It is common knowledge that the ozone hole forms in the upper atmosphere over Antarctica each September owing to the special atmospheric and chemical conditions that exist there. But the declining levels of the depleting chemicals brought in some good news this time around as confirmed by the scientists from NASA and National Oceanic and Atmospheric Administration (NOAA).

Despite colder-than-average temperatures in the Antarctic stratosphere, which created ideal conditions for destroying ozone, the hole was not as large as it would have been 20 years ago.

However, the findings showed that the ozone hole that forms in the upper atmosphere over Antarctica each September was slightly above the average size in 2018.

Ozone hole is smaller than it would have been without ban on depleting chemicals

"This year's colder temperatures would have given us a much larger ozone hole if chlorine was still at levels we saw back in the year 2000," said Paul A. Newman, chief scientist for Earth Sciences at NASA's Goddard Space Flight Center in Greenbelt, Maryland, reported IANS.

According to NASA, the annual ozone hole reached an average area coverage of 22.9 square km in 2018.

NOAA scientists said colder temperatures in 2018 allowed for near-complete elimination of ozone in a deep, 5km layer over the South Pole. This layer is where the active chemical depletion of ozone occurs on polar stratospheric clouds.

"Even with this year's optimum conditions, ozone loss was less severe in the upper altitude layers, which is what we would expect given the declining chlorine concentrations we're seeing in the stratosphere," added NOAA scientist Bryan Johnson.

Nations of the world began phasing out the use of ozone-depleting substances in 1987 under an international treaty known as the Montreal Protocol.

Also read | Recovery of Ozone layer could take more time than expected

Stats on ozone hole year-wise

The 2018 ozone hole was strongly influenced by a stable and cold Antarctic vortex -- the stratospheric low-pressure system that flows clockwise in the atmosphere above Antarctica.

In 2016 and 2017, warmer temperatures in September limited the formation of polar stratospheric clouds and slowed the ozone hole's growth. In 2017, the ozone hole reached a size of 19.7 square km before starting to recover.

In 2016, the hole grew to 20.7 square km.

However, the current ozone hole area is still large compared to the 1980s, when the depletion of the ozone layer above Antarctica was first detected. Atmospheric levels of man-made ozone-depleting substances increased up to the year 2000.

Since then, they have slowly declined but remain high enough to produce significant ozone loss.

What is the ozone layer and why there is a hole in it?

Ozone layer or say ozone shield is a region of Earth's stratosphere at an altitude of about 10 km. The layer carries a high concentration of ozone, which absorbs most of the harmful UV rays. The Ultra-violet radiations can damage the DNA of skin cells and cause skin cancer if exposed.

Every year in winter, the ozone hole forms above Antarctica. Cholorofluorocarbons (CFC) is the main chemical responsible for this depletion.

CFCs (used in refrigerants, foams, packing materials, blowing agents and propellants back then) rises up in the atmosphere and reacts with the UV rays resulting in the release of chlorine atoms which destroy the ozone molecules.

What is the Montreal Protocol?

Two years after the hole in the ozone was found in 1985, the Montreal Protocol was signed by almost all nations in 1987 pledging to get rid of chemicals causing the ozone depletion mainly CFCs.

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