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Weather Optics

"Red sky at night, sailors delight. Red sky at morning, sailors take warning."

This version of the old adage brings to mind the beautiful colors of many sunrises and sunsets that we see. They have been the topic of music, poetry, paintings and photography. The sky is a palette of many colors and many optical mysteries. Most can be explained through science and the study of the atmosphere. As sunlight or moonlight passes

through the atmosphere, the light rays react with particles of dust, water droplets (clouds), rain, or ice crystals. These particles cause the light particles to be reflected, refracted, diffracted and/or scattered; many times light is affected by a combination of these events.

What causes the red skies of many sunsets? Imagine yourself looking at a sunset. You are looking directly across to the horizon - a very long distance through the lower atmosphere, which is filled with particles. When the atmosphere is loaded with particles and the sun is low on the horizon, the longer wavelengths (yellow and red light) are able to penetrate the dense atmosphere, while the other wavelengths of visible light are scattered away. The reds and yellows that are left are the colors we see. The same is true of a red morning sky.

Why is the sky blue? We have all asked this question at some time in our lives either out loud or to ourselves. What answer did you get from the person you asked? The color of the sky is the result of a phenomenon called Rayleigh scattering. White light from the sun is made up of all the colors of visible light. You know! ROY G BIV - red, orange, yellow, green, blue, indigo and violet. The particles in the atmosphere scatter white light into its various colors. The blue part of the solar spectrum is scattered more easily than the other colors. So blue is the predominate color in the light we see in the sky on a sunny day. And the sky looks blue to us.

Mie scattering occurs when the wavelengths of visible light are approximately, equally scattered. Water droplets, in even small clouds, effectively scatter all wavelengths of visible light in all directions, thus making clouds appear white. It also explains why on a humid, summer day, the sky appears a milky white.

Rainbows are by far one of the most spectacular atmospheric optical events. All those colors in perfect layers within a perfect arch! Folklore says there is a pot of gold at the end of the rainbow. A primary rainbow has a single arch and is the most common type of rainbow. Sunlight is refracted when it enters a raindrop, then is reflected back from the other side of the raindrop and is refracted again as it leaves the raindrop. Each time refraction takes place, the many colors of white light are separated into the component light rays of red, orange, yellow, green, blue, indigo and violet. Usually red is on the outside and violet on the inside of the rainbow. You must be standing between the sun and the rain and the sun must be low in the sky (less than a 42° angle) to see a rainbow. We almost always see rainbows in the late afternoon or early evening when we are

facing east and a shower has passed. Occasionally a secondary rainbow can be seen above the primary rainbow. Light that forms this rainbow, reflects twice inside the raindrop and which causes the color sequence to reverse and violet is on the outside and red on the inside. A secondary rainbow is never as bright as a primary rainbow.

Rainbows are rarely seen in the morning except on the west coast where it is possible to stand between the rising sun and storm approaching from the west.

Halos around the sun or the moon are caused by the refraction or bending of light through ice crystals of a cirrus type cloud. They look like bright rings but may have some color. Sundogs are bursts of color on either side of the sun when it is low in the sky near the horizon. In this case, ice crystals floating

toward the Earth, act like prisms refracting the light as it passes through the crystals. These optical phenomena can be seen at any time of the year and can be quite spectacular.

Always remember: Do not look directly at the sun. If you want to observe weather optics, wear protective eyewear. The sun's rays are harmful to your eyes.

Classroom Activity

Analyzing Light Through a Compact Disc Spectroscope

A spectroscope is an optical instrument for separating light into spectral lines and measuring the wavelengths and intensities of the different colors of light that emerge. It is an efficient tool for the study of the dispersion of light.

Some spectroscopes use prisms to separate light into various colors. Others separate light with a diffraction grating; a film that has thousands of microscopic grooves lined up next to each other. A compact disc (CD) also has thousands of microscopic grooves; therefore, you can use it as a simple type of spectroscope to analyze light.

NOTE: It's best to do your light analysis at night so direct sunlight does not impact your analysis. If you want to examine the spectrum of sunlight, you can use the moon as your light source (the moon shines by reflecting sunlight).

Materials Required

• Compact disc (preferably one you don't care too much about)
• Different sources of light:
• Standard light bulb
• Fluorescent bulb
• Candlelight
• Street light
• Neon light
• Moonlight (reflected sunlight)

Instructions :

1. Start by examining the spectrum of a standard light bulb.
• Hold the CD in your right hand, at your waist, and stand so the light source is to your left.
• Tilt the CD back and forth at an angle until you see a rainbow stripe on the disc. (Closing one eye often helps.)
• As you tilt the CD back and forth, you should be able to see the entire spectrum -- red, orange, yellow, green, blue, indigo and violet.
• If you have a dimmer switch for your lamp, observe what happens to the spectrum as you dim and brighten the bulb.

When it's dim, the red end of the spectrum will be more prominent. When it's bright, the blue end of the spectrum will be more visible.

2. Now use your CD to look at the light of a street lamp.
• Again, make sure the lamp is in the distance to your left and tilt the CD in you right hand. Does the spectrum look the same as it did for the standard bulb?
• Continue analyzing different sources of light, including a fluorescent bulb, candlelight, neon light, and moonlight. Note the differences and the similarities in the spectrums.

You'll notice that the spectrum of a streetlamp is not a continuous smear of red-orange-yellow-green-blue-violet light, but rather a line of separate, distinct colors. If the street lamp is a typical mercury vapor lamp, those colors will be: red, orange, green, and blue.


Our sun us not the only sun in the universe. Most of the "stars" we see in the sky are other suns. Others "stars" are the planets in our solar system. A star (sun) in the sky is like the filament of a light bulb. Suns are made up of burning gases. As the gases burn, they give off light similar to a fire. Using a spectrometer, stars can be identified by the color of their light. As light from a star passes through a spectrometer, the spectrum of light from the star is

produced and analyzed. Scientists have found that hotter stars are brighter and bluer! The spectrum of light emitted by a star can provide us with evidence about its temperature.

Weather Wizard

Optic - Of or relating to the eye or vision.
Reflect - To throw or bend back from a surface
Refract - To deflect from a straight path
Diffract - To change in the directions and intensities of light rays after passing by or through an obstacle.
Wavelength - A measurement of light where red has a very short wavelength and violet a very long wavelength.
Spectrum - The visible distribution of energy emitted by a light source, arranged in order of wavelengths.
Spectroscope - An optical instrument for producing spectral lines and measuring their wavelengths and intensities.
Prism - A transparent body, often of glass and usually with triangular ends, used for separating white light passed through it into a spectrum or for reflecting beams of light.