Why is the Sky Blue?

If you could go to the moon and look at the sky from there, you would see black. So the sky is not blue at all--what we see is simply the air glowing blue. This is caused by reflected sunlight, and it is blue because tiny air molecules reflect blue light more efficiently than the other colors. If the reflecting molecules were larger, however, such as the large water drops in a cloud, then all colors would be scattered. That is why clouds appear white.

Strangely enough, the blue hue of the sky depends on the reflectivity of the underlying surface. Thus, the sky looks bluer over the oceans than it does over a reflecting ice or snow-covered surface. This tact was used by early polar explorers to detect open water. Haze or pollution causes the sky to look washed out. The bluest skies are seen in pure mountain air at locations far from urban pollution.

Sadly enough, as industrial haze continues to build up, it is rarer all the time to see a really blue sky in many parts of the world. Perhaps the bluest sky in the world can be seen from the summit of Mauna Loa, Hawaii (14,000 feet). From this location on a clear day, the sky looks almost violet, the surrounding oceans, the altitude and the clear air all contributing to the hue.