Why does the sky appear blue?

The sky is one of the most beautiful natural masterpieces of the universe. Even if some people are deprived of other aspects of nature, such as green spaces and trees, due to urban sprawl, the sky remains accessible to all, allowing us to observe its details clearly. For those who love to meditate on the sky, it’s natural to wonder: why is the sky so colorful? Why is it blue at times, and at others, it appears light indigo, reddish, or slightly orange, especially during sunset? At night, it turns black and dark. What causes these phenomena?

Let's first dispel some common misconceptions about why the sky is blue. Some believe it is due to the presence of seas and oceans, suggesting that the sun’s rays reflect off the water and color the sky blue. However, if this were true, the sky would only be blue near oceans, which is not the case. The sky remains blue even in arid regions far from any body of water. Others claim the sky's blue color is due to water droplets in the air, but water is colorless and cannot impart any color to the sky. Moreover, clouds, which contain the highest concentration of water droplets, are white, not blue. The sky is also blue in dry weather when there are no water droplets in the air.

 

So, what is the real reason for the sky’s colors? The answer lies in a natural phenomenon called Rayleigh scattering. This effect occurs when light or other electromagnetic radiation travels through a transparent medium, such as gas, liquid, or solid. The largest transparent gas medium we have is the Earth's atmosphere. Rayleigh scattering explains why we see different colors in the sky depending on the time of day and atmospheric conditions.



We understand that the sky isn’t inherently blue. The appearance of the sky as blue is due to the scattering of sunlight in the atmosphere. This explanation involves two key elements: sunlight and the gases in the atmosphere.

 

Firstly, sunlight, which appears white, is actually a mixture of seven primary colors: red, orange, yellow, green, blue, indigo, and violet. Each of these colors has its own frequency and wavelength, with violet having the shortest wavelength and red the longest. The other colors fall in between these two.

 

Secondly, the atmosphere is composed mostly of nitrogen (78%) and oxygen (21%), with traces of other gases like carbon dioxide. The blue color of the sky results from the interaction between sunlight and these atmospheric gases. Oxygen and nitrogen molecules are very small, and when sunlight passes through the atmosphere, it is the shorter wavelengths (blue and violet light) that are scattered more by these small molecules.

 

This scattering causes the sky to appear blue, but you might wonder why it doesn’t appear violet, given that violet light has an even shorter wavelength than blue. The answer lies in two factors:

 

1. The human eye is less sensitive to violet light compared to blue light.
2. Much of the violet light is absorbed by the upper atmosphere and doesn’t reach our eyes.

 

Thus, even though violet light is scattered, our eyes are more responsive to the blue light, making the sky appear blue. If green light had the shortest wavelength, the sky would appear green. Similarly, if atmospheric particles were larger and could scatter longer wavelengths, the sky would appear in the color corresponding to those wavelengths.

 

In conclusion, the sky appears blue because blue light is scattered in all directions by the small molecules in the atmosphere, and our eyes are more sensitive to blue than to violet light.


And green cones are responsible for perceiving the color green. When light reflects off objects and enters the eye, it is received by the retina and processed by the cones, which then send signals to the brain. Here’s where it gets interesting: the human eye is more sensitive to blue light than to other colors. This heightened sensitivity means the blue cones in our eyes are more strongly stimulated than the other types of cones. Therefore, we perceive the sky as blue, even though there is also a significant amount of violet light. The increased stimulation of the blue cones gives the brain the impression of a mixture with more blue than other colors. This results in the light blue color we see in the sky.

 

This sensitivity also explains why the violet band in a rainbow is the hardest for us to see, even though it is present like the other colors. Additionally, the amount of violet light emitted by the sun is much lower compared to blue and indigo light. Even though violet light is scattered more due to its shorter wavelength, blue and

 

indigo light are more prevalent in sunlight. These factors together explain why we see the sky as blue rather than purple.

 

Before concluding, let’s address another question: why are clouds white if the sky is blue? The primary reason the sky appears blue is due to the Rayleigh scattering effect, where gas molecules in the atmosphere are similar in size to the wavelength of blue light. However, clouds contain larger particles, such as dust, water droplets, and fumes. These particles are much larger than the wavelength of light, causing all colors of light to scatter equally. When these scattered lights combine, they produce white light, which is why clouds appear white.

 

During sunrise and sunset, the sun is low on the horizon, causing sunlight to travel a greater distance through the atmosphere. As a result, most of the blue, indigo, and violet light is scattered out before reaching the Earth's surface. The remaining light consists of the less scattered red and orange wavelengths, which mix to create the red and orange hues we see at these times. So, why is the sky blue? The answer lies in the interplay between the scattering of sunlight by atmospheric molecules and the sensitivity of our eyes to blue light.