Why Is the Sky Blue?Move the Sun, change the air, and watch the sky follow.
The Sun is high.Blue light scatters across the whole sky.
The sky is worked out from sunlight scattered once by air, using the standard figures for Earth's atmosphere, with a small allowance for light scattered more than once. Real skies add haze, dust and clouds. The hills are for scale.
The sky is blue because air scatters blue light far more than red. Blue light at 450 nanometers scatters about 5.9 times as strongly as red at 700 nanometers. At sunset, sunlight crosses about 38 times as much air, so most of the blue has scattered away by the time it reaches you, and what remains is orange and red.
Try this.
- Press Play a sunset. When does the blue begin to go? What color lasts longest?
- Choose No air. Why is the sky black with the Sun still up?
- Choose All colors scatter alike. What does the sky turn? What in the real sky looks like this?
- Set the Sun to Dusk and choose Face away. Find the dark band low in the sky and the glow above it.
- Choose Three times the air and move the Sun down. How would a sunset look on a planet with a thicker atmosphere?
Will tonight's sunset be worth seeing?
Clear air and high clouds make the best sunsets. Sol reads the forecast and tells you hours ahead, in one of four words, for the place you choose.
What's happening.
Why is the sky blue?
Sunlight is a mix of every color. As it crosses the air, gas molecules scatter short wavelengths far more than long ones, by the fourth power of the wavelength. Blue is scattered about 5.9 times as much as red, so blue reaches your eye from every direction, and the whole sky looks blue.
Why are sunsets red and orange?
When the Sun is low, its light crosses much more air: about 38 times as much at the horizon as from overhead. Almost all the blue scatters out of the beam on the way, and what comes through is orange and red. That same blue is lighting someone else's midday sky.
Why isn't the sky violet?
Violet scatters even more than blue. But the Sun gives off less violet, the upper air absorbs some of it, and our eyes are much less sensitive to it. The mix we see reads as blue.
Why are clouds white?
Cloud droplets are far larger than air molecules, and they scatter all colors about equally. All the colors together look white. Choose All colors scatter alike in the simulator, and the sky turns the same white.
Why is the sky a deeper blue overhead than near the horizon?
Looking up, you see through the least air. Near the horizon you look through much more, and the blue has been scattered many times, which washes it out to a paler color.
What is the band of dark blue opposite the sunset?
That is Earth's own shadow rising in the air, and the pink glow above it is the Belt of Venus. Choose Face away at dusk to see them. On a clear evening you can see the real thing in the east, just after sunset.
Common mix-ups.
It's the other way round. The ocean looks blue partly because it reflects the sky, and partly because water absorbs red light. The sky is blue over deserts too.
Clean air alone makes a sunset orange and red. Dust and smoke can deepen the color, but the best sunsets need clean air and high clouds to catch the light.
Sunlight fills space. Without air to scatter it toward your eye, the light travels straight past, and the sky looks black, as it does on the Moon.
From space the Sun is white. It looks warmer from the ground because air has scattered some of its blue.
For teachers.
Grades 4 to 9. 15 to 25 minutes. Students need no account, and the page collects nothing about them.
Standards
- MS-PS4-2
- Model how waves are reflected, absorbed or transmitted through materials
- 4-PS4-2
- Model how light reaching the eye lets objects be seen
Discussion questions, with answers
- How much more does blue scatter than red, and why?
- About 5.9 times, for 450 and 700 nanometers: scattering goes with one over the wavelength to the fourth power, and 700 divided by 450, to the fourth, is 5.9.
- What color would the sky be on a planet with no atmosphere?
- Black, as on the Moon.
- Why does the Moon turn red during a lunar eclipse?
- The only sunlight reaching it has passed through Earth's air at the edge of the planet, which has taken out the blue, as at sunset.
- Why is the Sun bright white when high and orange when low?
- Sunlight crosses little air when the Sun is high, and little of its blue is lost. Low on the horizon, it crosses about 38 times as much.
Embed it in your class page
Paste this into Google Classroom, Canvas, Schoology or your own site.
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<p><a href="https://initial.app/why-is-the-sky-blue">Why Is the Sky Blue?</a> from Initial</p>Keep exploring.
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