Diffuse sky radiation

Proceeding from analyses of the aftermath of the eruption of the Philippines volcano Mount Pinatubo (in June 1991) and other studies:[2][3] Diffused skylight, owing to its intrinsic structure and behavior, can illuminate under-canopy leaves, permitting more efficient total whole-plant photosynthesis than would otherwise be the case; this in stark contrast to the effect of totally clear skies with direct sunlight that casts shadows onto understory leaves and thereby limits plant photosynthesis to the top canopy layer, (see below).

Daily at any global venue experiencing sunrise or sunset, most of the solar beam of visible sunlight arrives nearly tangentially to Earth's surface.

The eruption of the Philippines volcano - Mount Pinatubo in June 1991 ejected roughly 10 km3 (2.4 cu mi) of magma and "17 million metric tons"(17 teragrams) of sulfur dioxide SO2 into the air, introducing ten times as much total SO2 as the 1991 Kuwaiti fires,[8] mostly during the explosive Plinian/Ultra-Plinian event of June 15, 1991, creating a global stratospheric SO2 haze layer which persisted for years.

[2][14] Surprisingly, a 3-4 year[15] increase in global Agricultural productivity and forestry growth was observed, excepting boreal forest regions.

[16] The means of discovery was that initially, a mysterious drop in the rate at which carbon dioxide (CO2) was filling the atmosphere was observed, which is charted in what is known as the "Keeling Curve".

[17] This led numerous scientists to assume that the reduction was due to the lowering of Earth's temperature, and with that, a, slowdown in plant and soil respiration, indicating a deleterious impact on global agriculture from the volcanic haze layer.

[2][18][22][14] This diffused skylight, owing to its intrinsic nature, can illuminate under-canopy leaves permitting more efficient total whole-plant photosynthesis than would otherwise be the case,[2][14] and also increasing evaporative cooling, from vegetated surfaces.

[23] In stark contrast, for totally clear skies and the direct sunlight that results from it, shadows are cast onto understorey leaves, limiting plant photosynthesis to the top canopy layer.

In Earth's atmosphere , the dominant scattering efficiency of blue light is compared to red or green light. Scattering and absorption are major causes of the attenuation of sunlight radiation by the atmosphere. During broad daylight , the sky is blue due to Rayleigh scattering , while around sunrise or sunset, and especially during twilight , absorption of irradiation by ozone helps maintain blue color in the evening sky. At sunrise or sunset, tangentially incident solar rays illuminate clouds with orange to red hues.
The visible spectrum, approximately 380 to 740 nanometers (nm), [ 1 ] shows the atmospheric water absorption band and the solar Fraunhofer lines . The blue sky spectrum contains light at all visible wavelengths with a broad maximum around 450–485 nm, the wavelengths of the color blue.
A clear daytime sky, looking toward the zenith
A Space Shuttle (Mission STS-43 ) photograph of the Earth over South America taken on August 8, 1991, which captures the double layer of Pinatubo aerosol clouds (dark streaks) above lower cloud tops.
Under more-or-less direct sunlight, dark shadows that limit photosynthesis are cast onto understorey leaves . Within the thicket , very little direct sunlight can enter.
Well lit understorey areas due to overcast clouds creating diffuse/ soft sunlight conditions, that permits photosynthesis on leaves under the canopy.