Why is the Sky Blue?

This is actually a very deep and intelligent question. This website is dedicated to answering that question in the clearest and most thorough manner possible. We are not going to only give you the answer, but hopefully we will make you think as well. So let's figure out Why is the sky blue?

This website is presented at the high school or college level. If you want a one page quick version, or a good version for kids, see Why is the Sky Blue - For Kids.

My first question to you is: Why is the sky not black? Let's do a quick thinking experiment. Imagine you are sitting out in space on a green marble, and someone shines a flash light at you. Or suppose you are in a very large blacked out room and someone far away turns on a flashlight. This is illustrated in Figure 1:

Figure 1. An experiment in outer space.

What would you see in this situation?

If the flashlight is off, you would see nothing - only black, as shown in Figure 2(a). If the flashlight was on, and there were no reflections from walls or objects, you would just see the light from the flashlight, as shown in Figure 2(b). Now, this situation is certainly different than what we see in the daytime on Earth, as shown in Figure 2(c):

Figure 2. What You Would Observe in Different Situations.

If we think about Figure 2, we see that something is a bit off. Why is the background black in the first situation, but the background (the sky) is blue as seen on Earth? Both of these situations involve a single light source shining on someone. Let's dig deeper as we try to figure out why the sky is blue.

If you look directly at the sun (don't ever do this!), you would observe a very bright white light from the light rays travelling directly at you. This is illustrated in Figure 3(a). However, if you are out in the daytime, and you look towards the sky away from the sun, you observe a blue color. This means there must be some blue light coming from the direction you are looking in, as shown in Figure 3(b):

Figure 3. Seeing Blue Means Blue Light is Coming From That Direction.

Where does this blue light come from? We know that the blue light must come in some way from the sun, because the sky is only blue during the daytime. But there must be something else involved, or the situation would be the same as the flashlight situation of Figure 2(b). The only thing between a person and the sun is the Earth's atmosphere. Hence, there must be some interaction of the sun's light with the Earth's atmosphere that makes the sky blue. This is shown in Figure 4:

Figure 4. The Atmosphere "Scatters" Light in All Directions.

Figure 4 is very important. Imagine you are standing at location of the red dot on the Earth's surface in Figure 4. It is noon, so the sun is directly overhead of you (to the right in Figure 4). The sun's strong light travels and hits the Earth's atmosphere. The rays that are directly between you and the sun pass in a straight line through the atmosphere. This is illustrated by the thick white line hitting the Earth.

The interesting thing is what happens when you look away from the sun. When sunlight hits the atmosphere, the light scatters in all directions due to the properties of the atmosphere. Hence, when you look away from the sun, you are seeing indirect (scattered) light from the sun. It travels towards the Earth, hits the atmosphere, and then changes direction (and turns blue) and then arrives at your eye. It is the small blue rays shown in Figure 4 that give the sky the blue color.

Now we have an idea of what is happening. But we have some very large questions remaining. For instance,