Twilight is light produced by sunlight scattering in the upper atmosphere, when the Sun is below the horizon, which illuminates the lower atmosphere and the Earth's surface. The word twilight can also refer to the periods of time when this illumination occurs.
Midtown Manhattan during civil twilight, demonstrating blue hour
Civil twilight in a small town in the Mojave Desert
Evening nautical twilight on Lake Ontario, Canada
Long exposure of nautical twilight in a small town in the Mojave Desert
Diffuse sky radiation is solar radiation reaching the Earth's surface after having been scattered from the direct solar beam by molecules or particulates in the atmosphere. It is also called sky radiation, the determinative process for changing the colors of the sky. Approximately 23% of direct incident radiation of total sunlight is removed from the direct solar beam by scattering into the atmosphere; of this amount about two-thirds ultimately reaches the earth as photon diffused skylight radiation.
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.
