Blue Sky. Bluer Sky. Red Sky. And Why Isn’t The Sky Violet?
Boulder (blue), Colorado
Christmas Day 2020
We probably all remember that light from the Sun looks white, but it is really made up of all the colors of the rainbow. And we remember how our science teachers showed us when white light shines through a prism, the light is separated into all its colors.
Like energy passing through the ocean, light energy travels in waves, too. Some light travels in short, "choppy" waves. Other light travels in long, lazy waves. Blue light waves are shorter than the other colors (except violet) that make up white light.
All light travels in a straight line unless something gets in the way and does one of these things:
reflect it (like a mirror)
bend it (like a prism)
or scatter it (like molecules of the gases in the atmosphere)
Sunlight reaches Earth's atmosphere and is scattered in all directions by all the gases and particles in the air. Blue light is scattered in all directions by the tiny molecules of air in Earth's atmosphere. Blue is scattered more than other colors because it travels as shorter, smaller waves. This is why we see a blue sky most of the time.
Closer to the horizon, the sky fades to a lighter blue or white. The sunlight reaching us from low in the sky has passed through even more air than the sunlight reaching us from overhead. As the sunlight has passed through all this air, the air molecules have scattered and rescattered the blue light many times in many directions.
Also, the surface of Earth has reflected and scattered the light. All this scattering mixes the colors together again so we see more white and less blue.
The Bluer Sky
Another way of saying all of this is in the daytime you are looking at a black sky (space) that is "illuminated" by mostly blue scattered sunlight. The higher your altitude the less dense the air (less particles in the air) meaning there is less scattering of the blue light and therefore the dimmer is the blue scattered illumination and you start to see more of the basic black of the sky. So instead of being “bluer,” the sky is actually blacker. 🤪
Humidity, Temperature and The Color Of The Sky
The higher the amount of humidity (more particles in the air), the more blue light scattering) with the result that the sky looks more whitish, or even greyish.
With lower temperatures in winter the amount of water vapour in air is lower, hence the sky appears bluer.
In desert climates, at high temperatures the sky is very blue due to a very low humidity (= amount of water molecules in air).
What makes a red sunset?
As the Sun gets lower in the sky, its light is passing through more of the atmosphere to reach you. Even more of the blue light is scattered, allowing the reds and yellows to pass straight through to your eyes.
Sometimes the whole western sky seems to glow. The sky appears red because small particles of dust, pollution, or other aerosols also scatter blue light, leaving more purely red and yellow light to go through the atmosphere.
More Detail, Including Why The Sky Is Not Violet Instead of Blue
Why is the sky blue again?
The sun emits energy as beams of light which are electromagnetic waves. When the beam of light approaches the earth the most harmful components of the beam, which have the shortest wavelengths and highest frequency (gamma, x-ray and ultraviolet waves), are prevented from passing through the stratosphere by the ozone layer. The ozone layer still allows radio and visible light waves, which have a larger wavelength, to pass through.
The visible light that passes through the atmosphere is made up of all the colours of the rainbow. So why doesn’t the sky appear multicoloured? This is due to the scattering of the visible light waves as they collide with nitrogen and oxygen molecules in our atmosphere. The degree to which the visible light wave is scattered is dependent on the wavelength of its component parts. Similar to the waves you see on the beach some are larger while some are smaller. The colours red, orange, yellow and green have larger wavelengths while blue and violet have a smaller wavelength and a higher frequency. The smaller the wavelength of the light the more the light is scattered by the particles in the atmosphere. Thus, the light with the higher wavelengths pass through the atmosphere with little or no scattering, while blue and violet waves are more scattered.
Finally, the blue and violet light waves which are scattered across the sky enter our eyes making the sky appear blue.
However, this begs the question if violet light has a shorter wavelength and is scattered more, then why does the sky not appear violet?
This is because the sun emits a higher concentration of blue light waves in comparison violet. Furthermore, as our eyes are more sensitive to blue rather than violet this means to us the sky appears blue