Remember the ozone hole? Last year it closed by November but scientists report it's back and for the second year in a row, it appears to be reaching record size.

Cold weather, giant areas of low and high pressure, high levels of human-made gases and approaching spring in the Antarctic have again provided the right mix of conditions to broaden a yawning gap of the protective layer of ozone in the stratosphere.

Scientists report the hole has cracked open to more than 15 million square miles or about the size of the entire continent of North America. That's only two to three million square miles short of the hole's record size last year, and it's expected to keep on growing.

The stratosphere is a layer of atmosphere about 6 to 30 miles above the surface that contains the chemical compound known as ozone. While at ground level ozone is an unhealthy pollutant, in the stratosphere it plays a vital role in absorbing harmful ultraviolet radiation from the sun.

Over the past 20 years, concentrations of this ozone layer have thinned due in part to human-made gases released into the atmosphere including those known as chlorofluorocarbons or CFCs.

Could Open Over Chile

It was about this time last year that locals in southern Chile were warned to don wide-brimmed hats and sunglasses to shield themselves from radiation beaming through a widening ozone hole that had stretched over southern portions of the country. This year scientists say Chileans might again need to seek cover.

"Conditions are ripe for a large hole," says Paul Newman, an atmospheric scientist at NASA Goddard Space Center in Maryland. "There's a good chance one lobe could pass over the southern coast of Chile this year."

Since the 1970s, scientists have observed the ozone hole begin to open by late August and then close by November or December. Its annual reappearance is due to a combination of cold air and sunlight that are both present in the Antarctic during that window.

Cold weather in the Antarctic triggers ice clouds to form in the stratosphere. The clouds provide a surface layer that converts CFCs, accumulated after decades of widespread use of items like aerosol spray cans and CFC-containing air conditioners and refrigerators, into new forms. Add sunlight and these forms become chlorine atoms which then break down the ozone layer.

Scientists believe that last year pressure systems swooped in over the Antarctic and expanded ozone destruction by pushing cold temperatures and ice clouds to more sunlit regions. Then in November other systems erased clouds over the Antarctic and closed the hole faster than expected.

Hole Going Away, In Theory

Although the hole has widened in recent years, there is some good news: it's expected to go away — at least eventually. Since the issue was addressed at a 1985 world conference in Vienna, nations have effectively reduced their use of CFCs.

"Basically chlorine molecules in the stratosphere peaked a couple years ago," says Susan Solomon, a senior scientist at a National Oceanic and Atmospheric Administration research center in Boulder, Colo. who was among the first scientists to discover the correlation of CFCs and ozone depletion. "But because these things live in the atmosphere for many decades, we'll have an ozone hole until at least the 2040s."

Newman says he and his colleagues remain only "cautiously optimistic" that the hole will go away in the next 40-50 years.

Earlier this month, NOAA scientists measured high concentrations of ozone-harming chemicals near large towns along Russia's Trans-Siberian Railway. The team is uncertain of the chemicals' sources, but the discovery is one indication that there may not be strict enforcement of CFC bans in all countries.

Warming Could Make Hole Linger

Another wildcard is global warming. One ironic aspect of global warming, as scientists understand it, is the trapping of greenhouse gasses like carbon dioxide may lead to warmer temperatures on Earth's surface, but in the stratosphere, carbon dioxide radiates more energy out to space and temperatures are cooled.

Cooler temperatures in the stratosphere contribute to more clouds over the Antarctic and could possibly contribute to a widening ozone hole.

"We're very very worried about how global warming may change things," said Newman.

On top of concerns about CFC emissions and global warming is the inconvenient fact that ozone monitoring has become more difficult in recent months. NASA officials announced in August they would be switching off the Upper Atmosphere Research Satellite by Sept. 30 to cut costs. For 10 years this satellite has provided scientists with critical data on CFC levels in the stratosphere.

Another satellite, which Newman and others now use to gauge the size of the ozone hole, has begun to malfunction, leading to data uncertainties of more than 2 million square miles. A replacement is due to launch in two week's time. Until then, Newman says the faltering satellite should be able to at least monitor the rough size of the hole.

"The ozone hole is so massive," says Newman. "It's not hard to detect — even a crummy instrument can see it."