What is Solar Radiation?

The radiation sent by the sun is spread across a broad range of wavelengths at different intensities. The electro-magnetic solar radiation imposing on the upper edge of the atmosphere is termed as extra-terrestrial radiation. The mean integral for the full spectrum is 1,367 W/m² (the Solar Constant).

The standard measurement of the wavelength of solar and atmospheric radiation is the nanometer (µm, 10-9 m) and for infrared radiation is the micrometer (µm, 10-6 m). The range is illustrated in the table below. In astronomy and older books, one may see wavelengths in Ångström (Å, 10-10 m).

The meteorologically important spectral range extends from 300 nm to 3000 nm (short-wave radiation). Approximately 96% of the full extra-terrestrial radiation is located within this spectral range. The highest radiation intensity of the solar spectrum happens at 500 nm, towards the blue end of the visible range.

Full Spectrum of Solar Radiation

The full spectrum consists of the visible (Vis), ultraviolet (UV), and infrared (IR) wavelengths. However, these wavelength ranges have to be sub-divided based on the separate application fields. Widely known are the prismatic colors of visible light, the colors of the rainbow. IR is divided into far-infrared (FIR) and near-infrared (NIR).

UV is usually sub-divided into UV-A, UV-B, and UV-C radiation. Approximately 6% of the full solar radiation falling on the earth is UV. Shorter wavelengths (higher frequency) have higher energy, thus increasing the effect on chemical and biological systems.

The attenuation of solar radiation passing through Earth’s atmosphere is because of the following processes:

Ultraviolet Range

Scattering by aerosol particles and molecules and absorption by ozone, nitrogen dioxide, sulfur dioxide, and trace gases.

Visible Range

Scattering by aerosol particles and molecules, slight absorption by aerosol particles, ozone, and other trace gases.

Infrared Range

Absorption by water vapor and aerosol particles but slight scattering.

Molecular ozone in the upper layers of Earth’s atmosphere functions as a filter for UV radiation and the effect increases with shorter wavelengths. Whereas nearly all UV-A radiation reaches the ground surface, almost 90% of the UV-B radiation is absorbed by the ozone, and the entire UV-C. During this process, new ozone (O³) is also generated from atmospheric oxygen and this reinforces the process.

Variations of the atmospheric ozone content will particularly impact the amount of UV-B. An increasing ozone layer concentration means diminishing UV-B radiation, and a dwindling ozone layer means increasing UV-B radiation reaching the surface.

Effects of Solar Radiation

Solar radiation is the driver for a number of biological, chemical, and physical phenomena on the ground, in the atmosphere, and in the seas.

A huge effect of solar radiation making contact with the Earth’s surface is that it is warming it up, which is essential for man’s existence. About 30% of the extra-terrestrial radiation solar radiation (shown in yellow in the illustration below) is reflected back into space but approximately 51% is absorbed by water and land and another 19% is absorbed by the atmosphere and clouds.

Long-wave far infrared radiation (FIR) is illustrated in red and is generally transformed short-wave energy that is re-radiated from the water, clouds, land, and atmosphere. Just a small quantity of the total energy remains on the earth but this is sufficient to maintain all the biological processes on Earth’s planet and to stimulate the weather systems.

Differences in the reflected and re-radiated energy do, indeed, impact the energy balance between atmosphere and ground. This energy balance, in turn, impacts meteorological conditions and other processes, for instance, the growth of plants.

Growing concern for the survival of Earth’s environment and way of life, along with the need for higher crop yields, makes it necessary to have available clear-cut information on incoming and reflected radiation. With this regard, the distribution of the radiation intensity at different wavelengths can be of the greatest importance.

Conclusion

These days, measuring solar radiation is very crucial in numerous different fields of application, such as meteorology, climatology, hydrology, agriculture, solar energy, pollution forecasting, and material testing.

This information has been sourced, reviewed and adapted from materials provided by Kipp & Zonen.

For more information on this source, please visit Kipp & Zonen.

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