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Measuring Solar Radiation
In 1958, Threlkeld and Jordan developed a model for the amount of solar radiation reaching the Earth’s surface under clear sky conditions which accounted for variations in atmospheric moisture, and annual changes in both earth to sun distance and the suns altitude above the horizon. Figure 1 shows irradiance values predicted by this model for direct beam solar irradiance under clear-sky conditions with sunlight at normal incidence to the collector surface. One notable aspect of this graph is that it predicts a relatively modest +/- 5% variation in noon-time solar irradiance throughout the year.

These curves represent the direct beam component only. A measurement of the global irradiance would include the beam component as well as any diffuse and reflected components. The diffuse component may contribute up to an additional 15% to the total depending on sky conditions. The reflected component will vary depending on the reflectivity and relative angle of surrounding surfaces.

For more northern latitudes the intensity of sunlight reaching the Earth’s surface decreases because of the greater atmospheric path length through which the sun must travel. Also, the variation in solar radiation intensity with latitude is minor in the summer months but very pronounced in the winter months. This is a result of the lower angle of the sun to the horizon during winter months. The greatest earth to sun distance occurs in July and this coincides with the minimum solar irradiance for latitudes of 40 degrees and below.