When the sun is out, the Earth’s atmosphere scatters sunlight in every direction. This is why during the day, some parts of the Earth are brighter than others-the Sun’s light has been reflected off of the ground and other objects.
However, when there is complete cloud cover over the planet, no light reaches Earth at all. This means that during a day with complete cloud cover, neither sunrise nor sunset will occur-the Sun will be completely hidden behind the clouds! On a cloudy day, the visible light headed toward the earth is redirected by the clouds and darkens. This is because the shorter waves of visible light (those with shorter wavelengths) are scattered more than the longer waves. This phenomenon is known as Rayleigh scattering.
Rayleigh scattering is caused by the particles in the atmosphere, specifically water molecules. The shorter wavelengths of visible light are scattered more than the longer wavelengths because the shorter wavelengths of light have a smaller wavelength than the longer wavelengths. This is because the shorter wavelengths of light have more energy than the longer wavelengths.