Why the sky is blue (and red at dusk)

Sunlight looks white, but packed inside it are all the colours of the rainbow mixed together. When it enters the atmosphere it collides with air molecules, and here's the trick: the colours don't scatter equally.

Blue, which travels in shorter waves, bounces around far more than red. So wherever you look during the day, blue reaches you from every corner of the sky. It's sunlight that's been deflected thousands of times before hitting your eyes.

At sunset the story changes. The sun sits low and its light has to cross much more air to reach you. Along the way it loses almost all its blue, and only the reds and oranges survive.

So the blue sky and the blazing sky at dusk are the same phenomenon seen from two angles. The colour isn't in the air: it's in how the air scrambles the light.