Experimental determination of cloud influence on the spectral UV irradiance and implications for biological effects

Measurements of UV spectra, total ozone, cloud cover, and cloud optical thickness, obtained at Lampedusa (central Mediterranean), are used to investigate the influence of clouds on the spectral UV irradiance, through the cloud modification factor (CMF), and on five biological processes. The CMF decreases with cloud optical thickness (COT), from about 0.5 for COT∼15 to 0.25 for COT∼45, and decreases with increasing wavelength above 315–320-nm. Observations display an increase in the CMF from 295 to 320-nm, which is related to enhanced absorption by tropospheric ozone due to the long photon path lengths under cloudy conditions. The use of a wavelength independent CMF instead of the experimentally determined spectral curves produces an overestimation of the biological effects of UV irradiance. The overestimation may be as large as 30% for the DNA damage, 20% for vitamin D synthesis, 12% for plant damage, and 8–10% for phytoplankton inhibition and erythema.

► Cloud modification factor decreases with increasing cloud optical thickness.
► CMF decreases with increasing wavelength above 320-nm.
► CMF dependence on solar zenith angle is also a function of cloud optical properties.
► Upto 320-nm, CMF increases with wavelength due to the UV absorption by ozone.
► No wavelength-dependent CMF overestimates the biologically effective radiation.