We have been blessed with a sky of many colors. Often, it’s a vivid and energetic blue. As the sun rises and sets, you can see fantastic bursts of red, orange, yellow, pink, and purple. Sometimes, the sky is even green.
Yes, you read that correctly! Green skies are certainly not common, and it takes a special set of circumstances to create them. And if you are fortunate enough to witness a green sky in real-time, you’ll find that the intriguing color won’t last long. If you’re wondering what makes the sky green, here’s an in-depth look.
The Sky and the Scattering of Light
To understand how the sky can appear to be any color, you first need to be familiar with something called Rayleigh scattering. This is a phenomenon discovered by British physicist Lord Rayleigh in 1871. With Rayleigh scattering, very small particles can scatter light. This will happen even if the particle is much smaller than the wavelength of the light.
You probably already know that the “white” light we see coming from the sun is actually made up of every color in the rainbow. And as you can see in the diagram above, different colors have different wavelengths.
Generally speaking, high-energy light with shorter wavelengths scatters a lot more easily than light with longer wavelengths. Blue light scatters as it hits the countless tiny molecules in the Earth’s atmosphere. Scattering the light spreads it throughout the atmosphere, making the sky look blue.
If you looked closely at the diagram, you may have seen that violet light has a shorter wavelength than blue light. But because our eyes are less sensitive to purple and the sun produces more blue light than violet light, the sky looks blue.
However, during sunrises and sunsets, wildfires, dust storms, and other events, the sky sometimes looks red, orange, yellow, or pink. Rayleigh scattering is behind that, too. When the sun rises and sets, it’s low on the horizon. That means light needs to travel through more of the Earth’s atmosphere to get to us.
Recall that shorter wavelengths of light scatter most quickly. Normally, that scattering happens within our field of vision. But when light has to get through more of the atmosphere (and therefore, more atmospheric particles), scattering happens faster. As a result, all (or most of) the blue light is scattered away before it gets to us. The colors with longer wavelengths — red, orange, yellow, and pink — remain, coloring the sky.
The same result happens with dust storms and human-made air pollution. When there are more particles in the atmosphere, blue light scatters faster. That means most of the light that reaches us has longer wavelengths, so the sky will look red, orange, yellow, etc.
The Perfect Storm: Creating the Green Sky
So now you know how the scattering of the sun’s rays makes the sky look blue (and sometimes other colors). But in most cases, the storm clouds that indicate a thunderstorm or tornado is coming aren’t blue — they’re gray.
However, very heavy clouds (like the ones that produce hail) sometimes have a bluish tint. The large volume of moisture and/or ice present will scatter blue light to create this color.
Of course, this is only part of the puzzle. The clouds are blue, but what gives the sky a greenish cast? A 1993 study found that if one of these bluish storm clouds appeared near sunrise or sunset when the sun’s rays appeared red, orange, and yellow, it usually looked green. The longer wavelengths of sunlight crossed the blue of the cloud, creating a greenish look.
While the authors of the study agreed that both of these factors needed to be present, they disagreed on the exact lineup. One believed that the sun’s light needed to be ahead of the storm. The other believed the sunlight needed to be behind it.
Think of it like this: you have two clear glasses filled with water. You put blue food coloring in one and yellow food coloring in the other. If you put one glass right behind the other and look through both at once, the water will look green.
Essentially, to create a green sky, we need to have (1) heavy storm clouds and (2) red, orange, and/or yellow sunlight line up perfectly. The best chance of this happening is before (and sometimes during) heavy storms that happen near sunrise and sunset.
As you can see, green skies tend to form when a storm is headed your way. And plenty of people will tell you that a green sky means a tornado is coming. But can these strangely-colored skies actually predict storms?
A green sky is a sure sign of bad weather to come. However, although research into green skies is limited, researchers haven’t found a correlation between green skies and tornados or hail.
So in short, a green sky generally predicts stormy weather (though not necessarily a tornado). But very few tornados start with a green sky. In fact, part of the reason there isn’t much research into green skies is because they’re so rare.
What Should You Do If You See a Green Sky?
There isn’t necessarily a correlation between green skies and tornados. However, a green sky does signal that some type of severe storm is coming. It might be a tornado, a hailstorm, or even an unusually bad thunderstorm.
Even severe thunderstorms can damage property, cause severe injuries, and sometimes even cause death. So if you do see a green sky, it’s a good idea to take cover. Once you’re in a safe place, check your local weather station to see if there’s any more information on the storm.
Aurora Borealis: Another Type of Green Sky
In most parts of the world, green skies are a cause for concern. But in some areas around the North Pole, there’s an especially beautiful type of green sky — the aurora borealis, or northern lights. There’s a similar phenomenon around the South Pole, but virtually nobody chooses to go to Antarctica in the winter (or ever, really).
The aurora borealis looks like a stretch of vast, colorful waves in the sky. These waves can be green, red, blue, or purple. Thousands make the trek to see them each year, but not everyone knows what causes this all-natural light show.
The northern lights happen during a specific type of solar storm called a coronal mass ejection. This is when the sun suddenly sends out a massive bubble of electrified gases. Sometimes, those gases head straight toward Earth at high speeds.
The Earth’s magnetic field is strong enough to keep out many of the gases and other particles that come from the sun. But these storms are so strong that they follow the lines of the magnetic field to enter the atmosphere at the North and South Pole.
Once they’ve arrived, they start to interact with different molecules in the atmosphere. Their interactions with oxygen create red and green light. When they interact with nitrogen, they create a purple or blue glow.
Unlike the green skies that happen before storms, green lights in the aurora borealis appear very frequently. The northern lights happen almost every day, but the conditions need to be right to see them. Most importantly, it needs to be dark enough outside.
Green Skies: Beautiful But (Often) Threatening
Unless you’re looking at the northern lights, green is one of the scariest colors the sky can be. But if you can admire a green sky from afar (or in pictures), you’ll see that this surreal phenomenon is strikingly beautiful. It’s just one of the many ways the Earth continues to amaze us!