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Why Is the Sky Blue? And Other Questions About the Sky

We look at the sky almost every day, but if we stare at it for a little longer than usual, it starts raising a lot of questions. While we generally know the answers to a lot of the immediate queries (Where does the sun rise and set? What is the brightest star in the sky?), many phenomena remain a mystery. Our atmosphere is a big science fair, and once you start digging in, you’ll find demonstrations of color waves, states of matter, and the speed of light. Here’s an overview of some of the most pressing questions about the sky above us.

Why Is the Sky Blue?

A desert highway road.
Credit: John King/ Unsplash

To understand this question, think about the atmosphere as a prism. In a prism, white light refracts through its polished surfaces and separates into the colors of the rainbow. The sun produces white light, so when its light travels through the atmosphere, it refracts a rainbow of colors.

But then why do we mostly see blue? Each color comes from an electromagnetic wave. While red has the longest, slowest wavelength, blue and violet move in quick, short waves. As these colors pass through the atmosphere, they oscillate charged particles in air molecules like oxygen and nitrogen. Blue and violet are scattered in all directions at around 10 times the efficiency of red light, so they get the highest coverage area in our sky. Our eyes are more sensitive to blue than violet, which is why we see the sky as blue.

By contrast, on the moon, the sun just looks like a glowing disc traveling through the dark, night sky. This is because the moon doesn’t have an atmosphere, so there’s nothing to scatter the sun’s light and reveal individual colored wavelengths.

Why Are Sunsets and Sunrises Colorful?

A sunset over the ocean.
Credit: Quino Al/ Unsplash

Sunsets and sunrises are colorful for a similar reason that the sky is blue: It’s about how light scatters in our atmosphere. During the day, the sun is close enough that we see blue in all directions. But as the sun rises and sets, there’s more atmosphere for the light to travel through. This longer journey gives yellow, orange, and red waves, which naturally take longer to scatter through the atmosphere, a chance to shine. This also explains the golden, or magic, hour, when the sun covers everything in a soft, diffused glow shortly after rising and before setting.

Why Does Outer Space Look Black?

Night sky filled with stars.
Credit: Kai Pilger/ Unsplash

Scientists still don’t know for sure why outer space appears black, but there are a few ideas. In scientific circles, many astronomers wrestle with Olbers’ paradox: If the universe is endless and full of infinite stars, why are we not bathed in the glow from this blanket of stars on Earth? Some theorize that light from distant stars doesn’t have time to reach the Earth in a way that’s visible to our eyes because the universe is expanding faster than the speed of light.

There’s also a lot of light in space that we can’t see. Think back to the blue sky and color wavelengths. There are plenty of light waves that are above or below the threshold for what our eyes can see. Slow, long radio waves and quick, short gamma rays aren’t visible to the naked eye. Stars give off all kinds of invisible light, including infrared, ultraviolet, and other colors we can’t see with our eyes alone.

How Do Clouds Form?

A grassy field during the daytime.
Credit: Rodion Kutsaev/ Unsplash

First, a quick recap from your school science class: Water can exist as a solid, liquid, or gas. When ice melts, it turns into a liquid; when water freezes, it turns into a solid; when water evaporates, it turns into a gas that’s held in the air. One of the ways gas turns back into liquid is precipitation, like rain or snow.

Clouds form when the air saturates, meaning the air is holding too much moisture. When this happens, condensation can occur. It’s the same phenomenon that causes the outside of a cup of water to become wet on a hot day, but instead of glass, that moisture binds to tiny particles in the air like dust, ash, and salt. As a result, that moisture becomes visible as clouds or fog.

The air’s capacity for water depends on atmospheric pressure, which changes with temperature, so clouds can also form when temperatures suddenly cool. Rain, snow, and hail happen when the clouds become too heavy and that moisture gives in to gravity.

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