AN ozone scare is now occurring twice a year. The problem, as we all know, is due to a build up over the years of various man made pollutants in the upper atmosphere: these are transformed into compounds like chlorine and bromine. When these are exposed to sunlight, they cause chemical reactions which reduce the amount of ozone in the protective layer some 15 to 20 miles above our heads.
The less ozone there is in the ozone layer, the more ultraviolet light that can penetrate to earth. And this latter is potentially harmful to humans and to other forms of life upon our planet.
The problem first became apparent with the discovery of the famous "Ozone Hole" over Antarctica in 1985. Scientists quickly spotted that it occurred every year during the southern spring, when the temperature many miles above the south pole was at its lowest.
In these very cold conditions, ice clouds formed in the high atmosphere. It was the photo chemical effect on these clouds of the spring sunshine peeping above the polar horizon, and reacting with chlorine and bromine, that resulted in destruction of the ozone.
Every year since, the Antarctic ozone hole has been getting larger and "deeper" - in the sense that more ozone is destroyed it receives a great deal of media attention each October and November before ozone levels begin to recover through December.
The second ozone event each year occurs in February and March - the northern late winter and early spring. Having spotted the Antarctic phenomenon, scientists began to look to see if something similar occurred elsewhere. In due course they discovered that a similar "hole" appears each winter in the Arctic, albeit only a pale shadow of its antipodean counterpart.
But this year it has been particularly bad. During the second half of February, and so far during the month of March, ozone levels have been 20 to 30 per cent below normal values over Greenland, northern Europe and western Siberia.
The cause is to be found in exceptionally low temperatures of nearly minus 80 Celsius that have persisted for several weeks in the high atmosphere over these regions.
This has facilitated the formation of high level ice clouds which, in the presence of the chlorine and bromine compounds, and combined with the increasing sunlight, bring about the destruction of the ozone.
We can be grateful, however, that "ozone holes" like these in the northern hemisphere last only for a few weeks unlike their Anatarctic counterparts which persist for several months.