Why sky is blue?

The blue colour of sky is due to the scattering of light by small particles of the atmosphere (air molecules) when the light is incident on particles whose size is smaller than the wavelength of light, it is scattered.

According to Rayleigh law, the intensity of scattered light is inversely proportional to the fourth power of wavelength.

As the wavelength of blue colour is smallest and that of red light is longest, the blue light is scattered most and the red light is scattered least.

The scattered blue light reaching the eye gives the appearance of a blue sky. The sky will appear black in the absence of earth’s atmosphere because no scattering of any colour takes place in that case. While flying in an aeroplane one can observe the sky to be black at high altitudes.

Source: http://www.icbse.com/2009/sky-appears-blue-colour/