Why Is the Sky Blue at Noon but Red at Sunrise?

The sky changes color because sunlight gets filtered differently depending on how much air it has to travel through to reach your eyes.

When the sun is high overhead, its light takes a short, straight shot through the atmosphere. Think of the air like a giant bucket of water. If you shine a flashlight through a small cup of water, it looks clear. But the air is full of tiny gas molecules. These little bits act like a sieve. They catch the blue parts of sunlight and scatter them everywhere. So when you look up, you are seeing that scattered blue light bouncing around the sky. It is like looking through a blue-tinted window.

When the sun is near the horizon, its light has to travel through a much thicker layer of air. Imagine walking through a crowded room full of people. If you walk straight across, it is easy. But if you have to walk the long way around, you bump into way more people. The sunlight bumps into far more gas molecules on this long journey. These molecules are very good at scattering blue light. By the time the light reaches you, the blue has been scattered away. The red and orange parts of light are like big, strong swimmers. They can push through the crowd of air molecules without getting scattered as easily. So you see the reds and oranges that survive the long trip.

It is like watching a flashlight beam through thick fog or dusty air. The thick air acts like a filter that removes the blue, leaving the warm colors behind.