Why is the sky red at sunset or sunrise?

One of my greatest pleasures is catching a beautiful sunrise or sunset. In the Autumn and Winter months when the nights start to draw in earlier some of the sunsets on my drive home are simply breath-taking!

As discussed in a previous blog explaining why the sky is blue. Blue light is scattered more by oxygen and nitrogen molecules in the Earth’s atmosphere than other colours of the visible spectrum. This is because blue and violet light have smaller wavelengths.

At the end and beginning of the day the light must pass through more of the Earth’s atmosphere to reach us. This means that a large percentage of the blue light is scattered away leaving yellow and red light. Red light is not scattered as much as the remaining yellow light so tends to dominate the early evening or morning sky.

This effect is seen to greater effect in the Autumn and Winter months. This is because the Sun effectively sits lower in the sky anyway so before the sun sets or rises the light travels much further through the Earth’s atmosphere than in the Summer months.

Clouds also play a part in this beautiful tapestry of hues. They are made of water droplets that are much larger than visible light’s wavelengths . They scatter all colours with a similar intensity. This makes the cloud appear the same colour as the light incident on it. Therefore they will appear white at midday reflecting all colours from the sun but during a sunset will project to us that familiar pallet of oranges and reds.