Agricultural and Forest Meteorology 120 (2003) 101–111 The inﬂuence of cloudiness on UV global irradiance (295–385 nm) I. Foyo-Moreno a , I. Alados b , F.J. Olmo a , L. Alados-Arboledas a,∗ a Dpto de F´ ısica Aplicada, Facultad de Ciencias, Universidad de Granada, Granada 18071, Spain b Dpto de F´ ısica Aplicada, Universidad de Málaga, Málaga, Spain Abstract Two years of continuous measurements of UV (295–385 nm) irradiance recorded at Granada (37.18 ◦ N, 3.58 ◦ W, 660 m a.m.s.l.), Spain, were combined with concurrent synoptic cloud observations to establish the relative inﬂuence of clouds on UV irradiance. A marked inﬂuence of cloud cover on UV irradiance is evident, but negligible for fractional cloud coverage below 3octas. The ratio of UV irradiance to the total solar irradiance increases with cloud cover, especially for cloud cover greater than 4 octas, highlighting the different effects of cloud on total solar irradiance and UV irradiance. In order to determine the effect of cloud on UV irradiance, we considered a cloud modiﬁcation factor, deﬁned as the ratio between the UV measurements and the corresponding clear sky UV irradiance that would be expected for the same time period and atmospheric conditions. It is shown that the effect of cloud for UV wavelengths is less than that for the whole solar spectrum and less than that for the visible part of the spectrum. On the other hand, the small inﬂuence of cloud cover for intermediate cloudiness was accompanied by low variability and variability increases with increasing cloud cover. © 2003 Elsevier B.V. All rights reserved. Keywords: Ultraviolet solar irradiance; Total solar irradiance; Cloud radiative effect 1. Introduction In recent years there has been a substantial increase in attempts to model the ultraviolet radiation, UV, reaching the surface of the earth. UV irradiance at the surface is a result of the combined effects of solar el- evation angle, surface elevation, cloud cover, aerosol loading and optical properties, surface albedo, and the vertical proﬁle of ozone. At a speciﬁc site, the main factor causing variation in UV irradiance is the change in solar elevation angle. The observed daily and yearly variations are dominated by this factor. Total ozone and clouds are of second-order impor- ∗ Corresponding author. Tel.: +34-958-244024; fax: +34-958-243214. E-mail address: alados@ugr.es (L. Alados-Arboledas). tance for the variation of this radiative ﬂux. Atmo- spheric aerosols also affect the UV irradiance, but their inﬂuence is small relative to clouds (Lorente et al., 1996; D´ ıaz et al., 2001; Wenny et al., 2001). Among the factors inﬂuencing the UV irradiance, clouds and aerosol present a high temporal and spatial variabil- ity. It is certain that clouds can cause large year to year variability in UV radiation and therefore possi- bly play an important role in determining long-term trends (Seckmeyer et al., 1996). The effect of clouds on instantaneous UV levels can vary from small en- hancements to near total elimination (Matthijsen et al., 2000). Cloud and aerosol characteristics determining the radiative transfer are seldom known, because ob- servational networks, especially for aerosols, are rather sparse. For the effect of clouds, it is necessary to have knowledge of cloud optical thickness and drop size 0168-1923/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.agrformet.2003.08.023