The lowest levels of ozone in the Southern Hemisphere generally occur every October. This interactive graphic shows October ozone concentrations in the Southern Hemisphere for odd-numbered years from 1979 to 1991. Scientists consider areas with ozone levels of less than 220 Dobson Units to be part of the ozone "hole."

  !   Use the color code and the measuring tool to measure the length and width (the major and minor axes) of the ozone hole on each image. Check the color code to estimate the minimum ozone level observed each year.