ADEOS TOMS Animation of Northern Hemisphere, 1997

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In March, 1997 average ozone amounts in a stable, nearly circular region region over the North Pole were 40 percent lower than the average March amounts observed between 1979 and 1982. This follows ozone amounts in March 1996 that were 24 percent lower than the 1979-82 average, although this low was off center of Earth's pole toward the North Atlantic. The minimum in total column ozone fell to 219 Dobson Units on March 24, 1997, from values near 280 units earlier in March.

The black circle in the middle is due to polar night, when TOMS cannot measure ozone. Notice that it's size decreases and then it disappears as the apparent position of the sun crosses over the equator (signalling the start of spring in the northern hemisphere) and more and more of the Arctic becomes sunlit.

The discontinuity that is sometimes seen at the international date line (180 degrees longitude) is due to the fact that data on the east side of the line (negative longitudes) are taken up to 24 hours later than the west side of the line (positive longitudes).