Since 1998, only the ozone holes of 2002 and 2004 have been smaller than this year's hole, said the UN's climate and weather agency.
The ozone hole over Antarctica has shrunk 30 percent as compared to last year's record size. Measurements made by the European Space Agency's Envisat satellite show that this year's ozone loss peaked at 27.7 million metric tons, compared to the 2006 record ozone loss of 40 million tonnes.
The 2007 ozone hole is relatively small both in terms of area - about 25 million square kilometers - and the amount of ozone destruction, which occurs when ozone reacts with chlorine, bromine, nitrogen, hydrogen, and oxygen gases.
Most ozone resides between 10 and 40 kilometers above the Earth's surface. This atmospheric region is called the stratosphere and it contains about 90 percent of all the ozone in the atmosphere.
'The stratosphere still contains more than enough chlorine and bromine to cause complete ozone destruction in the 14 to 21 kilometer altitude range,' the WMO said.
Scientists emphasize that the smaller ozone hole this year is not a sign of recovery.
'Although the hole is somewhat smaller than usual, we cannot conclude from this that the ozone layer is recovering already,” said Ronald van der A, a senior project scientist at Royal Dutch Meteorological Institute.
'This year's ozone hole was less centered on the South Pole than in other years,' he said, 'which allowed it to mix with warmer air, reducing the growth of the hole because ozone is depleted at temperatures less than -78 degrees Celsius.'
During the southern hemisphere winter, the atmospheric mass above the Antarctic continent is kept cut off from exchanges with mid-latitude air by prevailing winds known as the polar vortex. This leads to very low temperatures, and in the cold and continuous darkness of this season, polar stratospheric clouds are formed that contain chlorine.
As the polar spring arrives, the combination of returning sunlight and the presence of polar stratospheric clouds leads to splitting of chlorine compounds into highly ozone-reactive radicals that break ozone down into individual oxygen molecules.
A single molecule of chlorine has the potential to break down thousands of molecules of ozone, scientists say.
First recognized in 1985, the Antarctic ozone hole is caused by the presence of ozone destroying gases in the atmosphere such as chlorine and bromine. These originate from manufactured products like the refrigerants chlorofluorocarbons, CFCs. They have still not vanished from the air but are on the decline as they are banned under the Montreal Protocol, signed in 1987.
The Antarctic ozone hole typically persists until November or December, when the winds surrounding the South Pole, known as the polar vortex, weaken, and ozone-poor air inside the vortex is mixed with ozone-rich air outside it.
The amount of ozone-depleting gases reached a maximum in the Antarctic stratosphere around the year 2000. This amount is now declining slowly at a rate of about one percent per year.
The stratosphere is expected to contain enough chlorine and bromine to cause severe ozone holes for another 10 to 20 years, the WMO says.
During this period, the severity of the ozone hole will be determined by the meteorological conditions of the stratosphere during the Antarctic winter. These conditions are related to global warming, says the WMO.
Increasing amounts of greenhouse gases in the atmosphere will lead to lower temperatures in the stratosphere. This increases the risk of severe ozone holes in upcoming decades.
Ozone in the stratosphere absorbs some of the Sun's biologically harmful ultraviolet radiation, a beneficial role.
By contrast, excess ozone at Earth's surface that is formed from pollutants is considered bad ozone because it can be harmful to humans, plants, and animals.