The atmospheric absorption It is the decrease in the light intensity of a celestial source, caused by the gases that make up the atmosphere. It grows rapidly in the lower layers of the atmosphere, whose density is much higher than that of the upper strata.
The absorption experienced by the light of a star observed near the horizon, therefore, is greater than that of a star that is in the zenith, because the light rays of the first must pass through a larger mass of air.
The different colors that make up white light in serene sky conditions experience a variable absorption according to their wavelength: violet rays are absorbed more than red ones and this on the one hand causes "the redness" of the stars (especially in the proximity of the horizon), and on the other, the blue or violet coloration of the sky that can be observed on a clear and clear day. On the other hand, when the atmosphere is charged with particles of water vapor or other nature, there is no selective absorption and the sky appears whitish.
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