Welcome to You Ask Andy

Julia McCready, aged 10, of Portland, Maine for her question:

What makes the sky blue?

This is a very popular question. A lot of friends have asked Andy to explain it. The answer is not easy to understand. So you may have to read it two or three times.

The heavenly blue sky is caused by sunlight. And sunlight is full of tricks. Some of these tricks are very mysterious. Even the experts do not understand them all.

We cannot see the light of the sun. Or rather, we can see clear through it. We can also see clear through glass and clean water. It is hard to believe that sunlight is really full of colors. But it is. It is a blend of all the colors of the rainbow. Did you ever blend blue and yellow paint to make green paint? Well, the sunlight blends the‑ rainbow colors ‑‑to make ‑no color at all. That is quite a trick.

You cannot separate your green paint into blue and yellow again. But sunlight can separate its lovely colors. This happens when a sunbeam is forced to bend. Each strand of color in the sunlight bends at a different angle. Some colors bend over just a little. Some bend over further. This makes the colors of sunlight open up like a fan.

Sunlight is a blend of seven colors. They are red, orange, yellow, green, blue, indigo blue and violet. The strand of red bends the least. Orange bends a little more and yellow bends still further. Green bends a little more than the yellow. Blue, indigo and violet each bend at still sharper angles.

Falling raindrops can sometimes bend the colors of sunlight. This happens when the sun shines on a raincloud from the opposite side of the sky. The radiant colors of sunlight fan out to make the rainbow.

The blue rays of sunlight are bent by the air. The atmosphere of air reaches high above our heads. Light from the sun must travel through miles of air before it reaches us,, The atmosphere is made of fine particles of gases and dust. These fine particles bend the blue rays of sunlight. These are the rays that bend at the sharpest angles.

The atmosphere bends them and scatters them all over the sky. That is why the sky looks blue to us.

At morning and evening the sunlight slants through the atmosphere. It must travel through an even thicker blanket of air. This means it must pass through many, many more particles of dust and gases. This gives the red and orange colors a chance. These are the rays that bend least. The extra particles in the air force them to bend and scatter also. The reds, yellows and oranges get a chance to show off their colors at sunrise and sunset.


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