The amount of UV at ground level is slightly reduced by clouds, but visible light is reduced far more. This applies especially to high and thin cloud (Chapter 8). Unfortunately, the reduced illumination makes people feel that it is unnecessary to seek shelter, though UV levels are still dangerous. Even in shade, one does not escape the UV entirely, since much of it is diffuse, coming from all angles, as Rayleigh scattering by air molecules is more intense at the short wavelengths of UV. Typically, half the UV is diffuse when the Sun is high in a cloudless sky.
Air pollution lowers the UV intensity at the ground, especially in the case of the ozone-rich photochemical smog found over large cities like Sydney and Santiago (Chapter 15). As a result,
UV-B intensities have decreased by 4-11 per cent in the USA and intensities in industrial Germany are half those at a place free of pollution of similar latitude in New Zealand. So reducing air pollution is likely to increase the harm from UV.
The intensity varies also with latitude, season and time of day. The annual UV dosage at Brisbane (27°S) is about 50 per cent higher than at Melbourne (38°S) and four times as much as at Macquarie Island (55°S).
The rarefied atmosphere at high elevations leads to less attenuation of the UV. Compared with the dose at sea-level, it is 20 per cent more at 1,500 m elevation, and almost twice as much at 3,000 m, when the Sun is at 50 degrees. The amount that people receive is further increased if there is snow present, reflecting UV upwards.
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