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Why is the Sky Blue?

Updated on April 22, 2011

For a simple explanation of why the sky is blue, why grass is green, and why sunsets in some areas or at certain times of the year are better than others--read on. If you're looking for a very detailed and hard science approach to the issue, there are plenty of great in-depth explanations at sites such as Science Made Simple.

I was lucky as a kid. I grew up with a world-class scientist for a father, and a mother equally interested and skilled in art, botany, and literature. Between them, my sister and I were able to ask virtually any question and receive a prompt, clear, and detailed answer. Now that I'm at the age to think about having children of my own, the idea of explaining things like why the sky is blue seems a bit more daunting than it did years ago. Parents are supposed to know everything, aren't they? Well, the simple fact is that no matter how much a person knows, there are always plenty of things they don't know. The answer to "Why is the sky blue?" though, is actually pretty simple to understand or explain.



Light and Color

The first thing to understand about why the sky is blue is how humans view the world. I'm not going to get into the nitty gritty, but essentially everything we see is just a small part of the light spectrum. The part of the light spectrum humans can see without scientific tools is called the visible spectrum. This includes all of the colors contained within a rainbow. White light, such as sunlight, is actually a combination of all the colors in the visible spectrum, and black (which technically isn't a color) is the absence of color altogether.

The other spectrums of light that most people are familiar with are the infrared and ultraviolet spectrums. Many insects, such as bees, see the world through the ultraviolet spectrum--which helps them to locate pollen and nectar on flowers--among other things. Other animals such as cats and deer can see much more of the infrared spectrum than humans, which gives them better night vision. If you've ever seen an animal's eyes shine in your headlights at night, that's why (they have an extra layer of reflective tissue at the back of their eyes).



So Why Do Things Have Colors?

Now the reason an object we see will have one color instead of another is actually quite simple. If you look at grass or other plants and wonder why they're green, it's because the plants reflect the green portion of the visible spectrum. What this means is that everything that exists has a structure that reflects some part of the visible spectrum. A red car is reflecting the red light, green grass is reflecting the green light, and a polka dot bikini might reflect white and red light (or yellow and blue--it depends on the pattern).

To make this easier to understand, think about sunlight. It's white light, which is a combination of all of the colors of the rainbow. How a green plant appears green to us is that sunlight (or any other form of light) strikes the plant and the plant absorbs all of the other colors, but reflects back the green part of the light. This makes it appear green to our eyes. So whenever we see a color, that color is actually the color that the object is reflecting back to us--the rest of the colors in the visible spectrum are being absorbed.

Now the sky is a little more complex--as we've all noticed, it isn't always the same color. Generally the sky is blue because the particles and gasses in the atmosphere reflect back blue light. As the sky approaches the horizon, less and less blue light is reflected due to the fact that the horizon is the furthest point away, and more light gets scattered by airborne particles and gasses before it can reach our eyes. Less reflected blue light means a more pale blue color--which is why the sky directly overhead is the deepest blue, the middle of the sky a medium shade of blue, and the part of the sky nearest the horizon the palest shade of blue (often close to white).



So What About Sunsets and Sunrises?

Ok, now that we've covered how humans see light, why objects appear as one color or another, and why the sky is blue--what about sunsets and sunrises?

It's really quite easy to understand. The Earth is a sphere, and the horizon is the widest visible part of that sphere at any given point. The further away the sunlight is away from us, the more particles and gasses the light has to travel through before it reaches our eyes. A sunset or sunrise is full of beautiful reds, oranges, yellows, and other colors because the sun is near the horizon and light has to travel through more air, particles like dust, water, pollution, and different gasses before we see it.

Since the sunlight is coming from a low angle near the horizon, much or all of the violet and blue spectrums of light are absorbed completely by the air before it can reach us, and so we see the spectrums of light reflected by the particles and gasses in the air much more than during the rest of the day.

Now there are two main reasons that some areas have better sunsets or sunrises than others--and some times of the year produce more colorful sunsets and sunrises than others.

  1. The closer to the equator a location is, the further light has to travel before we see it. Places further north or south of the equator are at the thinnest parts of the planet, and light doesn't have to travel as far to reach our eyes, so more of the violet/blue spectrum reaches us. Places closer to the equator are at the widest part of the planet, and therefore the horizon is much further away and more of the violet/blue portion of light is blocked before it can reach us--making for more reds, yellows, oranges, etc.
  2. The other thing that can create better sunsets and sunrises is the amount of particles in the air. The more particles in the air (pollution, dust, water vapor, etc.), the more other colors will be reflected back to us and the more vivid and colorful a sunset or sunrise will appear. During the Summer, more dust, dirt, water vapor, and pollution are in the air, so the sunsets and sunrises are more colorful. Some of the best sunsets and sunrises in history have been after volcanic eruptions when the amount of volcanic ash in the air created an amazing display of color.

Ok, now you should have a pretty good basic understanding of why the sky is blue, why grass is green, and how sunset and sunrise colors work. If you have any questions or comments, just write them below, and I'll make sure to address them either in the comments section, the article itself, or both. Thanks for reading!


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    • Tracy Lynn Conway profile image

      Tracy Lynn Conway 6 years ago from Virginia, USA

      Excellent hub, really well done! Voted up and beautiful.

    • Morgan Orion profile image

      Morgan Orion 6 years ago from Minnesota

      @Tracy Lynn Conway- Thank you so much, I really appreciate it.

    • Faceless39 profile image

      Faceless39 5 years ago from The North Woods, USA

      This is an awesome hub, and explains everything in a way that most people will be able to understand. I'd forgotten how these things worked, so this was a great refresher course. Wonderful explanations and images. Voted up, interesting, useful, and awesome.

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