Cloudless days are not common in some parts of the world. We are sure you already know about low and high pressure areas? A barometer registers the atmosphere's pressure. When some parts of the world are in a low pressure area, the needle will be on 'rain'. When the pressure rises a little, the needle moves towards 'variable'. When a high pressure area arrives, the needle moves to 'Fine', which means clear blue sky, and little or no wind. Sunlight is a mixture of the colors of the rainbow :red, orange, yellow, green, blue, indigo and violet. These seven colors give you the answer to the question.
The yellowy-white sunlight is made of all the colors mixed with dust and other gases. Of all the colors in the sunlight, only sky-blue cannot be separated. Thus the blue is reflected and is visible. The air only holds part of the colors of sunlight, which is why it seems to be yellowy-white to us. Over a large expanse of water the sky can be clear blue. Why? when the sunlight falls on a place where there is deep clear water, only the blue is reflected to our eyes. 
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