The sky refers to the visible dome of the Earth's atmosphere. Its appearance is determined by the interaction of sunlight with atmospheric gases and particles. The dominant blue color is primarily due to Rayleigh scattering, a phenomenon wherein shorter wavelengths of visible light are scattered more effectively by nitrogen and oxygen molecules than longer wavelengths. At sunrise and sunset, the light must traverse a greater density of atmosphere, causing the blue light to be scattered away and allowing longer wavelengths, such as reds and oranges, to reach the observer directly. Variability in cloud cover, humidity, and particulate matter alters the scattering effect, resulting in a spectrum of visible celestial phenomena.