Measurements of the amount of ozone in the upper atmosphere have been made for over 15 years. In this activity you will learn how to set the scale for an image and use that scale to measure the length and area of different particles in an image. The images used for this activity come from NASA's TOMS (Total Ozone Mapping Spectrometer) satellite that was active from 1979 until 1992. Each of the images is a composite of the amount of ozone above Antarctica in October, when ozone is at a seasonal low.

Images for this activity are available at the SeaWIFS site at the Goddard Space Flight Center. Images from the last two years are also available from NOAA.

The colorbar allows you to determine what each color represents in an image. In this instance, purple colors represent the lowest levels of ozone recorded. This activity will measure the area of greatest ozone depletion. Once the area has been determined, students can plot the change over the course of several years.

  1. The Line Selection Tool is used to set the scale. Using an atlas, determine the distance between two obvious points on the image. Point the cursor at one point, click and hold the mouse button down. Move to the second point and release the mouse button. Choose Analyze/Set Scale, set the units, and enter the known distance in the box labeled Known Distance.
  2. Use the LUT tool to select color ranges from the image. To start click twice on the LUT tool in the toolbar or select Options/Density Slice. A red bar will appear over part of the LUT and over the corresponding color range in the image. You can increase the range of colors selected by clicking on either the top or bottom edge of the red bar and moving the cursor up or down. Narrow the range of values selected to include the area with the greatest ozone depletion over Antarctica.
  3. Select Analyze/Analyze Particles from the menu bar and NIH Image will measure the area of the particles and number them by default. To change the options chose Analyze/Options.... To show the results of the measurement chooseAnalyze/Show Results.
  4. If the measurement was successful save the information as a text file using theFile/Save As menu and marking the circle next to Text. Once data are saved as a text file, it can be opened in a spreadsheet program.
  5. On graph paper, or with a spreadsheet program, plot the change in area of the ozone hole and answer the following questions:
  • What year demonstrates the largest decrease in ozone concentration above Antarctica?
  • Is the decline in ozone constant or does it vary from year to year?

This activity can be adapted for use with daily or monthly ozone images to track the seasonal variation in the ozone layer.