Why do we have blue skies?
The sun consists of all the colors of the rainbow. Together, these colors appear white. This is why the sun looks white when it appears to be high over the Earth. Anyway, light travels in waves of different lengths with each color having its own unique wavelength. Normally, light travels in a straight line unless it hits something (i.e. molecules, dust, pollution, water droplets) that causes it to bounce off (or scatter) in different directions. The size of the particle determines how the light is scattered. If the particle is <.5 microns (1 micron = 1 millionth of a meter), the light is separated into a wavelength (a wavelength corresponds to a color). If a particle is > microns, then all wavelengths are reflected and the light appears white.
The molecules that make up the atmosphere are <.5 microns in size and cause light wavelengths to scatter. They send the shorter ones, mostly blue, off in a multitude of directions. The longer ones, like red, are not scattered.
As white sunlight enters the Earth's atmosphere, blue light is scattered by molecules in the air. These blue light waves scatter all over the sky and down in all directions. So, no matter which direction you look, blue light (or the scattered blue light waves) appears.
On hazy days, numbers of large water particles >.5 microns in size reflect all wavelengths equally making the sky appear either white or gray. Hazy air consists of moisture droplets.
Sometimes, pollution affects the skies' color. The sky can appear to be brown or yellow because particles of the right size scatter those colors. All colors seen in the sky are the result of scattering caused by molecules or particles in the atmosphere.