Why does the sky turn red at sunset?

Sunlight is a mix of many colors. Air molecules bounce short-wavelength blue light several times more than red light, so blue spreads across the whole sky. That's why the daytime sky looks blue even when you look away from the sun.

Then why isn't the sky violet, since violet scatters even more than blue? Sunlight contains less violet to begin with, and human eyes are less sensitive to it, so the mix looks blue to us.

Around sunset, light passes at a slant through a long stretch of the atmosphere. Blue light drops out first along that long path, while orange and red, which scatter less, make it all the way to your eyes. So the area around the sun and the western sky turn red, and clouds catching that light glow pink or orange. The sun itself looking like an orange disk on the horizon has the same cause.

The pink band you can see low in the eastern sky, opposite the setting sun, is that same red light reflected off high air.

A common misconception is that the air must be dirty for a sunset to be red. The red color forms even in clean air. In fact, when there are lots of large particles like fine dust, they scatter all colors about equally, which tends to make a sunset hazy and dull.

On the other hand, when very small particles spread widely high in the sky, as after the 1991 eruption of Mount Pinatubo in the Philippines, unusually vivid sunsets were seen around the world for months.

The old saying that a red evening sky means a clear next day is tied to the same principle. In mid-latitudes like Korea, weather generally moves from west to east. A red western sky at sunset means there are few thick clouds blocking sunlight in that direction, so clear weather is slightly more likely to arrive.

Still, it's only a tendency. To decide whether to pack an umbrella tomorrow, the weather forecast is far more reliable.