"Blue sky" refers to the clear, cloudless appearance of the sky during daylight, typically caused by the scattering of sunlight by the atmosphere. This scattering is more effective at shorter wavelengths, which is why we perceive the sky as blue. The phenomenon is most noticeable on sunny days when there are few or no clouds. The color of the sky can change due to various factors, such as the time of day or weather conditions. During sunrise and sunset, the sky may appear red or orange due to the longer path of sunlight through the atmosphere, which scatters shorter wavelengths and allows longer wavelengths to dominate.