Good question. Not too many people know the answer to this one, however, Richard F. gave this answer. He also tells you why the clouds are white. Check your school library for a book about this. I think I saw one titles "Why is the Sky Blue".
The light from the sky is light that has bounced off (the scientific word is scattered) the molecules that make up the air around the Earth. Sunlight is made up of light of all different colours, which looks white when they are all mixed together. The air molecules scatter blue light the best of all colours, so the light that comes from the sky looks blue.
There is a book in your school library called "Why is the Sky Blue?". It's by Chris Arvetis and Carole Palmer, and its catalogue number is 520 ARV. This book can help you learn more about this question. Older kids may enjoy reading "Exploring the Sky by Day", by Terence Dickinson, catalogue number 551.5 DIC. This book has answers to lots of other questions about the sky.
This next article came from NASA. This is what the scientists there had to say.
WHAT COLOR IS THE SKY? NASA EDUCATIONAL BRIEFS
For Secondary Classrooms
What color is the sky? It depends upon where you are. From Earth, the sky is blue. Why? The answer lies in the nature and interaction of sunlight and our atmosphere.
When sunlight is passed through a prism, it is broken down into a rainbow of colors. An examination of this color array will show the blue end to be deflected most from the direction of the originating sunbeam.
Our atmosphere acts somewhat like a prism, breaking down sunlight into its component colors, with blue light deflected the most. In fact, different air molecules or layers of the atmosphere deflect blue radiation repeatedly. Consequently, the blue rays come to Earth from all parts of the sky rather than directly from the Sun. This is why the sky away from the Sun looks blue.
In the vacuum of space, where there is no atmosphere, the Sun's rays are not deflected. Instead, they allow a direct line from the Sun to a viewer in space. As a result, to such a viewer, the sky appears jet black. This is the kind of sky that astronauts have seen while traveling in space or exploring on the airless moon.
People looking up at the sky from a spacecraft flying just above the clouds of Jupiter would see blue skies. This is due to a thick transparent atmosphere above Jupiter's clouds that scatters sunlight as does Earth's.
From Mars, the sky would look pink. Sunlight reaching Mars is reflected on a comparatively high concentration of dust particles in the Martian air. The particles contain a large proportion of red oxide, a principal constituent of Martian soil. If the Martian sky were clear of dust, it would be blue, but a much darker blue than Earth's because the atmosphere is much thinner and scatters light much less.
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QUESTIONS FOR DISCUSSION
Why is our sky blue?
Let sunlight pass through a prism. Describe the arrangement of colors.
The colors that you see are commonly called visible electromagnetic radiation, or visible light. Visible electromagnetic radiation is a band of frequencies or wavelengths to which our eyes are sensitive. Give the common names of other bands of electromagnetic radiation.
Which would be to the right of colors thrown by the prism? Which to the left?
Which are lower in frequency than visible light. Which are higher? Name electromagnetic bands in order of frequency, form the highest to the lowest.
Which frequencies are most affected by our atmosphere, high or low?
Why are sunsets and sunrises so red even though the Sun radiates all of the visible colors?
What color is the sky from the Moon? Why?
What color is the sky from Mars? Why?
What color sky would an astronaut soaring above Jupiter's clouds see? Why is the sky this color?
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NASA EDUCATIONAL BRIEFS For Secondary Classrooms, EB-80-1
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