What color is the sky? Even on gray or cloudy days, most people would still say blue. That might be the case most of the time, but the daytime sky doesn’t always look exactly the same.
Sometimes, you might wander outside or look out the window only to see something surprising — a sky that appears to be a silvery or milky shade of white. This phenomenon is more common than you might think. So what causes those white skies?
What Makes the Sky White?
There’s no singular reason the sky above appears white. Here are some of the most common causes of a white daytime sky.
Clouds or Fog
Sometimes, there’s a simple explanation for the whitish color of the sky — the clouds and/or fog are thick enough to obscure some of the sunlight (and the blue sky).
There are many different types of clouds, but stratus clouds commonly make the whole sky appear white. They often cover it like a veil or a sheet:
Stratus clouds are sometimes so uniform that they’re easily mistaken for fog!
Larger Water Droplets in Summer
Tiny particles in the atmosphere (like gas molecules) that are smaller than a wavelength of light tend to be best for scattering blue light throughout the sky. This phenomenon, called Rayleigh scattering, is responsible for creating the blue sky we know and love.
In Rayleigh scattering, tiny gas molecules absorb the sun’s light and radiate it outward in different directions. However, they don’t absorb and radiate all wavelengths equally. They show a preference for shorter-wavelength light that appears blue. When that blue light is radiated outward, countless other gas molecules capture and radiate it as well. This scatters blue light across the sky, making the sky itself appear blue to us.
However, different particles react differently with the sun’s light. On humid summer days, the air is filled with more water droplets than normal. These larger water droplets can absorb and radiate light just like gas molecules do. But unlike gas molecules, they absorb and radiate all wavelengths of light equally well. This is the same process that makes clouds appear white.
Unless the light from the sun is split into different wavelengths, it looks white to our eyes. So when the water droplets scatter sunlight through the atmosphere, it gives the sky a milky, whitish look.
Low Sun and Ice Crystals in Winter
The sky can appear white in any season, and white skies are also common in winter, even when the sky isn’t particularly cloudy. There are a few different reasons for this.
One is that the sun tends to sit lower on the horizon in the winter than in summer. This causes sunlight to be scattered more effectively in the upper atmosphere. As in the summertime, the blue wavelengths of light reach us. But assuming minimal interference from the rest of the atmosphere, the rest or most of the sun’s light also makes its way to us, making the sky appear white.
Just like water droplets in summer may make the sky look white, airborne ice crystals in winter can do the same. Sometimes, the winter sky is filled with thin, wispy clouds. These clouds aren’t thick enough to actually block us from seeing the blue sky. But they’re made with small crystals of ice, and like droplets of water, these crystals scatter all wavelengths of sunlight effectively. When this white light is scattered, the sky will look more white than blue.
There’s one last reason winter skies often look white. When the ground is covered in a blanket of snow, its surface reflects the white sunlight upward, creating a whitish glow.
Will We See Engineered White Skies in the Future?
As you can see, there are plenty of all-natural reasons the sky looks white sometimes. But in the future, it’s possible that humans may try to deliberately turn the sky white.
This might sound like something out of a science fiction plot, but it’s something the geoengineering community has discussed for years. If we want to stop global warming, we’ll need to drastically limit the greenhouse gases — like chlorofluorocarbons, methane, and nitrous oxide — we pump into the atmosphere.
These gases amplify the “greenhouse effect.” This is when the Earth’s atmosphere traps heat from the sun’s radiation, warming the planet. To some extent, the greenhouse effect is essential to life on Earth. But thanks to human-created emissions, the temperature of the planet continues to rise.
Clearly, reducing greenhouse gas emissions is the best way to combat global warming at the root. However, some scientists have proposed a backup plan that involves adding large amounts of sulfur dioxide to the atmosphere. These sulfur dioxide molecules would then reflect a portion of sunlight back into space, reducing the amount of heat that reaches the planet in the first place.
But like most other atmospheric particles, sulfur dioxide molecules don’t just reflect light in one direction. They also scatter it every which way. If we were to add sulfur dioxide to the atmosphere, the scattered white light would make the sky appear white, too.
What would happen if our skies turned a permanent shade of white? It might cool the planet slightly, but there’s a distinct possibility of other adverse effects. We don’t know how sun-blocking technology like this (euphemistically described as “solar radiation modification” or SRM) would impact plant growth and animal life. It also would likely have adverse psychological effects on humans.
Of course, if anything like this were to materialize, more extensive research would need to be done. The United Nations Environmental Program recently conducted a comprehensive review of this potential strategy and found that it wasn’t a viable or safe plan.
Andrea Hinwood, the Program’s Chief Scientist, noted that “the review concludes that SRM cannot replace reducing greenhouse gas emissions.” In short, experts may one day consider using SRM as an adjunct method to fight climate change, but if we want to address the actual problem at hand, we’ll need to reduce the amount of greenhouse gases we release into the atmosphere. For now, our blue skies remain unchanged.
White Skies: Part of the Rainbow of the World
You might think that bright blue skies are far more beautiful than the often-strange, washed-out look of white winter or summer skies. But it’s precisely this kind of variety that makes our world so magical. Whether you love the look of white skies or can’t wait to get back to blue, our planet’s dynamic atmosphere (and the fascinating science behind it) is certainly something to appreciate!