Why is it, that on a perfectly cloudless day, the sky is most often a deep blue color? The hue of the sky varies from time to time, sometimes going as far as teal. On those rare clear days (rare here, anyway), the color seems to be ideal deep blue found in the scenery in car commercials. Why exactly is this?

 

My guess would be that low levels of dust in the sky makes cloud formation less likely, and allows light to reach us with a minimal hazing effect. But I really don't know much about the weather, so I wouldn't be surprised if I'm way off.

It strikes me that it has to do with nitrogen. The blue color, you know. The more nitrogen, the deeper the blue color. That is: if you're in the center of a local high-pressure area, the sky will be cloudless and deep blue; and if you're in the center of a local low-pressure area, the sky will be cloudless and light blue; and if you're not in the center of either, the sky will be cloudy and an intermediate blue caused by a decrease in pressure and water vapor in the air.