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Chapter 2: Why do we need solar radiation data?

Chapter 3

What influences the amount of solar radiation?

The amount of solar radiation reaching the earth's surface varies greatly because of changing atmospheric conditions and the changing position of the sun, both during the day and throughout the year. Clouds are the predominant atmospheric condition that determines the amount of solar radiation that reaches the earth. Consequently, regions of the nation with cloudy climates receive less solar radiation than the cloud-free desert climates of the southwestern United States. For any given location, the solar radiation reaching the earth's surface decreases with increasing cloud cover.

Local geographical features, such as mountains, oceans, and large lakes, influence the formation of clouds; therefore, the amount of solar radiation received for these areas may be different from that received by adjacent land areas. For example, mountains may receive less solar radiation than adjacent foothills and plains located a short distance away. Winds blowing against mountains force some of the air to rise, and clouds form from the moisture in the air as it cools. Coastlines may also receive a different amount of solar radiation than areas further inland. Where the changes in geography are less pronounced, such as in the Great Plains, the amount of solar radiation varies less.

The amount of solar radiation also varies depending on the time of day and the season (see Understanding Seasonal and Atmospheric Variations .) In general, more solar radiation is present during midday than during either the early morning or late afternoon. At midday, the sun is positioned high in the sky and the path of the sun's rays through the earth's atmosphere is shortened. Consequently, less solar radiation is scattered or absorbed, and more solar radiation reaches the earth's surface. In the Northern Hemisphere, south-facing collectors also receive more solar radiation during midday because the sun's rays are nearly perpendicular to the collector surface. Tracking collectors can increase the amount of solar radiation received by tracking the sun and keeping its rays perpendicular to the collector throughout the day. In the Northern Hemisphere, we also expect more solar radiation during the summer than during the winter because there are more daylight hours. This is more pronounced at higher latitudes.

Both man-made and naturally occurring events can limit the amount of solar radiation at the earth's surface. Urban air pollution, smoke from forest fires, and airborne ash resulting from volcanic activity reduce the solar resource by increasing the scattering and absorption of solar radiation. This has a larger impact on radiation coming in a direct line from the sun (direct beam) than on the total (global) solar radiation. Some of the direct beam radiation is scattered toward earth and is called diffuse (sky) radiation (global = direct + diffuse). Consequently, concentrators that use only direct beam solar radiation are more adversely affected than collectors that use global solar radiation. On a day with severely polluted air (smog alert), the direct beam solar radiation can be reduced by 40%, whereas the global solar radiation is reduced by 15% to 25%. A large volcanic eruption may decrease, over a large portion of the earth, the direct beam solar radiation by 20% and the global solar radiation by nearly 10% for 6 months to 2 years. As the volcanic ash falls out of the atmosphere, the effect is diminished, but complete removal of the ash may take several years.

Chapter 4: What parts of solar radiation are measured?

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