For GSS Ozone chapter 9. Excerpt:  The ozone hole that forms each year in the stratosphere over Antarctica was slightly smaller in 2013 than average in recent decades, according to NASA satellite data. ...However, the size of the hole in any particular year is not enough information for scientists to determine whether a healing of the hole has begun. ...The ozone hole forms when the sun begins rising again after several months of winter darkness. Polar-circling winds keep cold air trapped above the continent, and sunlight-sparked reactions involving ice clouds and chlorine from manmade chemicals begin eating away at the ozone. Most years, the conditions for ozone depletion ease before early December when the seasonal hole closes. See image.... http://www.nasa.gov/press/2013/october/antarctic-ozone-hole-slightly-smaller-than-average-this-year/. NASA RELEASE 13-316.