The U.N.'s weather body says unusually cold high-altitude conditions have expanded the Antarctic ozone hole to one of the largest sizes on record, though the long-term trend remains one of shrinkage.

The World Meteorological Organizations notes regular, year-to-year variations in the hole, but says the expansion shows "we need to remain vigilant."

The agency said Thursday that NASA data on Oct. 2 showed the hole had reached 28.2 million square kilometers (10.9 million square miles), the widest recorded on that date, and has remained at a record size on the dates since.

The ozone layer sits high above Earth and protects against harmful UV rays. An international accord in 1987 to phase out use of ozone-depleting chemicals has underpinned hopes for recovery of stratospheric ozone worldwide by 2070.