Previously Asked Questions

Sky/Optical Effects

More information about rainbows and halos

Pictures of various types of rainbows and halos



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I am really wondering way the sky looks blue from Earth but not from space?
Elko, NV


Good question. The blue sky we see from Earth is the result of light interacting with our atmosphere.  Okay, so what is it about light, and what is it about our atmosphere that produces the blue color? Let’s start with a little about the physics of light.  Like sound, light is made up of waves.  When you hear a high note, you’re hearing a sound wave with a short wavelength.  Low notes come from sound waves with long wavelengths.  Light has short and long wavelengths too; but instead of determining notes, the wavelengths of light waves determine colors.  Think about the spectrum of colors that we can see.  At one end of the spectrum are the violets and blues, and at the other end are the oranges and reds.  All the other colors are somewhere in between.  A light wave with a shorter wavelength will produce violet or blue; one with a longer wavelength will produce orange or red.  The color of something is determined by the wavelength of light that we see.   

Now, about our atmosphere: The gases that make up Earth’s atmosphere tend to filter the shorter (blue) waves with a process known as "scattering." In our atmosphere, blue light is scattered more strongly than red light, and so that is the color that becomes more visible to our eyes. And, as you may have already guessed, when you have no atmosphere (like out in space), then you have no blue sky.

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We are 5th graders at Turkey Valley Elementary in Jackson Junction, Iowa. We saw the Northern Lights and were wondering if you could share with us any information you had about them.

Dear 5th Graders, 

The northern lights, or aurora borealis, as it is called in the Northern Hemisphere (aurora australis in the Southern Hemisphere) is a glow of green, yellow, red or even blue light that looks a little like bright curtains waving in the breeze.  You might see little points of light like a million fireflies dancing in the sky. Usually, only people who live very far north or south of the equator see this display, but often people as even in the southern United States can see it. 

An aurora happens when particles from the sun cause the earth’s upper atmosphere to act like a big neon sign.  Let me explain.  In a neon sign, an electric current runs through a wire into a glass tube filled with gas to make different colors of light.  The color depends on the kind of gas  .  The same thing can happen high above the earth.  Electrically charged particles speeding away from the sun (called the “solar wind”) are captured by  earth’s magnetic field and collide with atmospheric gases. Different gases give off different colored light. Since the earth’s magnetic field directs the charged particles toward the north and south poles, the colors are usually most vivid there. 

But sometimes the sun can release big bursts of energy. When these huge concentrations of charged particles reach earth, auroras can be visible much farther away from the poles.  

For more about auroras and lots of great pictures, go to

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Why are some clouds gray and others white?
Pittsburg, PA

This is a good simple question, but as is often the case with simple questions, this one has a complicated answer.
The color of a cloud depends on three things: the sun, how thick the cloud is, and where you are standing.
If the cloud is between you and the sun and the cloud contains a lot of moisture, then less sunlight will shine through it, so it will look darker. You may have seen big tall cumulonimbus clouds that are very dark at the bottom.  They are filled with moisture and can often bring heavy rain.  But even if a cloud is thick, it may actually look white if you watch the cloud with the sun to your back.  The cloud reflects the sun’s light and appears bright white. 

You also may see puffy cumulus clouds with the sun off to the side. Notice that the sides of the cloud facing the sun are bright white, and the sides away from the sun, especially the bottoms, are grayer. 

Usually the opposite is true of high thin cirrus clouds.  They appear bright white when you see them in the direction of the sun since they don’t block much sunlight.  And unlike most other clouds, they aren’t as bright when they’re in the direction opposite of the sun. 

If the sky is overcast with layers of stratus clouds, the sky appears gray for the same reason that bases of puffy clouds do—most sunlight does not make it through the cloud.  Also, a cloud may look white at first, but if a higher cloud moves in between the cloud and the sun, the higher cloud casts a shadow onto the lower cloud.  So the lower cloud may suddenly “change” from white to gray. 

Also, how white a cloud appears may depend on what is in the background.  Clouds against a blue sky may look very white, but if you put the same cloud against a background of a high white overcast, the clouds might look gray.

In any case, watch out for clouds that are very dark and very tall.   These usually can produce a lot of rain in a short time, so when you see them, it would be a good idea to head indoors! 

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My name is Christy and I am curious. I heard someone on the radio talking about seeing something that looked like two suns in the sky. She was told that it is called a "sun dog.”  I was hoping you may be able to explain it.
Thank you,

Dear Christy, 

A “sun dog” is a common name for an optical effect known as a “parhelion.”  It is also sometimes called a “mock sun” because the bright spot in the sky resembles a small sun to the right and/or left of the actual sun.  A parhelion is a type of halo, usually found in pairs, on the edges of a 22 degree halo on either side of the sun.  Parhelia are formed when plate-shaped ice crystals drift down through the sky with their flat faces almost horizontal.  Sunlight refracts or bends as it shines through the ice crystals, which act as tiny prisms, sometimes separating the light into colors.   More often, the parhelia are white as in the photograph below, sent to us by a weather enthusiast.   Notice the sun dog to the right of the sun.   Also notice that to see it or to photograph it, you need to shield your eyes from the sun’s rays with the edge of a building or other object.  Looking into the sun can cause eye damage, even blindness, so please always block the sun’s view when looking upward.  And when you do, you may see other optical effects around the sun or moon, including  circular halos, arcs and coronas.  It’s all the effects of light passing through differently-shaped ice crystals.  For more on sundogs and other optical effects in the sky, go to my Rainbows and Optical Effects pages.


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Why are sunsets red?
Auburn, WA


Although sunsets often display brilliant red and orange colors, not all sunsets are red.  To understand why sunsets are different colors, we first have to know what makes different colors of light.   This gets a little complicated, but stay with me. 

Like sound, light is made up of waves.  When you hear a high note, you’re hearing a sound wave with a short wavelength.  Low notes come from sound waves with long wavelengths.  Light has short and long wavelengths too; but instead of determining notes, the wavelengths of light waves determine colors.  Think about the spectrum of colors that we can see.  At one end of the spectrum are the violets and blues, and at the other end are the oranges and reds.  All the other colors are somewhere in between.  A light wave with a shorter wavelength will be violet or blue; one with a longer wavelength will be orange or red.  The color of the sunset (and everything else) is determined by the wavelength of light that we see.   

When the sun is high in the sky, it looks white.  That’s because all visible wavelengths (or all colors) of light reach our eyes, and all colors add up to white.  As the sun sets, its light travels through the atmosphere at a much lower angle, so the sun shines through the lower atmosphere which contains dust, salt, smoke and pollution.   These particles scatter away some of the shorter wavelengths of light (the violets and blues), leaving only the longer wavelengths (the oranges and reds.)  That’s why many sunsets are orange.  Where the air is cleaner, like in mountain regions, the sunsets will be white or yellow. When the concentration of particles is especially heavy, all the shorter wavelengths of light will be completely scattered away, and the sunsets will be very red.  You often see a red sunset at the seashore because there are so many salt particles in the air over the ocean.  Volcanic ash can scatter most of the blue light away too.  After the eruptions of Mount St. Helens in Washington State in 1980 and Mount Pinatubo in the Philippines in 1994, the ash drifting through the air turned sunsets around the world more red than usual. 

So as you watch your next sunset, you might think about all that goes into making nature’s colorful light show every single time the sun goes down.

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Dear Nick,
I have an inquisitive 10-year old who asks the following: Why does it sometimes look yellow outside after it rains?
Jo Donna
Riverview, FL

Jo Donna,

The sky does sometimes turn yellow or even green when thick storm clouds are present.  The color is not completely understood, but one theory states that clouds will turn a yellow-green if 1) The clouds contain a very high amount of liquid water drops and perhaps hail, and 2) the thunderstorm forms early or late in the day near times of sunrise or more typically sunset. 

Here’s how it works.  Liquid water (as well as ice) is actually slightly blue in color (meaning that it absorbs red light weakly.)  However, this color is so weak that it requires a thickness of tens of feet before it becomes apparent (much larger than the dimensions of a glass of water, for example).  However, scuba divers can vouch for this, as objects seen beneath the surface of the water by more than ten or twenty feet do indeed appear bluish. 

As you know, the sky turns a shade of orange or red at sunset.   But why?  It’s because the distance that sunlight travels through the atmosphere is much longer when the sun is low in the sky.  Because atmospheric particles (dust, salt, smoke, pollution) scatter blue light more than red, the longer path of the sunlight through the atmosphere leaves sunlight depleted of blue and therefore rich in orange and red.  This alone could be enough to account for an orange or yellow sky. When the reddish light from a horizon-hugging sun combines with the slightly blue color of clouds filled with water, the result can be an eerie greenish-yellow light. 


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My son saw an amazing sight: a rainbow ring around the sun, and he asked me if I knew what this was.  Can you help?JoEllen
Williamstown, MA

What your son saw was a halo, a single ring of color around the sun or moon.  Believe it or not, halos actually occur more frequently in nature than rainbows do.  It is formed by sunlight shining through ice crystals between the sun and the viewer.  A 22-degree halo is the most common; formed when light refracts (bends) around the edges of long ice crystals at a 22 degree angle.  Sometimes the halo is white.  Sometimes you can see red and orange in the middle, with yellow and blue at the outer edges.

If a thin cloud's ice crystals are in the right position, you might see arcs just above or below the halo. The arcs form when light refracts inside long pencil-shaped ice crystals. Flat ice crystals can produce an effect high above the halo called a circumzenithal arc -- or an upside-down rainbow. Also when light refracts through flat horizontal ice crystals, you might see bright spots of light along the right and left sides of a halo. These bright spots are commonly called sun dogs; their scientific name is parhelia. 

To see these optical effects, do not look directly into the sun. You can damage your eyes and you won’t see the colors anyway. Instead, block the sun from your view with your hand, a car visor, the edge of a building or other object so you can just see the clouds around it. Sunglasses also may help you see a halo -- but even with sunglasses, you'll need to block the sun from your eyes.


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Hi Nick,

On Dec.7, 2003 at about 7:30pm I saw a white rainbow while riding in the car. It was the full size of a rainbow, clear from end to end, very beautiful. Could you please tell me more?
Lake County, California

Most likely the phenomenon you saw is what is known as a "moonbow." Count yourself fortunate, for the kind you saw is rare.  I saw one a couple of years ago at Cumberland Falls State Park in Kentucky, and it attracted quite a crowd.

Moonbows are like rainbows, except that instead of the sun illuminating the drops of water in the air, the moon does it. The reason a moonbow is white is because of the way our eyes work. Moonlight is much less intense than sunlight, and our vision which perceives such low light levels is primarily rod vision, which is not color sensitive. So we see the bow as white instead of multi-colored.  

Conditions have to be just right to see a moonbow.  Usually the night is clear and the moon is full. To get conditions like this, you usually don't have raindrops in the air, so moonbows are most common when the full moon shines on big waterfalls that send up a lot of spray. The situation you encountered was rarer.  The moon was full that night, and it was evidently behind you, unobstructed by clouds.  The moon illuminated raindrops that were in the air ahead of you, creating the spectacular sight. 

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