Why does the sky appear blue during the day?

The sky appears blue because of a phenomenon called Rayleigh scattering. When sunlight enters Earth's atmosphere, it collides with gas molecules. Sunlight contains all colors of the visible spectrum, each with different wavelengths. Blue light has a shorter wavelength than other colors like red or yellow, and shorter wavelengths scatter much more easily when they hit air molecules. As sunlight travels through the atmosphere, blue light gets scattered in all directions across the sky, while longer wavelength colors pass through more directly. When you look anywhere in the sky, you see this scattered blue light coming at you from all angles. This is also why sunsets appear red and orange, as light travels through more atmosphere at low angles, scattering away the blue and leaving the longer wavelengths.