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Blue Sky

 

Overview

Why is the sky blue? People have been asking this question for centuries. The generally accepted scientific explanation for blue sky was first proposed by Lord Rayleigh, a British physicist and mathematician of the late 1800's. Rayleigh's theory was unique. He agreed with other scientists of the time that dust and other large particles in the atmosphere could scatter light, and that when this occurred, the spectral colors red and blue were revealed. Rayleigh also agreed that no light was absorbed by large particles, but he took this concept of scattering in the atmosphere one step further. Rayleigh concluded that as light traveled from the Sun to an observer, it encountered molecules, mostly of nitrogen and oxygen, in the atmosphere. Rayleigh then calculated a mathematical formula that demonstrated that, even in an atmosphere without smoke and dust, gas molecules like oxygen could redirect sunlight and scatter it in many directions. Sunlight is a form of visible light that contains all of the colors. When it is scattered, it is perceived by the human eye as having a specific color. When sunlight encounters gas molecules in the atmosphere, high frequency blue light is scattered out first. The most intense blues are usually seen between 10:00 a.m. and 3:00 p.m. on cloudless days, if we look at the sky about 45 degrees above the horizon with our with our backs to the Sun. As the Sun begins to set, the sky at the horizon often appears to be red. Sunlight entering at the horizon level travels through more atmosphere than sunlight entering overhead. Most of the shorter wavelength light has been scattered out, allowing longer wavelengths of light to reach our eyes. When particles of dust provide additional opportunities for scattering, sunsets have a red glow. The brilliance is often enhanced in the sky by clouds.

Activity

By keeping daily records of weather conditions, you will be able to study how particles in the atmosphere scatter light and change the color of the sky. Materials:
  • Log book
  • Weather news or reports
1. Begin a two to four week log to record your daily observations of the sky where you live. 2. Choose two times per day that you will record your observations in the log. Always look at the sky with the sun behind you, and choose two areas of the sky to monitor. Your observation points should be at a 45 degree angle above the horizon. Never look directly into the sun! 3. Decide what words you will use to describe the different hues you may see. Pick a number to correspond to the intensity of the various hues. Include descriptions of the clouds and their brightness. 4. Write a brief description of what you see at both observation points each time you observe the sky. Include your notes about the weather at the bottom of each day's entry.

Resources

  • Asimov, Isaac. How Did We Find Out About Sunshine?. New York: Walker and Company, 1987.
  • Bohren, Craig F. Clouds in a Glass of Beer: Simple Experiments in Atmospheric Physics. John Wiley and Sons, 1987.
  • Gallant, Roy A. Rainbows, Mirages and Sundogs: The Sky as a Source. New York: MacMillian, 1987.