The solar UV radiation flux reaching the Earth is one of important factors to evaluate the planetary energy balance. The UV flux (200 – 400 nm) at the top of atmosphere is about 8% of the total flux from the Sun. But the amount of UV measured at ground level is lower, this because the UV, crossing the atmosphere, undergoes both strong spectral absorption and diffusion. The spectral part 200 – 280 is practically negligible at ground.

The normal irradiance, and the radiation on a horizontal surface, as global spectral irradiance is expressed by vertical component of direct radiation and the diffused one. They are obtained by instruments calibrated in physical units, or evaluated by models.

The work shows the problems linked to a correct understanding and interpretation of solar UV data sampled at ground, several practical and theoretical problems of measurement are shown. In particular, the comparison among different sites or instruments is analyzed, and the effects due to O₃ level, albedo and cloudiness are discerned. The impact of clouds and their position respect to the Sun on the observation carried out both by spectral and broad-band instruments are also examined by models.