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Lots of people comment that you have to use more sunscreen in Australia compared to European countries due to the hole in the ozone layer. (Even for roughly equivalent latitudes).

What would cause this imbalance? It can't be gravity, since that comes from the centre of the earth. Perhaps magnetic poles could have an influence, but I don't understand how.

My question is: Why is the Ozone Layer 'hole' more pronounced in the Southern Hemisphere?

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Polar stratospheric clouds are the answer. These clouds contain solid nitric acid (mixed with water ice and sulfuric acid), and when they form they remove gaseous nitric acid from the air. This impacts the other chemical cycles in the stratosphere and leads to ozone depletion. This powerpoint is kinda long, but I think it explains the chemistry fairly well. It's written from a historical perspective, contrasting the different theories for ozone loss.

These kinds of clouds only occur at extremely low temperatures, though. You can see them sometimes in the Arctic, but not nearly as often as in the Antarctic. Oddly enough, this is caused (mostly) by there being more mountain ranges in the northern hemisphere. Mountain ranges can induce atmospheric waves that travel up into the stratosphere and perturb the stratospheric vortex. (The Wiki page image shows what a perturbed and unperturbed vortex look like.) Without that perturbation, the stratosphere can just keep cooling down by radiating to space all winter while there's no sunlight. This is why the Antarctic is so much colder than the Arctic, and why polar stratospheric clouds form there much more often.

So...The ozone hole is worse in the southern hemisphere because there are fewer mountain ranges there. Earth science is weird!

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Two significant factors that influence the size of the ozone hole over the southern pole are temperature and the polar vortex.

A number of sources confirm that the ozone hole increases in size when the temperature over Antarctica is colder. (Parrondo et al, The Ozone Hole, Sam Houston State University),

The polar vortices that form over the northern and southern poles are different because the northern vortex forms over water that is surrounded by land, whereas in the southern hemisphere the situation the opposite; the vortex forms over land surrounded by water. Because of this, Antarctica "is meteorologically isolated from air at higher latitudes during winter".

This physical condition forms an isolated air mass swirling over Antarctica called the south polar vortex. In the air of the polar vortex, the temperatures drop to very low levels in the stratosphere, below -80 degrees Celsius. At these temperatures, chemicals present in the stratosphere freeze out and form polar stratospheric clouds (PSCs). It is the chemical reactions that occur on the PSCs that result in the large decrease in ozone during each austral spring over Antarctica that we called the Antarctic ozone hole.

Chlorine and fluorine oxides, produced by stratospheric photolysis (sunlight induced decomposition) of Chlorofluorocarbons (CFCs) also contribute to the depletion of ozone. During the southern winter, Antarctica is predominately dark due to the lack of sunshine. With no sun light photolysis of CFCs does not occur. During the southern summer when Antarctica is continuously sunlit CFCs decompose, further reducing ozone levels within the polar vortex.

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  • $\begingroup$ "Because of this Antarctica is meteorologically isolated from air at higher latitudes during winter". Maybe should be "air at lower latitudes", as I believe such terminology is based on the number of the latitude rather than "level up the map". It's isolated from the warmer air/water to its north. $\endgroup$ – JeopardyTempest Feb 11 '17 at 7:24
  • $\begingroup$ Also, to follow through on WHY it is isolates: if I remember right, a main reason is that water flows. So the Arctic interacts with ocean waters from different latitudes, preventing any great "snowballing" effects where the cold can isolate and deepen. ... $\endgroup$ – JeopardyTempest Feb 11 '17 at 7:38
  • $\begingroup$ ... On the other hand, Antarctic land remains protected from this intrusion, and the more-quickly-coolable land only further helps the surface high strengthen, and thus also the connected UPPER LEVEL low (a TRUE polar vortex). This, plus the more consistently oceanic SH geography [leading to more zonal storm paths, and thus even less energy from warmer environs to interrupt Antarctic weather], helps the vortex become more and more entrenched in winter, allowing much colder temperatures. $\endgroup$ – JeopardyTempest Feb 11 '17 at 7:38
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The south pole has a continent covering it, which allows the retention of relatively long and cold storms, compared to the North Pole. The north pole is covered by water and is not an elevated land mass, so it is a bit warmer on average. Polar stratospheric clouds have chemistry on the ice particles that leads to catalytic ozone depletion (mainly caused by human emissions of CFCs). The colder it is... the more of this can occur.

From https://en.wikipedia.org/wiki/Ozone_depletion

Reductions of up to 70% in the ozone column observed in the austral (southern hemispheric) spring over Antarctica and first reported in 1985 (Farman et al.) are continuing. Since the 1990s, Antarctic total column ozone in September and October continued to be 40–50% lower than pre-ozone-hole values.[1] In the Arctic, the amount lost is more variable year-to-year than in the Antarctic. The greatest Arctic declines, up to 30%, are in the winter and spring, when the stratosphere is coldest.

Reactions that take place on polar stratospheric clouds (PSCs) play an important role in enhancing ozone depletion.[8] PSCs form more readily in the extreme cold of the Arctic and Antarctic stratosphere. This is why ozone holes first formed, and are deeper, over Antarctica.

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