Why Is the Sky Blue?

Often, people talk about the color of the sky as if it is just a fact of the universe. Water is wet, the Queen of England was British, and the sky is blue.

But sometimes we wonder, "Wait, what is it that actually makes the sky blue?

In this article, we will explore what makes the sky look the way it does.

 

Basic Principles of Light

What we see as light is actually electromagnetic radiation that forms waves of different lengths to create what our eyes see as different colors. When light is white, it is a combination of all colors, which can be separated by slowing the light down by passing it through an obstacle like a prism. [2]

Spectrum of visible light and its colors

The wavelengths of light our eyes can detect (and thus we can see) is known as visible light. The colors within the visible light spectrum are separated by the length of their waves, which can also be thought of as the speed of the wave. [2]

The fastest light, with the longest wavelength, is red light. The slowest light, with the shortest wavelength, is blue light. [2]

It is easy to see the visible spectrum by shining white light through a prism.

 

Rayleigh Scattering

Rayleigh scattering is what happens when white light from the sun enters our atmosphere. The composition of the atmosphere and why this happens will be covered in later sections.

Relationship to size of particles

In essence, Rayleigh scattering is a physics theorem that explains how the light entering the atmosphere is scattered by small particulates in the atmosphere, and separated out by size of wavelength. [3]

Rayleigh scattering increases with the size of the particle, so different-sized particles in the atmosphere can lead to scattering of different sizes of wavelengths, resulting in different colors being seen. [4]

Relationship between wavelength and the degree of scatter

The degree to which light will be scattered partially depends on the size of the wavelength. Blue light is scattered most because is the shortest wavelength, whereas red light can stay together and avoid this scattering. [5]

 

Role of the Earth’s Atmosphere

Composition of the atmosphere

The atmosphere of the Earth can be thought of as a thick blanket of gas that sits over us to protect us from outer space. The atmosphere is mostly made up of only four gasses.

These gasses include nitrogen as the most prominent, followed by oxygen, argon, and carbon dioxide. All these gasses have important roles in ensuring we have all the requirements for life. [6]

Relation of composition to light we see

"The composition of the atmosphere strongly influences the size of particles within it, and these particle sizes determine which colors of light are scattered more than others, as the particle size correlates with the wavelength of light that is most scattered. [5]"

 

Variations in Sky Color

Color changes at sunrise and sunset

At sunrise and sunset, the sun is sitting closer to the horizon. This means the light travels through the thickest part of the atmosphere; blue light is scattered more than usual, leaving longer wavelengths like red light. [7]

Pollution

Pollutants are sometimes released into the atmosphere, often through natural disasters like wildfires. When this happens, you may notice color changes in the sky.

This is because the pollutants are a different particle size than what is normally in the atmosphere. As such, the Rayleigh scattering is different, and the wavelength of light that gets scattered is different. [5]

Latitude

Latitude describes your position on the Earth from the Equator, and since this relates to where the sun is, it can have an effect on the color of the sky. In places where the sun never gets above the horizon, the characteristic blue color of the sky may never be achieved, with oranges or reds being the color instead.

 

Other planets

Other planets don’t have the same color skies that Earth does. [9]

This might seem strange, but actually makes sense when you think about it. The other planets have different atmospheres, so the amount of light scattering and the length the light has to travel are different. [5,7].

For instance, Uranus has a cyan sky, while Mercury has a black sky. [9]

 

Predicting Blue Skies With Sonuby

Predicting blue skies is what most people use weather apps and weather forecasts for. There are already a lot of apps out there that do a pretty good job of predicting good weather.

Here is why you should give my weather app Sonuby Weather a try anyway:

Let's say you want to know when the sky will be blue without having a specific date in mind. Sonuby offers pre-built weather reports for different activities like skydiving, surfing but also for your daily life.

In our case we would choose a location, let's say Berlin and the "Daily Life" report which contains forecasts for your everyday life.

Since we are not looking for a specific date, we could scroll down to the "7-day meteogram" which shows different graphs for different weather variables. In our case, we would be looking for sunshine percentage and cloud cover. If the sunshine percentage is high (above 50% per time period) and the cloud coverage is low (below 25%), like in the second screenshot above chances are good that we will see at least a little bit of blue sky.

We could also use the cloud cover weather map below the "7-day meteogram" which will allow us to get an idea of where clouds will be in and around Berlin.

The variety of different forecasts makes Sonuby a perfect companion for people who want to dive a bit deeper and be more certain when it comes to predicting great weather.

 

Resources:

[1] https://futurenow.com.ua/en/why-is-the-sky-blue

[2] https://www.amnh.org/explore/ology/physics/see-the-light2/the-color-of-light

[3] https://link.springer.com/referenceworkentry/10.1007/978-3-642-27833-4_1349-2

[4] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6211534

[5] https://www.britannica.com/science/Rayleigh-scattering

[6] https://education.nationalgeographic.org/resource/atmosphere/

[7] https://www.metoffice.gov.uk/weather/learn-about/weather/optical-effects/why-is-the-sunset-red

[8] https://weatherworksinc.com/news/the-skys-colors