During the day
Except for light that comes directly from the sun, most of the light in the day sky is caused by scattering, which is dominated by a small-particle limit called Rayleigh Scattering. The scattering due to molecule sized particles (as in air) is greater in the forward and backward directions than it is in the lateral direction. Scattering is significant for light at all visible wavelengths but is stronger at the shorter (bluer) end of the visible spectrum, meaning that the scattered light is bluer than its source, the sun. The remaining sunlight[clarification needed], having lost some of its short wavelength components, appears slightly less blue.
Scattering also occurs even more strongly in clouds. Individual water droplets exposed to white light will create a set of colored rings. If a cloud is thick enough, scattering from multiple water droplets will wash out the set of colored rings and create a washed-out white color.
The sky can turn a multitude of colors such as red, orange, purple and yellow (especially near sunset or sunrise) when the light must pass through a much longer path (or optical depth) through the atmosphere. Scattering effects also partially polarize light from the sky and are most pronounced at an angle 90° from the sun. Scattered light from the horizon travels through as much as 38 times the atmosphere as does light from the zenith, causing it to lose blue components, causing a blue gradient: vivid at the zenith, and pale near the horizon. Because red light also scatters if there is enough air between the source and the observer, these longer wavelengths of light will also scatter significantly, making parts of the sky change color during a sunset. As the amount of atmosphere nears infinity, the scattered light appears whiter and whiter. Read more