The colours seen at sunrise and sunset arise from the same effect that produces blue sky, namely scattering. In their path through the atmosphere the shorter wavelengths are preferentially scattered out of the line of sight. If the air is clean and dry, the rising (or setting) Sun appears yellow, but if there are numerous dust particles or tiny water droplets suspended in the air, it will appear orange or even red. Varying mixtures of scattered light at shorter wavelengths and the light illuminating them from the Sun cause the varied cloud colours that are visible. Similar considerations apply to the colours seen in the sky itself (see twilight arch). See also alpine glow; green flash; purple light; shadow of the Earth.
Subjects: Meteorology and Climatology.