The area of the Antarctic atmosphere called the "ozone hole" opens up each year in mid-August and peaks in September. It appears blue and purple in this clip created with data collected from satellite.

In 2007, the ozone hole reached its peak on September 13, covering a surface just larger than the size of North America.

This extent is about average when compared to measurements from the last few decades but it is very impressive, compared to the 1970's when the hole did not yet exist.

Question: Why do we need remote sensing to measure ozone ?