Kids: Why is the sky blue?
Blue skies image © NOAA.
Space is black, but Earth’s sky looks blue. It wouldn’t be blue if Earth didn’t have an atmosphere – or if the light in ordinary sunshine didn’t have some blue color in it.
Molecules of air in Earth’s atmosphere happen to be just the right size to send sunlight flying in all directions, as the light strikes the air. This is called “scattering.”
It happens that the blue part of sunlight is scattered more strongly than the red part. The red light takes a straighter path through the atmosphere, which is why – when you’re looking through a lot of atmosphere at sunset or sunrise – the sun looks red.
But, at most times of the day, the blue part of sunlight is scattered most – and that’s why the sky looks blue. If you were to go more than about a dozen kilometers above Earth’s surface, the sky would begin turning dark. Up that high, there are fewer molecules of air to scatter sunlight.
The mixture of colors in sunlight also explains why clouds look white. If you look at a single water droplet from a cloud, it’s clear. Clouds look white because their water droplets are big enough to scatter light of all colors – and all colors combined make white.
Clouds can also look dark, when they’re in the shadow of other clouds – or when the top of a cloud casts a shadow on its own base.
