The molecules of air and other fine particles in the atmosphere have a size smaller than the wavelength of visible light. These are more effective in scattering light of shorter wavelengths at the blue end than the light of longer wavelengths at the led end. When sunlight passes through the atmosphere, the fine particles in the air scatter the blue color more strongly than red. The scattered blue light enters our eyes. Since we see the blue light from everywhere overhead, the sky appears blue.