Abergavenny sunset

As a ray of white sunlight travels through the atmosphere to an observer, some of the colours are scattered out of the beam by air molecules & airborne particles, changing the final colour of the beam the viewer sees. Because the shorter wavelength components, such as blue & green, scatter more strongly, these colours are preferentially removed from the beam. At sunset, when the path through the atmosphere is longer, the blue & green components are removed almost completely leaving the longer wavelength orange & red hues we see at those times. The remaining reddened sunlight can then be scattered by cloud droplets & other relatively large particles to light up the horizon red & orange.