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
Photograph by Lamont Poole, NASA Langley Research Center
Polar mesospheric clouds (also known as noctilucent, or “night-shining” clouds) are transient, upper atmospheric phenomena that are usually observed in the summer months at high latitudes (greater than 50 degrees) of both the Northern and Southern Hemispheres. They appear bright and cloudlike while in deep twilight. They are illuminated by sunlight when the lower layers of the atmosphere are in the darkness of Earth’s shadow.