When our normally-blue sky changes color, most people notice. You’ve probably seen a completely or partially orange sky before in the form of a sunrise or sunset.
But have you ever thought about why sunsets have such brilliant colors? Or realized that there might be other reasons why the sky is orange? Let’s take a closer look.
A Quick Look at the Colors of Light
The sun shines white light down on us, but that “white” light is actually made up of the various colors of visible light. You can see these different colors when you refract light with a prism.
Each of the colors has a corresponding wavelength. As the picture above illustrates, red light has the longest wavelength. These wavelengths become progressively shorter as you move through the colors of the rainbow toward violet.
You don’t need to memorize the wavelengths of the different colors. But in order to appreciate the many possible reasons the sky could be turning orange, you’ll need to know which colors of light have longer wavelengths and which colors have shorter wavelengths.
What Makes the Sky Orange? A Few Possible Reasons
Looking up and seeing an orange sky can be interesting. Depending on the circumstances, it can even be alarming. If you’re wondering why the sky is orange, it may be due to one of these possible causes.
Sunlight Is Traveling Further Than Normal
During the day, the sky generally looks blue. When the sun’s rays don’t have to travel especially far (like when the sun is high in the sky), the short wavelengths of blue light hit the various particles in the atmosphere and scatter. This scattering (called Rayleigh scattering) makes that light even more noticeable. This all happens within our field of vision, so the sky appears blue.
When the sun is closer to the horizon (when it rises and sets), sunlight has further to travel. When this happens, the blue light coming from the sun is scattered long before it reaches our eyes. The longer wavelengths of visible light (red, orange, and yellow) reach our level instead.
Depending on the exact distance the sunlight is traveling and the number of clouds, dust, pollen, and other particles in the air, we might see more or less of each particular color. Sometimes, that means that the entire sky takes on a glowing orange hue.
There’s a Wildfire Nearby
Wildfires cause enormous destruction. But they also cause the sky to turn interesting and unusual colors. In many cases, the air above a wildfire appears yellow or orange. Some people might think that’s the light of the fire itself, but it’s actually something much different.
Particles of smoke are larger than most of the other particles floating around in the atmosphere. They interfere with the rays of the sun, making the blue wavelengths of light scatter before we see them.
This is essentially the same process as when the sun rises and sets — when the sunlight travels longer distances, it runs into more interference in the atmosphere, causing only longer wavelengths to reach us. Wildfires cause a large amount of interference over a shorter distance, so the sun doesn’t have to be further away to make the sky look red, orange, or yellow.
There Are More Clouds Than Usual
Clouds are made out of water droplets and/or tiny crystals of ice. Both of these interfere with sunlight, scattering blue light before it reaches us. During the day, interference from clouds is generally not enough to make the whole sky turn orange. But when you add a sunrise or sunset to the mix, the presence of clouds can result in some really spectacular colors!
A Storm Is On Its Way
You may have noticed that before particularly bad storms, the color of the sky seems to change. In some cases, the sky will look orange before a thunderstorm. This is for the same reason the sky can look orange if there are more clouds than usual.
The Air Around You Is Filled With Dust and Pollutants
By now, you’ve seen that when sunlight has to travel through a large number of particles in the atmosphere, it’s more likely to make the sky look orange (or yellow, or red). Sometimes, dust storms cause a sudden influx of dust that makes the whole sky look orange.
A relatively recent example of this happened in 2022 when parts of southern Spain appeared to be blanketed by fiery orange skies. Naturally, citizens were concerned and curious, so several news outlets ran stories explaining the phenomenon.
Storm Celia had drawn dust from the Sahara Desert north. With that many dust particles in the air, blue light scattered long before it reached our field of vision. More orange wavelengths of light got through than any other color, so the sky looked orange for a brief period of time.
Various forms of air pollution will unfortunately cause a similar effect. Human-made aerosols pollute the air in many parts of the world. But as you can imagine, the air tends to be more polluted in more populated areas.
That pollution may not be enough to turn the sky into a permanent shade of orange. But cities with major pollution problems tend to have sunsets with more red and orange. These sunsets might look more spectacular than most. But unfortunately, they aren’t truly natural — they’re a sign that we’re harming the environment.
What If the Moon Is Orange?
During daylight hours, you might sometimes find that the sky is orange. But once the sun has set and the night sky takes over, you may occasionally find that the normally-white moon appears to be a shade of orange! Here are a few possible reasons why:
It’s the Harvest Moon
The first full moon in autumn is generally called the Harvest Moon. And sometimes, it even looks like it’s tinted in rich, autumnal orange. There are a couple of reasons why. At this time of the year, the moon rises at almost the same time as the sunset. The moon doesn’t produce any light — it just reflects the light of the sun.
That means that when the Harvest Moon is on the horizon, it will look orange for the same reason the sky looks orange. The moon reflects light back to us, but because that light is traveling further, only the longer wavelengths of light reach our eyes.
There’s More Dust or Air Pollution Than Normal
If you see an orange moon that’s high in the sky, extra dust (or other particles) in the air may be to blame. Just as these extra particles scatter away blue wavelengths of the sun’s light, they scatter away shorter wavelengths of moonlight (or technically, reflected sunlight) as well. As a result, it’s mostly longer wavelengths of light (like orange light) that reach us.
Orange: Part of the Rainbow of Light
This lively shade is just one of many you may see when you look up at the sky. You might see it as part of a brilliant sunset or as a sign of an impending dust storm.
Orange skies can be beautiful, but outside of sunsets and sunrises, they’re fairly rare. If you happen to see one, take the time to take it in — it’s likely not an experience that will happen again soon!