19 January 2011
Ask a meteorologist “why is the sky blue?” and they will (at least they should) quickly give you the proper answer. The basics are this: shorter wavelengths of blue light are scattered by the air molecules much more than longer wavelengths of red light. The blue light gets scattered across the sky and makes the sky look blue. The technical term is Rayleigh scattering.
This is actually a question that any weather person gets rather frequently. Often, it’s from parents who have been stumped by their three year old! Kids really do ask the best questions, and probably because they aren’t afraid to ask something that sounds silly. I mean really good questions, like why is water wet and how come it’s colder on mountains.
Here is another one that’s related to the blue sky question.
Why does the sun appear yellow?
Ask any weather nerd and they will likely tell you that when the blue light is scattered from the sun’s white light, what’s left is yellow. I’ve given that same answer countless times.
Apparently it’s wrong.
Astronomer Phil Plait writes the Bad Astronomy blog, and has a great book about common astronomical myths. It’s also called BAD ASTRONOMY, and well worth a read.
I was skimming through the chapter on the blue sky when I spotted something very surprising. It seems that not enough blue light is scattered away from the incoming white sunlight to turn the sun yellow. Furthermore, no one really seems to know why it’s yellow!
I’ve been thinking about this, and I think the first thing to ask here is this: Does the sun really look yellow? Yes, it does at sunset and sunrise, when dust and a thick atmosphere scatter all but the red and yellow light away, but what about at high noon? It’s hard to look at it then, so do we just assume it’s yellow?
I looked through some snaps I took in Antarctica and guess what, the sun looks pretty white to me. Maybe it’s an illusion caused by our eyes when looking at the sun surrounded by a blue sky. Phil Plait says this is not likely the case, although that would be most peoples first guess. It was what I first thought of as well.
Here’s my take. The sun is white in the sky except when near the horizon. It can be yellow when there is enough dust/pollution to increase the Rayleigh scattering but most of the time this is not the case.
The link at the beginning of this post on Rayleigh scattering goes to the excellent article on the subject on Wikipedia, except that it too says the sun is yellow because the blue light get scattered away!
Feel free to disagree! If you want more details on it, pick up Bad Astronomy.
PS: The sun can be dangerous to look at when high in the sky, you can permanently damage your vision! So look a little away from the sun if you are curious about it and be careful!