It is well known that the atmosphere scatters shorter wavelengths of solar energy much more than longer wavelengths, and that the scattering increases with SZA (Iqbal, 1983). Usually, more than half of the UV-B irradiance arriving on earth is from diffuse radiation from the sky. The greater fraction of radiance from the sky has profound implications for the amount of UV-B irradiance in urban ecosystems.
Not only the sky radiance fraction, but also the distribution of sky radiance, is important in determining irradiance in urban ecosystems where often much of the sky is in view. Models of radiance distributions have been developed for use in predictions of tree and building effects on irradiance (Grant and Heisler, 1996; Grant et al., 1997a, b). The distribution for photosynthetically active radiation (PAR, essentially visible radiation) has a large gradient in radiance in the sun half of the sky and a decided dark portion opposite the sun in the other half of the sky (Grant et al., 1996; Grant and Heisler, 1997). The pattern for UV-A is similar to the pattern for the PAR, though the gradient is smaller. The UV-B distribution has a generally smaller gradient, that is, the irradiance is relatively equal from all parts of the sky, owing to the greater scattering of the energy in this waveband (Grant and Heisler, 1997; Grant et al., 1997a).
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