NASA says a combination of extreme cold temperatures, man-made chemicals and a stagnant atmosphere caused a significant hole in the arctic ozone layer in 2011. Although both the Earth's poles experience decreases in ozone during the winter, the arctic's ozone depletion tends to be milder and shorter-lived than Antarctica's, researchers said. The reason, they said, is because the three key ingredients needed for ozone-destroying chemical reactions -- chlorine from man-made chlorofluorocarbons, frigid temperatures and sunlight -- are not usually present in the arctic all at the same time. Yet in 2011 ozone concentrations in the arctic atmosphere were about 20 percent lower than average, researchers said. While chlorine in the arctic stratosphere and uncommon atmospheric conditions blocked wind-driven transport of ozone from the tropics, the main culprit was unusually low temperatures, NASA atmospheric scientist Susan E. Strahan said. "You can safely say that 2011 was very atypical: In over 30 years of satellite records, we hadn't seen any time where it was this cold for this long," she said. Most ozone depletion in the arctic occurs inside the so-called polar vortex, the researchers said, a region of fast-blowing circular winds that intensify in the fall and isolate the air mass within the vortex, keeping it very cold. In 2011 an unusually quiescent atmosphere allowed the arctic vortex to remain strong for four months, the said, maintaining frigid temperatures even after the sun reappeared in March, and promoting the chemical processes that deplete ozone. "Most ozone found in the arctic is produced in the tropics and is transported to the arctic," Strahan said. "But if you have a strong vortex, it's like locking the door -- the ozone can't get in." "It was meteorologically a very unusual year, and similar conditions might not happen again for 30 years," she said.