Solar Radiation and the Sun

The sun is the center of gravity of the solar system, and supplies energy to the planet. The energy emitted by the sun enables the existence of life on earth, directly or indirectly.

The solar energy which is emitted is 3.72 X 1020 MW — equivalent to a radiative power of 63 MW/m² of its surface. At the mean distance between sun and earth, this radiation reaches the exterior of the earth’s atmosphere with an intensity of 1.367 kW falling onto a 1 m² surface oriented perpendicularly to the sun’s beams. This is known as the Solar Constant.

The outermost and comparatively thin 400 km layer of the sun is known as the Photosphere and has a temperature of about 5,770 Kelvin. This Photosphere layer emits the spectrum of radiation called ‘light’ and is visible to the human eye.

The mean distance between the sun and earth is around 150 million kilometers. In the beginning of January every year, the earth reaches the point closest to the sun (Perihelion - 147 million kilometers). After a period of six months, on the 4th of July, it reaches the farthest distance from the sun (Aphelion - 152 million kilometers). As a result of these different distances, the direct solar radiation that reaches the earth’s atmosphere is 7% less intense on the 4th of July than it is in January (see Figure below). However, these varying distances between the sun and the earth only have a negligible effect on the earth’s seasonal temperatures.

The sun

The Atmosphere

The atmosphere of the earth has a significant effect on the intensity of solar radiation reaching the ground. It has a height of about 70 to 80 km and it mostly composed of Oxygen (~21%) and Nitrogen (~78%). About a dozen other gases and water vapor together constitute only 1% but can have a major influence on the environment and the climate, for example, ‘greenhouse gasses’ such as Carbon Dioxide and Methane.

The individual layers of earth’s atmosphere are different from one another and are thus classified by specific terms as represented in Figure 2.

The Atmosphere

Figure 2. The Atmosphere

The bottom layer is referred to as the Troposphere. It is the cloudiest layer and extends up to the Tropopause, which is the boundary between the Stratosphere and the Troposphere. It is present at altitudes of 11 km to 16 km based on the location above the earth and the atmospheric conditions.

The Stratosphere is almost cloudless because of its very low air humidity. The ozone layer is mainly located in the stratosphere at altitudes of 15 to 85 km. The layer above Stratosphere is called the Mesosphere, starting at a height of around 50 km above the ground surface. The Ionosphere, also called the Thermosphere, is the layer above the Mesosphere. It reaches up to a height of approximately 640 km. Above the Ionosphere lies the Exosphere, which is the outer boundary and reaches up to 9,600 km. Beyond this is inter-planetary space.

Half of the entire mass of the earth’s atmosphere is located in the first 5 to 6 km above the ground surface. When extra-terrestrial solar radiation passes through the atmosphere, it is decreased by scattering and absorption by aerosol particles, air molecules, ice crystals, and water droplets in clouds.

Scattering of solar radiation occurs within the full spectral range. However, the scattering can take place in different ways:

  • Scattering by ice crystals and/or water droplets in clouds quite uniformly across the entire spectral range
  • Scattering by molecules (Rayleigh-Scattering), mostly of radiation at shorter wavelengths
  • Scattering by aerosol particles (Mie-Scattering) at wavelengths dependent on the distribution and size of particles.

Aerosols and gaseous molecules cause a fairly high absorption of solar radiation. Absorption and scattering in the atmosphere impact the spectral balance of the solar radiation reaching the ground.

Figure 3 depicts the normal spectrum of solar radiation at sea level in ideal atmospheric conditions.

Solar Radiation at Sea Level

Figure 3. Solar Radiation at Sea Level

Differences of temperature and air pressure within the atmosphere influence absorption and hence impact the spectrum at sea level and at varying heights above sea level.

The Earth

The type and amount of radiation reaching the ground surface depend upon the atmosphere’s varying characteristics. Other key factors are the size of the earth and its location within space; however, the composition of the ground surface is the decisive factor for the amount of radiation being absorbed by the earth, or reflected from it.

The sun gives more than 99.98% of all energy to the earth’s surface, and the rest is internal geothermal energy. This leads to an average surface temperature of 14 °C, even though extreme variations may take place temporally and locally. The highest temperature noted was 57.3 °C in Libya and the lowest was - 89.2 °C in Antarctica.

Angle of Incidence of Solar Radiation

Figure 4. Angle of Incidence of Solar Radiation

As the earth is revolving around the sun and also rotating around its own axis, the angle of incidence of the solar radiation is varying continually. The ratio of the angle of incidence and radiation intensity may be represented as a cosine function, which is also known as Lambert’s law (see Figure 4).

In addition, the 23.5º inclination of the earth’s axis has an influence as shown in Figure 5. The change of the angle of incidence during the different times of the day is the all-important factor

Inclination of the Earth

Figure 5. Inclination of the Earth

The earth is nearly spherical in shape, and not a flat disk, with gravitational force binding the atmosphere like a shell. Thus, the curvature of the surface and the effective thickness of the atmosphere influence the intensity of the solar radiation at a point on the surface.

The solar radiation attains its highest intensity when the sun is at its apex the angle of incidence is 90°, and the thickness of the atmosphere is at its least. If the sun’s position in the sky is lower, the radiation has to travel through more amounts of atmosphere, and hence more radiation is scattered and absorbed by the atmosphere and less radiation reaches the ground surface.

Atmospheric Depth refers to the effective thickness of the atmosphere. The Atmospheric Depth just above the horizon is about 11 times greater than at the shortest path, at 90º (solar zenith), as shown in Figure 6.

Atmospheric Depth

Figure 6. Atmospheric Depth

The composition of the ground surface also influences the effects of solar radiation. It is not hard to understand that a surface covered with snow reflects more amounts of radiation than one covered with black rock or with trees. The fraction of the incident solar radiation that is reflected by the surface is known as the Albedo.

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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