Question: what are the harmful effects of the portion of uv radiation called uvb

title: Ultraviolet
text: However , at ground level sunlight is 44 % visible light , 3 % ultraviolet ( with the Sun at its zenith ) , and the remainder infrared . Thus , the atmosphere blocks about 77 % of the Sun 's UV , almost entirely in the shorter UV wavelengths , when the Sun is highest in the sky ( zenith ) . Of the ultraviolet radiation that reaches the Earth 's surface , more than 95 % is the longer wavelengths of UVA , with the small remainder UVB . There is essentially no UVC . The fraction of UVB which remains in UV radiation after passing through the atmosphere is heavily dependent on cloud cover and atmospheric conditions . Thick clouds block UVB effectively ; but in `` partly cloudy '' days , patches of blue sky showing between clouds are also sources of ( scattered ) UVA and UVB , which are produced by Rayleigh scattering in the same way as the visible blue light from those parts of the sky .