The Ozone Hole: Why It Opens Over Antarctica Every Spring
A dark winter, icy clouds and the first spring sunlight work together to thin the ozone layer over the South Pole. In 2026 the hole again grew bigger than Antarctica.
By Saurabh Dhakad, PhD scholar, Atmospheric Chemistry and Climate Change Lab, School of Environmental Sciences, Jawaharlal Nehru University · · 3 min read
In November, north India watches the haze, and ozone comes up only as a smog gas. Far to the south, the good ozone high above Antarctica has just been through its yearly thinning. On 12 September 2026 the ozone hole covered about 25 million km2, close to twice the size of Antarctica itself. Why there, and why in spring?
What the "hole" really is
It is not an empty patch of sky. It is the area where the total ozone column falls below 220 Dobson Units (DU). In the 1950s, Antarctic spring values were around 300 DU. In May 1985, British Antarctic Survey scientists at Halley station reported in Nature that spring ozone there had fallen to about two-thirds of its old thickness.
Step 1: a cold, closed box
In the dark polar winter, strong winds circle the pole and form the polar vortex, which keeps the air inside from mixing. With no sunlight it cools hard: average daily minimum temperatures reach about −92 °C in July and August, and stay below −78 °C for about five months.
Step 2: chemistry on icy clouds
Below about −78 °C, nitric acid and water condense on tiny sulphuric acid droplets and form polar stratospheric clouds (PSCs). Near −85 °C, pure ice clouds also form. Most chlorine from old CFCs normally sits in two harmless reservoir gases, hydrogen chloride (HCl) and chlorine nitrate (ClONO2). On the cloud particles they react:
HCl + ClONO2 → Cl2 + HNO3
The chlorine leaves as Cl2 gas, while nitric acid stays in the particles and falls out with them, so little NO2 is left to lock the chlorine away again.
Step 3: the spring sun
When the sun returns in September, light splits Cl2 into two chlorine atoms. Each Cl atom turns ozone into ClO and O2. Two ClO join into a dimer (Cl2O2), sunlight breaks it, and the chlorine is free again. The net result is that two O3 become three O2, while the chlorine survives to repeat the cycle. The fast loss stops by mid-October, when the air warms above the PSC limit.
Why Antarctica, and why 2026 was large
- The Arctic is warmer and its vortex is disturbed by mountains and land–sea contrasts, so PSCs are fewer.
- In early September 2026, the temperature about 20 km above the South Pole suddenly dropped, just as the hole grew fast.
- In 2025 the hole was the fifth smallest since 1992, so such swings come mostly from stratospheric weather.
What this means for us
At India's Maitri station in Antarctica, ozone loss reached 59% in the cold winter of 2006. As in Delhi's winter smog, cold still air, particle surfaces and sunlight together do the damage. The good news is that ozone-depleting gases over Antarctica peaked around 2000 and have since fallen by about a third, and the hole is expected to recover by the late 2060s. Old fridges and air conditioners still hold such gases, so servicing them through trained technicians who recover the refrigerant, and keeping long-term ozone measurements going, help keep that recovery on track.
References and further readings
- Copernicus Atmosphere Monitoring Service (16 Sep 2026). Ozone hole reaches notable threshold early again as area increases above average in 2026.
- NOAA CSL / WMO (2022). Twenty Questions and Answers About the Ozone Layer, Question 9: Why has an ozone hole appeared over Antarctica?
- NASA and NOAA, via ScienceDaily (Dec 2025). New data reveals one of the smallest ozone holes in decades.
- UKRI–NERC. The story behind the discovery of the ozone hole (Farman, Gardiner and Shanklin, Nature, 1985).
- Kuttippurath, J. et al. (2021). The ozone hole measurements at the Indian station Maitri in Antarctica. Polar Science, 100701.
- Finlayson-Pitts, B. J. and Pitts, J. N. Chemistry of the Upper and Lower Atmosphere, Chapter 12.