WHY IS THE SKY BLUE? by SCIENCE MADE SIMPLE

Introduction

What makes the sky have color? During the day, why is the color blue? What makes the colors of the sunset and sunrise? Why are they red and orange instead of blue? Why are the sunset and sunrise different? Does the amount of polution in the sky really make the colors of the sunset more vivid and varied? As children we all thought about many of these questions. The problem is that we settled for the answer of "That is just how it is." Instead there is a scientific answer behind the colors of the sky and what allows us to see all the variety of colors that we do in a day.

Background

The colors of the sky are created because the atmosphere between the sun and the earth consists of many different particles and chemicals. The sky directly surrounding the earth though, mainly consists of nitrogen and oxygen molecules as well as the dust and dubree kicked up by humans and animals on the earth's surface. So, when the light hits these particles it is broken up and the different wavelengths are scattered in different directions. This process is called scattering. So with our atmosphere the sky surrounding the earth would be black and only lit up by the white light of the sun. But instead we have an atmosphere that provides us with the colors we can enjoy.
During the day the sky is seen as blue unless clouds are in the way or the weather is not nice. But why is it that the sky is blue instead of purple or orange? The answer to this question is that the wavelength of blue light is very small and can be broken up about ten times more than red light because red light has almost double the wavelength of blue light. So, blue light is scattered in more directions than the shorter wavelengths when it comes in contact with the nitrogen and oxygen molecules. Therefore, blue is the light that enters our eye. But, using this same scientific reasoning, it would seem logical that we would see a violet sky instead since it has the longest wavelength of all. But, human eyes are more sensitive to blue than violet so therefore we see a blue sky.
At dusk and dawn the colors of the sky change and the first portion of the visible spectrum is what appears to the viewer. During these times of the day the sun is positioned at a different angle to the earth than during the normal daytime. Because of this angle, the longer wavelengths are the ones that reach the eye. The blue and other shorter wavelengths are scattered millions of times and do not reach the viewer. When the sun does not shine directly on a certain portion of the earth there are more gas molecules between the viewer and the sun. Therefore, the normal blue color of he sky is no longer visible because the short wavelengths of blue and green are scattered millions of times and do not reach the viewer. Therefore the longer wavelengths that couldn't be scattered as many times are able to now hit the viewer in a more direct way.
But the question that now arises is that the sunrise and sunset should appear to be the same. In reality, they are actually quite different. The sunset tends to be brighter and contain more vibrant colors than the sunrise. This is due to the fact that during the day animals and humans are usually awake and kicking up dust and dubree that will reflect the light. This refection will make the colors in the sunset appear to be brighter than those in the sunrise.
Lastly, if the sunset or sunrise of a big city is observed, it will be evident that the colors are more varied and tend to appear prettier than a sunset or sunrise that one would see in the country. This fact is because the sky surrounding the cities is more polluted than that around a rural setting. The pollution in the air has many chemicals in it that will break the light up in different ways and scatter it differently than the normal atmosphere would do. Though it is unfortunate that some of the prettiest sunsets can be seen due to pollution, it is a fact.

Observations

These photos are a few examples of the sky through out the day. Pay attention to the different colors of the sky and how vivid each one is. Please take into account that these are only photographs and in many cases cannot capture the true colors.

	This is an example of a fairly typical sky. It is easy to see that the sky is definantly blue. There are clouds scattered across the sky that appear to be very white.
		This photo was taken of a sunrise. As you can tell the colors are not very vivid. In fact, i the colors seem to have almost faded out and close to white.
	This photo is also taken of the sunrise. The colors appear to be brighter since the photo was taken by a profesional. But, the basic facts about a sunrise are still evident. The sky is still dull and lacking the vivid, exciting colors of the sunset.
		Unlike the photos above, this sunset has bright, vivid colors. Also the sky above the sun has a purple tint to it due to the fact that the sky is fading from blue to red.

Conclusions

Most people go through their days without noticing what is happening in the sky above them. Even if they do wonder why our sky is the way it is the answer is not important enough to them to find. Most people just settle for the fact that this is how the world works, we have a pretty sky that changes colors due to the time of day and the weather. So, take a look at the sky and notice what is going on. Notice the difference between the sunrise and the sunset, a sunset in a city and a rural area.