At dawn and at sunset, the sun’s radiation reaches the ground at a very low angle of incidence, therefore the sun’s rays must cross a thick layer of atmosphere, in particular its lower layers, those which are more dense with dust and humidity. In these conditions, there is a greater diffusion of radiations with a longer wavelength, and therefore the sky appears of a red or orange colour.
The greater the amount of atmospheric dust, the greater amount of colours in sunsets and dawns.
When volcanoes erupt and introduce great quantities of ashes in the atmosphere, in the interested areas, dawns and sunsets of unusual colours are common. In the case of particularly violent eruptions the phenomenon may be noted even thousands of km away, as for example in the case of the eruption of the Krakatoa volcano in Indonesia that caused unusually colourful sunsets even in Europe.