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...The map above shows the concentration of stratospheric ozone over the Arctic—63 to 90 degrees North—on April 1, 2014. Ozone is typically measured in Dobson Units, the number of molecules required to create a layer of pure ozone 0.01 millimeters thick at a temperature of 0 degrees Celsius and an air pressure of 1 atmosphere (the pressure at the surface of the Earth). Reaching 470 Dobson Units, April 1 marked the highest average concentration of ozone over the region so far this year. The average amount of ozone in Earth’s atmosphere is 300 Dobson Units, equivalent to a layer 3 millimeters (0.12 inches) thick—the height of 2 pennies stacked together....