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"Yesterday seems as though it never existed. Death greets me warm, now I will just say goodbye."

Why the sky is blue:

Not all colors are the result of the addition or subtraction of light. Some colors, like the blue of the sky, are result of selective scattering. Consider the analogous case of sound: If a beam of a praticular frequency of sound is directed to a tuning fork of similar frequency, the tuning fork is set into vibration and redirects the beam in multiple directions. The tuning fork scatters the sound. A similar process occurs with the scattering of light from atoms and particles are far apart from one another, as they are in the atmostphere.
((you research on Rayleigh scattering)) talk2hand

Atoms behave like tiny optical tuning forks and re-emit light waves that shine on them. Molecules and larger collections of atoms do the same. The tinier the particle, the greater amount of higher-frequency light it will re-emit. This is similar to the way small bells ring with higher notes than larger bells. The N and O molecules that make up most of the atmostphere are like tiny bells that ring with high frequencies when energized by sunlight. like sound from the bells, the re-emitted light is scattered. question

Of the visible frequencies of sunlight, violet is scattered the most by nitrogen and oxyen in the atmostphere, followed by blue, green, yellow, orange and red, in that order. REd is scattered only a tenth as much as violet. Although violet light is scattered more than blue, our eyes are not very sensitive to violet light. Therefore the blue scattered light is what predominates in our vision, and we see a blue sky! ((go figure... rolleyes ))

The blue of the sky varies in different places under different conditions. A principle factor is the water vapor content of the atmostphere. On clear, dry days, the sky is much deeper blue than on clear days with high humidity. Places where the upper air is exceptionally dry, such as Greece and Italy, have blue skies that have inspired painters for centuries. Where the atmostphere contains a lot of particles of dust and other particles larger than oxygen and nitrogen molecules, light of the lower frequencies is also scattered strongly. This makes the sky less blue, and it takes on a whitish appearance. After a heavy rainstorm when the particles have been washed away, the sky becomes a deeper blue.
((there are other blah blah, but I think this is the most important...))
Hope this helps. ninja