Erythemal dose rate under noon overcast skies

Abstract

The incidence of ultraviolet radiation (UVR) under noon overcast skies was investigated in an urban, tropical site. Overcast skies stimulate people to be outdoors more freely and carelessly than under a cloudless scenario, thereby representing a situation with UVR. 153 occurrences of noon overcast skies were reported for erythemal dose rates (EDR) of up to 0.395 W m(-2) (Extreme UV-Index) and average of 0.121 W m(-2) (Moderate UVI). Erythemal doses varied from 0.4 to 21.6 Standard Erythema Dose (SED) units and averaged 7.7 SED. 34% of the occurrences of Extreme UVI were events of radiation enhancement up to 19% above the summer peak value of 0.332 W m(-2). The elapsed time for such events ranged from 2 to 7 minutes and averaged 4.5 minutes. Cloud genera referring to EDR were predominantly Cu followed by As in summer, and Cu plus Sc and As in the other seasons. For events of radiation enhancement, cloud genera featured a reduction in the occurrence of Cu and an increase in the occurrence of As and Ac.

Daily Maximum Erythemal Dose Rates in the Tropics

“…Nonetheless, the quantitative contribution of such events to the ambient ED is still unknown. Note that, many days in Figure had overcast skies, and yet, their corresponding EDR max were in the Extreme UVI . The usual perception of clouds as solar radiation attenuators is a dangerous assumption to the public in terms of photoprotection in view of radiation enhancement events.…”

“…In fact, it is likely that daily exposures to increments of a few tens of percentage in EDR, as seen in Figures and owing to the scattering of solar radiation from clouds, can potentially produce significant harmful doses in the long‐term of years . In the short‐term of a few minutes, it could induce erythema in fair‐skinned people . For instance, the 45% surplus of the DEC292009 EDR max to the reference EDR max on JAN022010 correspond to an augmentation of 0.151 W m −2 (UVI = 6) in the incidence of erythemal UVR.…”

“…However, in contrast to other atmospheric contents, which only attenuate, clouds can also enhance the solar radiation reaching the Earth surface. Radiation enhancement events arise from augmentation in the diffuse component of global radiation (8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18). For the occurrence of such events, two main fundamental mechanisms exist, both based on the forward scattering of solar radiation from clouds (8,10,13,(19)(20)(21)(22)(23):…”

A Semiempirical Approach to the Determination of Daily Erythemal Doses

“…The values of r 2 shown in Table indicate the complex effects of clouds on ED on a daily basis. In fact, the behavior of clouds attenuating and enhancing UVR contributes significantly to r 2 ranging from 0.71 to 0.82. However, as the OMI parameters correspond to an area similar to the territorial area of Belo Horizonte, it is reasonable to assume the data of both TOC and R PC as climatological for the site.…”

“…In addition, only ground‐based equipment can detect local and particular aspects of the atmosphere influencing the incidence of UVR. For instance, the positioning of some genera of clouds relative to the solar disk can lead to radiation enhancement events, which are not at all reported by satellite measurements . As complete long‐term series of ground‐based measurements of UVR is of utmost importance, we introduce in this work a semiempirical approach based on measured daily erythemal dose (DED), TOC and R PC to the determination of the theoretical daily erythemal dose (DED t ) for a tropical site.…”