ozone hole

The name does more harm than good, and NASA says so directly: the ozone hole is not technically a hole where no ozone is present, but is actually a region of exceptionally depleted ozone in the stratosphere over the Antarctic that happens at the beginning of Southern Hemisphere spring. There is ozone throughout it. There is simply much less than there should be.

It also has a precise boundary rather than a vague one. Ozone in a column of atmosphere is measured in Dobson Units, and NASA uses 220 Dobson Units as the edge: the ozone hole is the region over Antarctica with total ozone of 220 Dobson Units or lower. That number was not chosen arbitrarily. Values below 220 were not observed anywhere before 1979, so the threshold marks a level that simply did not occur before the depletion began.

This is why the hole has an area that can be reported as growing or shrinking from year to year. It is the size of the region under that threshold, measured against a fixed line, which is what makes one year comparable with another. It is also seasonal, forming in the southern spring when returning sunlight drives the chemistry, so a figure only means something alongside the time of year it was taken.