On a clear day, the sky is a bright and energetic blue. But at sunrise, sunset, and even sometimes during storms, it may look as though it’s been painted with shades of red, orange, yellow, pink, and even purple.
Purple skies aren’t commonplace, but they’re certainly memorable. And when you know what causes them, they become even more amazing!
What Gives the Sky Its Color?
If you’ve stepped outside to see a purple sky (or even if you’ve just seen a photograph of one posted on social media), you might wonder what exactly creates this near-magical hue. But in order to understand what’s turning the sky purple, you first need to know why the sky has any color at all. After all, air is colorless — so why does the sky (usually) look bright blue?
In short, the sky gets its coloring thanks to a phenomenon called Rayleigh scattering. This means that when wavelengths of light come into contact with particles — even tiny, microscopic ones — in the atmosphere, they scatter in different directions. This scattering effectively spreads the light. If the light has a visible color, that color then becomes more noticeable.
As the image above illustrates, the white light of the sun is actually made up of several different colors, each with its own wavelength. Shorter wavelengths are higher-energy forms of light that scatter more easily. Blue light has a short wavelength, so it scatters easily, making the sky look blue.
So why aren’t sunsets blue? That has to do with the distance the sun’s rays need to travel through the atmosphere to reach us. Shorter wavelengths scatter first, and the sun is just the right distance from us to make that happen within our field of vision (at least during the day).
But when the sun is rising or setting, it’s low on the horizon. That means that sunlight has to travel further to reach us. That further distance means that the light runs into more particles, causing greater scattering.
When the sunlight goes on this comparatively long journey, the blue light scatters long before it reaches our eyes. Recall that longer wavelengths take longer to scatter. So when the light reaches us, it’s the longer wavelengths — like red, orange, and yellow — that are most visible. That’s why sunrises and sunsets often have vivid, fiery colors.
Why Isn’t the Sky Purple on a Regular Basis?
As you saw above, the sky appears blue because blue wavelengths of light are short and easily scattered. But as the diagram illustrates, violet wavelengths are shorter than blue wavelengths of light. So by that logic, shouldn’t the sky actually be purple all the time?
It might seem that way, but the sky looks blue and not purple for two key reasons: (1) human physiology and (2) the actual composition of the atmosphere.
Humans see color thanks to complex receptor cells called cones. There are three types: one for longer wavelengths of light, one for medium wavelengths, and one for short wavelengths (including blue and violet).
Research into cone cells in the eyes of deceased people has yielded a surprising discovery: short-wavelength cone cells respond the same way to a mixture of blue and violet light as they do to a mixture of blue and white light. In other words, our eyes have evolved to be far more sensitive to blue light than violet light.
Notably, the sun also produces far more blue light than violet light. So even though the shorter violet wavelengths might scatter more effectively, there aren’t enough of them to make a significant difference in the way we perceive the sky.
What Makes the Sky Purple?
You’ve seen the sky turn many colors. But as colors of the sky go, purple is not too common. So if you do see a purple sky, you’re probably wondering why that happens.
Here are some of the rare circumstances that can cause an unforgettable purple sky.
Pink Clouds and Blue Skies
Sunsets are often red, orange, or yellow. When the sun is lower in the sky, its rays need to travel through more of the atmosphere. Blue and violet wavelengths scatter before we see them, so only the longer wavelengths are present.
However, you’ve probably seen at least a handful of purple (or at least purplish) sunsets in your lifetime. You know that any violet light scatters along with blue light, so it’s not actual purple light creating this cool shade.
According to some experts, the purple color at sunset is something like an optical illusion. Sometimes, the longer wavelengths of red light become diluted and appear pink. And when the sun is very low in the sky, that pink light will sometimes appear to light up the base of a cloud. When you put those pink-lit clouds in front of a darkening blue sky, the result is a striking purple color.
Volcanic Eruptions
When you imagine a volcanic eruption, you probably picture hot lava flowing down the sides of a mountain. That’s certainly some of it. But when they erupt, volcanoes also release different types of aerosols. One of the most prevalent is sulfur dioxide.
Like any other particle in the Earth’s atmosphere, these aerosol particles scatter sunlight. Volcanic aerosols tend to amplify the scattering of blue light, making the sky look even bluer.
One of the best examples of this effect was the 2019 eruption of the Raikoke volcano on the Kuril Islands. This eruption was closely followed by that of the Ulawun volcano in New Guinea. With both volcanoes blasting massive amounts of sulfur dioxide and other aerosols into the air, the scattering effect was quite pronounced.
Of course, the sky generally looks blue anyway. So by itself, this phenomenon might have gone unnoticed. But for months after these volcanic eruptions, some areas of the world saw strange purple sunsets. As the sun was setting, red light was making its way to us. However, before it reached human eyes, it had to shine through the blue light scattered by the volcanic aerosols, producing a purple hue.
Some Types of Stormy Weather
Sometimes, the sky appears to turn a vivid shade of purple after a hurricane or typhoon. Occasionally, this phenomenon happens before a storm as well. A great recent example is the purple sky after Hurricane Delta — it was so striking that it made the news!
So what makes post-stormy skies turn a beautiful shade of violet instead of the typical ominous gray? Several conditions have to line up. After a storm, the large amounts of water vapor and different types of aerosols in the air can make it so only reddish wavelengths of light reach us.
If the sun is positioned just right in the sky, it may make it look like the entire sky is bathed in a pinkish glow. And if you put this pinkish glow right in front of a dark blue sky, it creates the dramatic purple skies you just might see posted all over social media.
Experience the Wonder of Purple Skies
It’s easy to become so accustomed to the sky that we don’t notice its everyday beauty. And while reds, pinks, yellows, and oranges are truly striking, the rarity of purple skies makes them truly spectacular. If you do happen to see one, consider yourself lucky!
