Answer:
(i) During winter months (June to August), Antarctic region receives no sunlight and is characterized by very low temperature (about - 85°C). Further Antarctic air is isolated from that of rest of world by polar vortex. These environmental conditions favour the formation of ice clouds which provide catalytic surface for the action of chlorine atoms, produced by the chlorofluorocarbons, on the ozone. But the ozone degradation occurs only in the presence of sunlight which is available only in spring (September to October). So the ozone hole appears in spring but disappears in summers.
(ii) Ill-effects. The thinning of ozone shield results in an increase in the UV-B radiations reaching the earth's surface. The increased UV-radiations increase the incidence of cataract and skin cancer. These also diminish the functioning of immune system of human beings, and inhibit the rate of photosynthesis in most of the phytoplanktons.
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