Why Is The Sky Blue...And Why Are Sunsets Red?
Why is the sky Blue? I remember from physics, something about "dispersion" but what specifically makes the clouds take on the hue of black before and during a thunderstorm, or better yet, green when it's really nasty out?
For more on cloud colors, check out WeatherSavvy on Clouds and Color
For more on why the sky is blue...Read On
Actually, dispersion is the break up of white light into colors like a rainbow. ( And on the cover to Pink Floyd's Darkside of The Moon)
Scattering is the real reason why the sky appears in color. Scattering occurs when the gas molecules that comprise our atmosphere, mostly nitrogen and oxygen, absorb select colors of white light and then radiate that particular color in every direction.
Let's get into the Savviness...and it all starts with a really smart dead guy named Tyndall.
John Tyndall showed that small, but non-dissolved, particles in a solution will scatter light. This is kind of like how at a movie theater you can see the light from the movie projector streaming towards the screen. The light gets scattered in all directions from small dust particles in the air.
This "Tyndall effect" can apply to our atmosphere. Some other really smart dead guy named Lord Rayleigh, took the Tyndall Effect one step further.
Rayliegh showed how different wavelenghts of light (which is the same thing as saying different colors of light) will scatter or not scatter depending upon the substance they are encountering. As sunlight encounters the Nitrogen and Oxygen which comprise over 90% of our atmosphere...the Blue, Indigo, and Violet colors of light get scattered. Red, Orange and Yellow are not being scattered. This is now called Rayleigh Scattering and explains why the sky is blue.
Note To Self:
Become Smart, discover something, then have it named after me, "The Steve Process".
But I digress,
So to understand this better, imagine you had 2 flashlights. One flashlight only shines red color and one only shines blue. Now, in a dark room you shine the red flashlight towards the celing, and all you see is the red beam of light going straight to the ceiling. Now try using the Blue flashlight, and all of a sudden, instead of just seeing the beam of blue light going straight to the ceiling, the whole room is illuminated as the blue light scatteres off of every gas particle in the air.
That is similar to what happens to our air, and why the sky is blue. This example is not meant to be exactly scientific, but simply meant for you to grasp the concept of scattering. And yes...that's the best I could come up with.
Some Moms out there know that their kids have selective hearing. You hear what you want to hear and block everything else out. So in this sentence, "Clean up your room, walk the dog, take out the garbage and by the way, we're getting Pizza for dinner tonight"...I might only hear the "Pizza for dinner tonight" part. That's because I magically ignored everything else.
Well, similarly, this Rayleigh Scattering is sometimes called slective scattering. It depends on the wavelenghts of the material that is doing the scattering with respect to the wavelength of the light. It just so happens that the relatively small particle size of the gases in our atmosphere scatter the shorter wavelenghts of light, i.e. Blue, Indigo, Violet.
So Why Is The Sky Red At Sunset/Sunrise?
During a sunset, light travels through more atmosphere (See the diagram at right).
Thus, as it travels through more atmosphere, the light is subjected to more and more gas molecules in the air.
Since we just got done talking about how air scatters blue light, the reult is that more and more blue light is scattered out of the beam of light that eventually reaches your eyes. So Blue is "scattered out" and the part of light that is left over is red, orange and yellow.
When the blue has been scattered out, even the clouds which scatter all colors, look reddish. The only color the clouds can reflect, is the color that shines on it. If blue is scattered out, only red will be reflected and scattered off of the clouds and in to your eyes.
In space, the sky appears black, because space has no atmosphere to scatter the blue. Further, all of the colors of the sun reach your eye, and the sun appears white. On Earth, the sun appears yellow, because most of the blue light is removed or "scattered out" from those direct rays of the sun.
I hope that answers your question Dave and thanks for the email.
If You Have Questions, Email Me