What color is the sky? Most people would say blue. That’s the case most of the time. But if you’ve ever watched a spectacular sunset, you know that blue is far from the sky’s only color!
Today, we’ll be taking a look at one of the sky’s most spectacular shades: yellow. This brilliant, energetic color is commonly seen in sunsets (and sometimes even before storms). But have you ever wondered what makes the sky yellow?
The Colors of Light: A Primer
In order to understand why the sky looks yellow (or any other color) at times, it’s necessary to have a basic understanding of the different wavelengths of visible light and their corresponding colors.
You may have heard the phrase “white is the presence of all color” before. In the case of white light, this is especially true. If white light is refracted through a prism, you see a rainbow. Each color of that rainbow represents a different wavelength:
As you can see in the illustration above, the colors closest to the “top” of a rainbow (starting with red) have the longest wavelengths. As you move toward violet, the wavelengths get progressively shorter.
When the sun shines down on us, it radiates all the colors of visible light (plus some invisible light like UV rays). But we don’t routinely see rainbows. Instead, the sky looks blue. Why?
The short answer is that it has to do with how light scatters throughout the sky. The Earth’s atmosphere is full of tiny particles and different types of gases. As light hits them, it scatters.
Think of it like spraying a hose. The water usually travels as a unified stream. But if you spray the hose right at a rock or other object, the water sprays in many different directions!
The blue light has some of the shortest wavelengths, so it scatters more than most other wavelengths. As a result, the sky looks blue.
But if you looked closely at the above graphic, you saw that violet light has even shorter wavelengths than blue light. So why isn’t the sky purple instead? The sun emits a lot more blue light than violet light, and our eyes tend to be more sensitive to blue light.
What Makes the Sky Yellow?
Now you see that the color of the sky is dependent on the way light scatters. Under normal circumstances, blue light scatters most easily. But sometimes, other events make it so yellow light is the most visible.
In some cases, you see yellow as one color of many (like in the case of a beautiful sunset). At other times, the sky above might seem to be a single shade of yellow. Here’s a glimpse into the science behind this interesting phenomenon.
A Low Sun in the Sky
When the sun is high in the sky during the day, blue wavelengths of light scatter easily. But at sunrise and sunset, when the sun is lower in the sky, light has to travel further before it reaches us.
That increased distance means there’s a greater opportunity for interference from clouds and other components of the atmosphere. In some cases, only certain wavelengths of light can get through. At sunrise and sunset, it’s common for only the longer wavelengths of light (like red, orange, and yellow) to be visible. Sometimes, one wavelength (in our case, yellow) is far more visible than the others.
Of course, the transitional periods of sunrises and sunsets are relatively short. Yellow skies at these times can be especially magnificent, but they won’t last long!
Have you ever noticed that the sky seems slightly yellow before a storm comes in? The reason is similar to why you sometimes see yellow skies at sunrise and sunset. More storm clouds mean more interference for the sunlight passing through. And as we saw above, when there’s a good bit of interference, you commonly see longer wavelengths of light coming through.
When the sky is filled with storm clouds (and especially when the storm happens in the late afternoon or evening), yellow light tends to be the most visible. Sometimes, these yellow, stormy skies look pretty and golden. But at other times, the sky has an ominous yellow glow.
Dust, Smoke, or Pollen in the Atmosphere
There’s always some level of dust in the atmosphere. But if there’s a very significant amount of dust, the sky might appear yellow. This is usually a darker shade than the bright, golden yellow seen at sunrise and sunset.
You might remember that in 2017, parts of the UK faced an interesting phenomenon: the sun looked reddish, and the sky was a hazy dark yellow. Scientists at the University of Nottingham analyzed the dust particles taken from the sky and found that they had come all the way from the Sahara Desert.
As it turned out, that dust was blown over the UK by Hurricane Ophelia. Some people thought that fires in Portugal and Spain might be partially responsible, but the researchers didn’t find any evidence that that was the case.
Smoke might not have been the cause of the yellow sky in that specific instance, but it can give the sky a yellowish hue. If you’ve ever watched news footage of wildfires, you might have noticed a yellow glow in the sky. That’s because particles of smoke are larger than most other atmospheric particles.
These large particles change the way the atmosphere filters light. The shorter wavelengths (green, blue, indigo, violet) are largely absorbed, so the red, orange, and yellow wavelengths are the main ones scattered down to us. In many cases, the yellow wavelengths are the most common (and the most visible).
A Possible Indicator of Poor Air Quality
As you can see, the presence of a lot of foreign particles in the atmosphere often leads to a yellow sky. So it follows that yellow skies can sometimes be a sign of air pollution. The most common example of this is “smog,” the thick haze that hangs over some large cities.
Smog is a type of ozone that forms when heat and light interact with two main types of particles: (1) nitrogen oxides that form as a result of combustion and (2) volatile compounds in human-made products like gasoline and hairspray. Because heat and light are needed to form it, smog (and the yellow sky it causes) is much more noticeable in the summer months.
So as you might imagine, seeing a yellow sky might be an indicator of poor air quality. If you are generally healthy, it may not pose much of a risk. But for children, pregnant women, elderly people, and anyone with heart or respiratory problems, very poor air quality can cause or exacerbate conditions.
However, breathing polluted air over time is not good for anyone. The misty yellow haze over a city skyline might be pretty, but it’s in our best interest to get pollution under control!
Yellow Skies: One of Nature’s Colorful Wonders
Even if you know nothing about how yellow skies come to be, you can still appreciate their beauty. But when you know the natural processes that need to happen to create that striking golden glow, you gain a new appreciation for the natural world. Whether the sky above you is pure yellow or a magnificent assortment of fiery shades, it’s a sight you won’t soon forget!