Why is the sky blue?
Introduction
Hi there! Ever looked up on a seemingly clear day and pondered why the sky is blue? That’s one of those questions that seem like they’d have an easy answer, but the explanation is actually pretty neat. Let’s crack open the world of light and atmosphere and see what’s inside.
The Basics of Light
What is Light?
Light is a form of energy. It moves in waves. The energy in those waves is spread out evenly along their paths. And because there must be something to spread it out over, the waves are actually made of something else as well, though very small. The smallest measure of these energy packets is known as its wavelength. The waves that make up what we call light have a variety of lengths. Among all these lengths, the ones our eyes can detect make up a little sliver of the whole electromagnetic spectrum that also includes radio waves, microwaves, infrared, ultraviolet, X-rays and gamma rays.
How Light Travels
At 299,792 km/sec (that’s 186,282 mps, or miles per second!) light is travelling fast. It hits things, it reflects off them, it is absorbed, and it is scattered. Scattering is the key to what makes the sky blue.
The Science Behind Blue Skies
Rayleigh Scattering
Definition of Rayleigh Scattering
Called Rayleigh scattering after the British scientist Lord Rayleigh, it is the scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of light.
How Rayleigh Scattering Works
When sunlight travels through the atmosphere of Earth — hitting molecules of nitrogen and oxygen along the way — the light scatters in all directions, with the shorter-wavelength blue light scattered more than other colours. It’s this scattered blue light that we see when we look up during the day.
Other Colors in the Sky
Why is the Sky Red at Sunrise and Sunset?
Image courtesy of ISAS/JAXAToday, the Sun sits low on the horizon at sunrise and sunset. When the Sun is higher in the sky, more of its light passes straight through, whereas when it’s lower, its rays have to ascend to the top of the atmosphere and plummet downward back toward you. On the ascent upward, it has light waves from the sky to contend with. Most of the blue and green light gets scattered out of your line of sight, allowing the reds and oranges to paint the rest.
Why Isn’t the Sky Purple?
Considering that violet light is scattered more than blue light, you might wonder why we don’t see a purple sky. For two reasons. First, our eyes are more sensitive to blue light than to violet light and, second, most of the violet light is absorbed by the upper atmosphere.
Common Misconceptions
Myth: The Sky is Blue Because of the Ocean
It’s a widespread myth that the sky is blue because of the ocean being blue. The ocean reflects the colour of the sky, it’s true, but the sky is not blue because of the ocean: it’s all about the scattering of sunlight.
Myth: The Sky is Blue Because of Atmospheric Gases
Another misconception is that it’s the gases in the atmosphere that make the sky blue. The blue that we see is indeed the result of light scattering off those gases, but it’s the scattering process, not the composition of the gases – all the gases we have around us scatter light in a similar fashion.
Interesting Facts About the Sky
Why is the Sky Different on Other Planets?
provided it has an atmosphere like ours, the same phenomenon that colours our own sky will be in action elsewhere. Different gases and particles scatter light to different degrees, giving skies on the various worlds of this vast universe their distinct hues.
The Sky on Mars
(Mars has a thin atmosphere of almost pure carbon dioxide.) The butterscotch sky is a deep effect caused by particulates in the Martian atmosphere, which make the dust ‘Rayleigh scatter’ sunlight differently than short wavelength light does on Earth.
The Sky on Jupiter
Jupiter’s clouds are dominated by hydrogen and helium, and its sky is blue at high altitudes. At lower altitudes, the sky is deep red, caused by ammonia crystals.
Conclusion
Thus, the next time you look towards the overhead blue sky and marvel at its beauty, it’s Rayleigh scattering that you’re admiring – the way light engages with the atmospheric molecules to turn the sky blue. And what’s more, the shade of blue can vary throughout the course of the day, and even from one planet to the next.
FAQs
Why is the sky blue only during the day?
The sky is blue during the day because light from the Sun is scattered by the atmosphere. At night, when there is no light, the sky is dark.
Can the sky be other colors?
Yes: the sky can, at times, be a red-orange, pink, or green, for instance, at sunrise, at sunset, or under the view of an aurora.
Do other planets have blue skies?
Some have blue-sky planets, such as Earth’s. But the colour of sky depends on atmosphere. The Martian sky is butterscotch.
Why is the sky sometimes gray?
It can be gray because there are deep, grey clouds blocking sunlight, then the light bounces in angles evenly but less in number as opposed to clear and bright sky. Essentially, light is redirected in evenly distributed angles throughout the visual space giving the illusion of an opaque, grey shade.
Is the sky always blue?
Not necessarily. The sky can appear blue for numerous reasons, including the time of day, the weather, and the variety of particles in the atmosphere. During sunny conditions, the blue hue is caused by Rayleigh scattering.
For a complete explanation of what makes the sky blue (and why it is blue rather than green or red), this article is the way to go, written in an approachable and friendly way that’s sprinkled throughout with interesting tidbits of information.
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