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Ozone is a gas, a form of oxygen, that is found in the layers of the atmosphere, most predominantly in the stratosphere . Here, 90% of the atmosphere's ozone is distributed in a ratio of five ozone molecules to every million molecules of other gases. This minute distribution serves as a shield that helps screen the sun's rays by absorbing some of the ultraviolet(UV-B)radiation . Depletion of the ozone in the atmosphere can have severe consequences on earth. Plants, animals, and humans all suffer when exposed to higher levels of ultraviolet rays. Food crops have stunted growth; marine phytoplankton can die off; and humans are more vulnerable to skin cancer. Atmospheric research in the mid-1980s indicated a serious thinning of the ozone shield, upsetting a natural balance between oxygen and ozone in the stratosphere. This thinning was evident from satellite pictures and showed up as a dark area; thus the term "ozone hole" was coined. It was apparent to scientists studying the ozone depletion that chemicals called chloroflourocarbons (CFCs) , used in spray cans, refrigerators and air conditioners, foam, plastics, and cleaning solvents, might be contributing to the problem. After being released, either during the manufacturing process or from consumer use, CFCs reach the stratosphere. There, chemical reactions break apart the CFCs. The chlorine then breaks down the ozone. A single chlorine atom can destroy 100 thousand molecules of ozone. The degree of ozone depletion has followed an annual cycle that corresponds to the amount of sunlight that reaches the Antarctic. The cycle begins every year around June when the vortex winds develop in the Antarctic. Cold temperatures produced by these winds create polar stratospheric clouds that capture the floating CFCs. For the next two months, a reaction occurs on the cloud surface that frees the chlorine in the CFCs but keeps the chlorine contained within the vortex area. In September, sunlight returns to the Antarctic and triggers a chemical reaction, causing chlorine to convert ozone to normal oxygen. Measured ozone levels usually are lowest in October. November brings a breakdown in the vortex that allows the ozone-rich air to combine with the thinning ozone. Wind currents carry this mixture over the southern hemisphere and carry the "hole" over other areas of the earth.


Analyze and interpret the same ozone TOMS images as those used by NASA. Before information about ozone is released to the general public, scientists spend many hours analyzing ozone data from earth and space. Here's your chance to be an ozone-research analyst. Materials
  • NIMBUS-7 TOMS Images: The Twelve Octobers. Order one for each two students by lithograph name and number (HqL-308) from: NASA Goddard Space Flight Center Greenbelt, MD 20771 (301) 286-8955
  • paper
  • pencil
Note: Instructor may wish to make photocopies of the TOMS lithographs so that the information on the back is not available until students have interpreted the data.
  1. Divide students into research-analyst pairs.
  2. Discuss why the Dobson Unit key will be important and note the range of the value for each color. (If paper copies have been made, allow time for the students to color in the key.)
  3. Use the questions below to analyze this data and to formulate predictions.


    Aeronomy Laboratory/NOAA
    325 Broadway
    Boulder, CO 80303
    (303) 497-5785
    Climate Protection Institute
    5833 Balmoral Drive
    Oakland, CA 94619
    Education Division
    Mail Code F
    Washington, DC 20546
    (Atlas 1 Teacher's Guide: Earth's Mysterious Atmosphere)
    Larain County, JVS
    15181 Route 58 S
    Oberlin, OH 44074
    (216) 774-1051 Ext. 293
    (videotapes from shuttle missions during which Blue Planet was filmed)
    National Air and Space Museum
    Office of Education
    Washington, DC 20560
    (202) 357-3133
    (Blue Planet educational booklet)
    PBS Environmental Resource Compendium
    1320 Braddock Place
    Alexandria, VA 22314
    (703) 739-5402