The atmosphere is full of tiny particles and molecules, such as gases, water, soot and ashes. Sunlight hits the atmosphere and is scattered into different wave lengths, which can be seen as colours. Red has a longer wave length than blue. A red sunset occurs because light has to travel further through the atmosphere. More of the shorter wave length light is absorbed or scattered by the atmosphere, allowing the ‘red wave length’ to reach you.
Short answer: The light you see has to travel further through the atmosphere to reach you.
