At sunset, clouds appear red/orange primarily because of what atmospheric filtering effect occurring along the sun's path?

Answer

Shorter wavelengths (blues/violets) are scattered away by air molecules.

When the sun is low, its rays travel through a greater atmospheric volume, causing shorter blue and violet wavelengths to be scattered away by air molecules, leaving only the longer red, orange, and yellow wavelengths to illuminate the cloud.

At sunset, clouds appear red/orange primarily because of what atmospheric filtering effect occurring along the sun's path?
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