Another atmospheric gas is ozone, a form of oxygen in which each molecule consists of three atoms instead of the usual two. Ozone within the lowest 2 km of the atmosphere results from air pollution and is dangerous to health (Chapter 14). But it occurs also at 15-40 km above sea-level, where it protects us from the Sun's ultraviolet rays (UV), and its current depletion by about 4 per cent per decade is a matter of great concern (Chapter 2).
The upper ozone is formed chiefly in the summer hemisphere and over the equator, where solar radiation is strongest, and then it circulates towards the poles. The highest concentrations are found at latitudes above 50 degrees, but even there they are less than one thousandth of a gram in each cubic metre, representing about one hundred-thousandth part of the air. If all the ozone in the atmosphere were separated out at sea-level, the layer would be only 3 mm thick. Nevertheless, that small amount is important because it shields us from UV, which inhibits photosynthesis and is dangerous to health. In addition, the absorption of the UV leads to warming of the upper stratosphere, discussed in Section 1.8 and Chapter 2.
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