Differences

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--- user_area:plane-parellel_versus_pseudo-spherical_versus_spherical [2008/10/27 13:00]
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+++ user_area:plane-parellel_versus_pseudo-spherical_versus_spherical [2008/10/27 13:13]
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 The radiation reaching the Earth's surface may be decomposed into a direct part and a diffuse part. The figure below shows the ratio of the plane-parellel direct irradiance versus the spherical direct irradiance (dotted lines) for two wavelengths in the UV. The solid curves are pseudo-spherical/spherical direct irradiance ratios. For the shortest wavelength the plane-parallel approximation underestimates the direct irradiance by 2% at a solar zenith angle of 60°. Depending on wavelength and application the pseudo-spherical approximation may be acceptable for the direct irradiance up to about 80°.
 {{:user_area:fig_dir_ps_pp_s_ratio.png?609x451}}
-For the total (=direct+diffuse) irradiance the similar ratios are shown in the figure below. When the diffuse irradiation is included both the plane-parallel approximation and especially the pseudo-spherical approximation, works well for larger solar zenith angles than the direct irradiance alone.
+For the total (=direct+diffuse) irradiance the similar ratios are shown in the figure below. When the diffuse irradiation is included both the plane-parallel approximation and especially the pseudo-spherical approximation, works well for larger solar zenith angles than the direct irradiance alone. At large solar zenith angles most of the radiation is diffuse. The diffuse radiation is driven by the direct radiation.
 {{:user_area:fig_total_ps_pp_s_ratio.png?609x451}}
 To understand why this is so the below figure shown diffuse ratios for the same conditions as above.
 {{:user_area:fig_dn_ps_pp_s_ratio.png?609x451}}