The most basic, least complicated form of natural light is the sun in a clear sky. Without clouds or other weather effects, the lighting conditions are predictable during most of the day.
The sun’s arc through the sky varies with the season and the latitude, but it is consistent. As the sun rises in the sky, the light levels increase. They rise quickly in the morning and fall quickly in the late afternoon, bu during the middle of the day they change very little. Once the sun is at about 40 degrees above the horizon, it is almost at its brightest. How long it spends above this height depends on the season and the latitude. In the tropics, this can be as much as seven hours; in a mid-latitude summer, eight hours; and in a mid-latitude winter, no time at all. On a winter’s day in the northern USA or Europe, light levels change steadily but slowly from sunrise to sunset. In the summer, the levels change more quickly at either end of the day, but very little for the hours around noon.
The thickness of the atmosphere affects the lights intensity – at higher altitudes, the light levels are higher. The atmosphere also scatters different wavelengths that are scattered the most, and these are at the blue end of the spectrum. So, looking away from he sun, you can see this scattering in the form of a clear blue sky. Looking directly at the sun, its color depends on how much blue has been scattered and lost; when the sun is high, very little is lost, but close to sunrise and sunset, when the light has to pass obliquely through a much greater thickness of atmosphere, the light is shifted towards red. The atmosphere close to the ground has the greatest effect on sunrise and sunset, and as these vary locally quite a lot, the colors can differ from place to place and from day to day, between yellow and red. Chroma key software can do some great effects for you.
The effect of the blue sky, which is also known as skylight, is so much weaker than direct sunlight that it is felt only in the shade. It acts as a reflector, and tints shadows. In addition to this on a clear day are the surroundings, which act as colored reflectors, and also the shadows: green grass; the walls of buildings, and so on.
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