On a clear day, the sky appears blue due to a phenomenon called Rayleigh scattering. When sunlight enters the Earth's atmosphere, it collides with air molecules and scatters in different directions. Shorter wavelengths of light, like blue, scatter more than longer wavelengths, making the sky predominantly blue to our eyes. This effect is especially noticeable when the sun is high in the sky, resulting in the vibrant blue color we associate with clear weather.