Chapter 3: What influences the amount of solar radiation?
As the name implies, direct beam radiation comes in a direct line from the sun. For sunny days with clear skies, most of the solar radiation is direct beam radiation. On overcast days, the sun is obscured by the clouds and the direct beam radiation is zero.
Diffuse radiation is scattered out of the direct beam by molecules, aerosols, and clouds. Because it comes from all regions of the sky, it is also referred to as sky radiation. The portion of total solar radiation that is diffuse is about 10% to 20% for clear skies and up to 100% for cloudy skies.
Some of the solar radiation entering the earth's atmosphere is absorbed and scattered. Direct beam radiation comes in a direct line from the sun. Diffuse radiation is scattered out of the direct beam by molecules, aerosols, and clouds. The sum of the direct beam, diffuse, and ground-reflected radiation arriving at the surface is called total or global solar radiation.
A station with a lower level of funding may only measure two quantities; the third is calculated. For example, the direct beam component can be derived by subtracting the diffuse radiation from the global radiation and applying trigonometric relationships to account for the position of the sun. The trade-off for this approach is that the calculated direct beam data are less accurate than if the direct beam data were measured.
Historically, many stations have measured only the global radiation on a horizontal surface. This necessitates calculating both the diffuse and direct beam solar radiation, which results in less accurate values for these two quantities than if they were measured.
In the absence of any solar radiation measurements, we employ models using meteorological data such as cloudiness and minutes of sunshine to estimate solar radiation. Although much less accurate, this is often the only option we have for locations where solar radiation is not measured. Cloudiness data are based on observations by a trained meteorologist, who looks at the sky and estimates the amount of cloud cover in tenths. A clear sky rates a cloud cover value of 0 tenths, and an overcast sky rates a cloud cover value of 10 tenths. Minutes of sunshine are recorded by an instrument that measures the time during the day when the sun is not obscured by clouds.
To investigate the spectral distribution of solar radiation, an instrument called a spectroradiometer measures the solar radiation intensity at discrete wavelengths. Spectroradiometers are complex and relatively expensive instruments, and their operation and maintenance require significant effort. Consequently, spectroradiometers are not routinely used for long-term data collection. Rather, they help establish data bases that have sufficient information to validate models that predict the spectral distribution based on meteorological data and the position of the sun.
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