Why do clouds look red at sunset?
Short answer
At sunset, sunlight travels through more atmosphere before reaching us and the clouds. Blue light is scattered along the way, leaving more red and orange light to illuminate the clouds.
How it works
Sunlight contains many colors. Air molecules scatter blue light more easily than red light. When the Sun is low, its light takes a longer path through the atmosphere, so more of the blue light is scattered away from the direct path.
The remaining red and orange light reaches clouds and is reflected toward our eyes. Cloud height, thickness, and particles or water droplets in the air can change whether the colors look red, orange, gold, or purple.
Key point
The clouds are not turning into a different substance at sunset. The changing color of the sunlight that reaches them makes the clouds look red.