radiation includes that obtained directly from the solar disk and diffuse radiation from the sky scattered through the atmosphere.
- The pyranometer measures global radiation. To measure only the diffuse component, the natural part is covered utilizing a screen or shading system.
3. Infrared Radiation Measurement
- Pyrometers measure infrared radiation.
- And also, most of these remove short wavelengths by filters that exhibit constant transparency at long wavelengths while being nearly opaque at shorter wavelengths.
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