What defines the ozone hole?

The definition of the ozone hole centers on the average concentration of ozone in the stratosphere. Specifically, the ozone hole is identified as an area where the average ozone concentration falls below 100 Dobson Units. This measurement is critical because it signifies significant depletion of the ozone layer, particularly over Antarctica during the Southern Hemisphere's spring (September to November). The Dobson Unit is a standard measure used in atmospheric science to express the amount of ozone in a vertical column of the atmosphere.

Understanding this measurement is vital as it relates directly to the impacts of increased ultraviolet (UV) radiation reaching the Earth's surface due to the reduced ozone concentration. Other factors such as the presence of chlorofluorocarbons (CFCs) do play a role in the formation of the ozone hole, but the defining characteristic is the low concentration of ozone itself. The significance of this threshold and how it impacts environmental health is why this answer is deemed correct.