Why Is the Sky Red? The Science Behind Red Skies

A fiery red sunset sky over silhouettes of trees

There’s something utterly captivating about a fiery red sunset. Red skies are far from common, but they’re certainly memorable.

But have you ever stopped to wonder what exactly makes the sky turn this beautiful and unusual hue? Here’s a look at why you sometimes see a red sky above.

Red Skies and Scattered Light

Before you really understand why the sky sometimes appears red, you need to understand what gives the sky any color in the first place. And to understand that, you need to know a bit about the wavelengths of visible light:

Diagram of the different colors of visible light and their wavelengths

The light from the sun appears white, but that white light includes all of the colors shown above. Because those colors have different wavelengths, they behave differently once they enter the Earth’s atmosphere.

When shorter wavelengths of light come into contact with the tiny gas molecules that make up the atmosphere, they are “scattered,” meaning those gas molecules radiate them in different directions. The shorter wavelengths of blue light scatter easily. Because the countless gas molecules in the atmosphere radiate blue light in every direction, the sky looks blue.

Once most of these shorter wavelengths have scattered away, that leaves the longer, reddish wavelengths of light. And as we’ll see in a moment, it’s these wavelengths that are responsible for unforgettable red sunrises and sunsets.

What Makes the Sky Red at Sunrise and Sunset?

A yellow sun setting among deep red clouds

During the day, the sun is at the perfect distance from Earth to make the sky appear blue. But at sunrise and sunset, it sits low on the horizon. That means that light has to travel through more of the atmosphere before it gets to us. And of course, the atmosphere is full of particles that scatter light.

When sunlight travels this longer distance, the blue wavelengths of light scatter early on. By the time we can see the light, the blue has been scattered off. That leaves the longer wavelengths of red, yellow, and orange light.

You may have noticed that some sunrises and sunsets are redder than others. This tends to happen when there are more particles in the atmosphere. Smoke and other larger particles are particularly good at scattering red light.

Unfortunately, air pollution from human-made aerosols also scatters red light very effectively. When the air is densely polluted, more of the shorter wavelengths of light scatter completely away before we see them. Yellow and orange light has shorter wavelengths than red light, so sunsets and sunrises in polluted areas often have a higher proportion of red light.

That isn’t to say that every red sunrise or sunset is the result of air pollution. Red skies can happen naturally — air pollution just makes them more likely (and often brightens the color).

Red Sky at Night, Sailors Delight

Red sunset over a quiet lake

You may have heard the familiar saying “Red sky at night, sailors delight; red sky at morning, sailors take warning” before. It’s an old adage that has roots in the Bible. Essentially, it means that seeing a red sunset is a sign of good weather to come, but seeing a red sunrise means that bad weather is coming.

Surprisingly enough, there’s actually some truth to that saying. At least in the middle latitudes, storms tend to travel from west to east. The sun rises in the east, so the light would give oncoming storm clouds a reddish hue — a red sky in the morning.

The sun sets in the west. So if the sunset is red, that may mean that it’s illuminating the departing clouds, indicating that the bad weather is moving away.

The Red Night Sky: A Sign of Changing Times

Glowing red evening sky

Sunrises and sunsets are probably the best-known causes of red skies. But thanks to a recent phenomenon, the night sky above urban areas sometimes glows red, too.

You might already be familiar with “skyglow.” This refers to the way light pollution makes skies above urban areas seem to glow. While studying skyglow, a team of scientists noted that the actual color of the night sky is changing as well. In a report creatively titled “Red Is the New Black,” they explored how cloud cover alters the color of skyglow in urban areas.

You might think that a cloudy night sky would appear darker. That might be the case in areas without much light pollution. But in this study, the researchers found that cloudy skies actually made the skyglow more noticeable.

This makes sense when you think about it. Light pollution in cities comes from a variety of sources — streetlights, outdoor lighting around homes and businesses, lights from shopping malls, billboards, etc. Upward-facing lights like the ones illuminating billboards are the worst offenders.

When the night sky is cloudy, the clouds reflect much of that light back down to Earth. Over the course of the study, the researchers found that those same clouds also make red light much more radiant and noticeable than blue light. Clouds increased the radiance of red light by a factor of 17.6. For the sake of comparison, they only increased the radiance of blue light by a factor of 7.1. That explains why skyglow tends to be more reddish than bluish.

This reddish nighttime glow isn’t as noticeable as a blood-red sky would be. But it’s a sign of our changing world. And in this case, that change isn’t for the better. When the night sky is filled with light, it disrupts the circadian rhythm of wildlife. Sleep-wake cycles are interrupted, and the excess light can cause major issues with migration. Many types of wildlife use the moon as a guide. When moonlight is drowned out by so many other forms of light, these migrating animals can become confused, get lost, and even die.

Excess light can also disrupt the habits of nocturnal animals. For example, during the mating season, some species of frogs croak and make other vocalizations at night. It’s an important part of attracting mates. Light pollution can make nighttime seem like daytime, so these confused frogs may not perform their ritual like they typically do. As a result, the species may suffer a population decline.

What About the Red Moon?

A blood moon suspended in a black sky

On most nights, the moon is a bright shade of silvery white. But every so often, you might see what’s sometimes called a “blood moon.” This is a moon that appears to glow red.

The blood moon happens during a total lunar eclipse — when the Earth is right between the sun and the moon. With this lineup, the moon is completely blocked from sunlight.

So how does it glow at all? The only light that reaches the moon is from the edges of the Earth’s atmosphere. And just like Rayleigh scattering is responsible for red sunrises and sunsets, it also causes the moon to glow red instead of blue, white, or any other color.

As you already know, the gas molecules in the Earth’s atmosphere scatter most of the blue light from the sun — that’s what makes the sky blue. That leaves the longest wavelengths, which are primarily red (with maybe a little orange mixed in). When that longer-wavelength light reaches the normally-white moon, it makes it turn a striking, glowing red.

A true blood moon comes as the result of a lunar eclipse. However, some people may mistakenly describe any moon that appears red as a blood moon. If the sky looks reddish as a result of excess dust or pollution in the atmosphere, the moon will usually have a red tint as well.

Will Future Skies Look Even Redder?

Tall grass silhouetted against a blood red sky

Even without air pollution, we would still experience red skies — at least to an extent. But as you’ve seen, excess pollution in the air can make sunrises and sunsets have a brighter red glow. It can even make the sky appear red at night or during the day.

As the atmosphere continues to get more polluted, will the sky continue to turn red? That’s a possibility. At the very least, we may start to see sunrises and sunsets that look even brighter. Red skies might be beautiful, but they’re also an indicator that if we want to keep our air clean and healthy, something needs to change.

The Colors of the Sky