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Stratospheric ozone concentrations in spring 2016 in the Southern (left, September 19–26) and Northern (right, March 21-27) Hemispheres. Each year, ozone amounts drop below 220 Dobson Units—the threshold that marks the start of an ozone hole—across a large area of Antarctica. Northern Hemisphere values are rarely that low. NOAA Climate.gov image based on satellite ozone data provided by NOAA View.