Dobson unit

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The Dobson Unit (DU) is a measurement of column ozone levels.

If 100 DU of ozone were brought to the Earth's surface, it would form a layer 1 millimeter thick. In the tropics, ozone levels are typically between 250 and 300 DU year-round. In temperate regions, seasonal variations can produce large swings in ozone levels. For instance, measurements in Leningrad have recorded ozone levels as high as 475 DU and as low as 300 DU. These variations occur even in the absence of ozone depletion, but they are well understood.

Ozone depletion refers to reductions in ozone below normal levels after accounting for seasonal cycles and other natural effects.

A graphical explanation is available at NASA's TOMS site.