Due to increasing artificial light, nighttime in urban areas is moving from black to red:
In places where natural light dominates the night sky, clouds typically make those skies darker, just as they do during daylight hours. But the researchers found that the opposite is true in urban areas: Clouds actually magnify the sky glow effect by reflecting more artificial light back down to Earth. They also discovered that sky glow doesn’t just affect night time brightness—it affects the color of the sky as well.
It’s well known that the daytime sky is blue because the atmosphere scatters shorter blue wavelengths of light more than longer red wavelengths. Similarly, the team found that the shorter wavelengths of artificial light are scattered more easily. But in the case of light coming from street lamps below, that means that much of the blue light is sent out into space, while more of the red light is reflected back down to Earth from the atmosphere and cloud cover. The result is that nights in urban areas have a reddish glow.
Besides ruining our stargazing, the move towards redder skies also messes with our circadian rhythms.