Scientists recently discovered that polar stratospheric clouds,
long known to play an important role in Antarctic ozone
destruction, are occurring with increasing frequency in the
Arctic. These high altitude clouds that form only at very low
temperatures help destroy ozone in two waysthey provide a
surface which converts benign forms of chlorine into reactive,
ozone-destroying forms, and they remove nitrogen compounds that
moderate the destructive impact of chlorine. In recent years the
atmosphere above the Arctic has been colder than usual, and
polar stratospheric clouds have lasted into the spring. As a
result, ozone levels have been decreasing. This photograph shows
polar stratospheric clouds lit from below near Kiruna, Sweden.
Photograph by Lamont Poole, NASA Langley Research Center