<p id="content">Persistent cold temperatures and strong circumpolar winds helped the formation of a large and deep Antarctic ozone hole that should persist into November, scientists have said.</p>
The annual Antarctic ozone hole reached its peak size at about 24.8 million square kilometres, roughly three times the area of the continental US, on September 20, according to the US National Oceanic and Atmospheric Administration (NOAA) and Nasa scientists.

Observations revealed the nearly complete elimination of ozone in a four-mile-high column of the stratosphere over the South Pole.

The scientists said year 2020 will go down as having the 12th largest ozone hole by area in 40 years of satellite records, with the 14th lowest amount of ozone in 33 years of balloon-borne instrumental measurements.

Ongoing declines in levels of ozone-depleting chemicals controlled by the Montreal Protocol prevented the hole from being as large as it would have been under the same weather conditions decades ago.

"From the year 2000 peak, Antarctic stratosphere chlorine and bromine levels have fallen about 16 per cent towards the natural level," Paul Newman, chief scientist for Earth Sciences at Nasa's Goddard Space Flight Center in Greenbelt, Maryland, said in a statement.

"We have a long way to go, but that improvement made a big difference this year. The hole would have been about a million square miles larger if there was still as much chlorine in the stratosphere as there was in 2000."

Ozone is composed of three oxygen atoms and is highly reactive with other chemicals.

In the stratosphere, roughly seven to 25 miles above Earth's surface, the ozone layer acts like sunscreen, shielding the planet from ultraviolet radiation that can cause skin cancer and cataracts, suppress immune systems and damage plants and sensitive plankton at the base of the global food chain.

By contrast, ozone that forms closer to Earth's surface through photochemical reactions between the sun and pollution from vehicle emissions and other sources, forms harmful smog in the lower atmosphere.

The Antarctic ozone hole forms during the Southern Hemisphere's late winter as the returning Sun's rays start ozone-depleting reactions.

Cold winter temperatures persisting into the spring enable the ozone depletion process, which is why the "hole" forms over Antarctica.

These reactions involve chemically active forms of chlorine and bromine derived from man-made compounds.

The chemistry that leads to their formation involves chemical reactions that occur on the surfaces of cloud particles that form in cold stratospheric layers, leading ultimately to runaway reactions that destroy ozone molecules.

In warmer temperatures, fewer polar stratospheric clouds form and they do not persist as long, limiting the ozone-depletion process.

The rate at which ozone declined in September has slowed compared with 20 years ago, which is consistent with there being less chlorine in the atmosphere, said Bryan Johnson, a scientist with NOAA's Global Monitoring Lab..