Cloud density (g/m2) 4urn 17fim
* For example, stratus cloud t For example, cumulonimbus (Chapter 8)
and wavelengths up to about 0.29 pm are absorbed by the dissociation of oxygen in the ozone layer (Section 1.4), so that only around 6 per cent of the Sun's radiation reaching the ground is ultra-violet radiation (UV), instead of 9 per cent at the top of the atmosphere. This includes UV-A (i.e. 0.32-0.40 [pm) and the shorter-wavelength UV-B (i.e. 0.28-0.32 pm) and even shorter wavelength UV-C (sometimes called 'vacuum' or 'far' UV). The last penetrates in negligible amount down to sea-level; it is a hazard only on high mountains. UV-B comprises about 1.5 per cent of extra-terrestrial radiation and most is absorbed in the ozone layer. Only around 0.5 per cent of the solar radiation at ground level is UV-B, but this small amount can be very harmful.
The damage caused by UV-B includes sunburn, skin cancer and eye problems such as cataracts, both in people and in cattle. Plants
90 60 50 40 30 20 10 0 10 20 30 40 5060 90 north south
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