The winter hole in the ozone layer above Antarctica appears to have grown from last year but is still smaller than in 2003, when it was at its largest, the World Meteorological Organisation (WMO) said. The UN agency's top ozone expert added that seasonal depletion of the protective gas layer, which filters harmful ultraviolet rays that can cause skin cancer, may become more pronounced in the near future before the problem diminishes. Large reductions in the ozone layer, which sits about 15-30 km (9-19 miles) above the earth, take place each winter over the polar regions, especially the Antarctic, as low temperatures allow the formation of stratospheric clouds that assist chemical reactions breaking down ozone.