Blue skies are beautiful as they are. But there’s something uniquely intriguing about a pink sky. Whether it’s a few clouds that appear to be tinted with blush or a magnificent sunset splashing hot pink above, pink skies are simply unforgettable.
But why is the sky pink? Here’s a bit of the science behind our color-changing sky and a few scenarios that lead to the fantastic phenomenon of temporary pink skies.
A Scattering of Color
You might already know that the spectrum of visible light contains a whole range of colors. And as you can see in the image above, each color has a different wavelength. Blue and violet have the shortest wavelengths, and the wavelengths gradually increase as you get closer to red.
Those wavelengths are important because they help explain why we see different colors in the sky. As light reaches through the atmosphere, it runs into various types of particles. When this happens, the light scatters through the atmosphere in a phenomenon known as Rayleigh scattering.
However, not all wavelengths of light scatter equally. Rayleigh’s scattering law holds that the amount a given type of light scatters is inversely proportional to the fourth power of its wavelength.
What this boils down to is that the shorter the wavelength, the more light scatters. Blue light and violet light have the shortest wavelengths. The sun produces much more blue light than violet light, and because the blue light scatters far and wide (and our eyes are much more sensitive to it), the sky appears blue.
Barring excessive clouds or other unforeseen weather events, the sky looks blue for much of the day. But during sunrise and sunset (the times when you’re most likely to see pink in the sky), the light of the sun has to travel further before it reaches us.
The short wavelengths of blue light still scatter, but that scattering tends to happen higher up in the atmosphere. By the time light reaches all the way down, longer wavelengths are the most visible. And as you saw in the diagram above, red light has the longest wavelengths of all visible light.
So why doesn’t the sky look red instead of pink? In some cases, you will see deeper red skies. But on the diagram, you can see that there’s no designated wavelength for pink light.
Some of the sun’s light that reaches Earth’s surface is white. And when that white light mixes with the red light that has filtered through, you get a brilliant, pink-tinted sky.
What Makes the Sky Pink?
To the casual observer, it may appear that pink (and all the other colors of the sky) show up at random. But certain events are more likely to cause the kind of light scattering that leads to a pink sky. Here are some of the most common ones.
The Sun Rising or Setting
Sunrises and sunsets are known for their breathtaking colors. But have you ever stopped to think about why they’re as colorful as they are?
During both sunrise and sunset, the sun is very low on the horizon. When the sun sits that low, light has to travel further to reach the surface of the Earth. When white light has to travel further, the blue wavelengths of light scatter long before they reach our eyes. In many cases, we only end up seeing the longer wavelengths of red, orange, and yellow light.
Depending on clouds, dust, pollution, and other particles in the air, some wavelengths may be more noticeable to us. You’ve probably seen some sunsets that were a mix of pink, red, orange, and yellow. But you’ve likely also seen some that were almost entirely pink!
Clouds and Natural Molecules in the Atmosphere
Like everything else in the world, the Earth’s atmosphere is made up of countless tiny molecules. And as we saw above, the scattering of light off of these molecules is responsible for the color of the sky.
During most normal daylight conditions, the sky looks blue. But when the sun’s light runs into more interference than normal, you’ll start to see different colors (normally those of longer wavelengths). That interference can come from clouds as well as the many types of gases in the atmosphere.
This is the same process that happens when the sun rises or sets. When light travels further, it runs into more interference along the way. And by the same token, if light is traveling a shorter distance but runs into more atmospheric particles, you just might see pink and other exciting shades.
Unfortunately, not all the atmospheric particles the sunlight hits are natural. Air pollution isn’t a new problem, but it is a worsening one. When various types of pollutants gather in the air, longer wavelengths of light are scattered down to us.
Air pollution is more concentrated in some areas than others. Some larger cities seem to have a haze of smog along the skyline. Los Angeles (pictured above) is a great example. As you can see, the sunlight through the smog sometimes gives it a pinkish, rosy glow. It can be a pretty sight from afar, but it doesn’t bode well for a city’s air quality!
An Impending Storm
In many areas of the world, the sky will take on an unusual (and sometimes eerie) color right before a storm. A great example of this was right before Typhoon Hagibis struck Japan. The clouds of this storm were so thick that the sky seemed to glow with a vivid magenta color.
Seeing a dramatic, nearly all-pink sky is a relatively rare occurrence. But even if the sky above doesn’t look completely pink, storm clouds may give it a pinkish tint.
The Pink Sky in Folklore
Pink skies have captivated us for centuries, so it’s only fitting that they’ve made an appearance or two in the world of folklore. You may have heard the phrase “red (or pink) sky at night, sailors’ delight; red (or pink) sky at morning, sailors take warning.” It means that if you see a pink or red sunset, the next day should bring calm and pleasant weather. But if you see a pink or red sunrise, the rest of the day might have storms or other types of bad weather in store.
This old adage has ancient roots — in the Bible, Jesus says “When it is evening, ye say, fair weather: for the heaven is red. And in the morning, foul weather today for the heaven is red and lowering” (Matthew 16:2-3). But experts say that there’s some truth to it.
At least in the middle latitudes of the world, storms generally travel from west to east. You already know that when the light of a rising or setting sun passes through heavy clouds, you’re more likely to see pink or red light.
The sun rises in the east, so it would illuminate any approaching storm clouds. Those clouds might make a sunrise pink or red. The sun sets in the west, and in this case, it would illuminate any departing clouds. That would create a red or pink sky at night, indicating that pleasant weather was coming.
The Color-Changing Sky
Most of the time, the sky appears to be more or less the same color. But as you’ve likely already noticed, no two sunrises (or sunsets) are exactly the same. Pink is one of the most spectacular colors of the sky — it creates cotton-candy clouds and electric sunsets alike. Whether you like to use the colors of the sky to predict the weather or just like to see beautiful and unusual colors, keep an eye out for stunning pink skies!