Evaluation of a ground-based sky camera system for use in surface irradiance measurement

This paper describes the evaluation of a ground-based sky camera system for studying the effect of clouds on the level of the ambient ultraviolet radiation. The system has been developed for research in the characterization of the effect of clouds around the sun. It is the first sky camera system to be used for the assessment of cloud conditions in the vicinity of the sun, rather than a whole-sky assessment. The system features a sun-tracking sky camera with an integrated measurement of horizontally received radiation at the same location. The image-processing algorithm uses solar radiation readings to reduce reflections from the sun on the camera system being mistaken for cloud in the images. Cloud amount was estimated in an angular region of between 12.5°and 37.5° around the sun. The algorithm also estimates the amount of solar obstruction by cloud (sun not covered, partially or totally covered). The system was evaluated during September 1997 at Toowoomba, Australia (27.6°S latitude). Compared to manual assessment of 592 images, 76.5% were identified correctly by the algorithm for the degree of solar obstruction and 81.9% for cloud amount. The behavior of ultraviolet radiation levels with cloud conditions is discussed

UV protection and shade structures

[Abstract]: Broadband field measurements were conducted beneath three different sized public shade structures at a sub-tropical Southern Hemisphere site for relatively clear skies and for a changing solar zenith angle (SZA) of 13o to 76o. These data were compared to the diffuse UV to quantify the relationship between diffuse UV and the UV in the shade of the structures. On the horizontal plane, the ultraviolet protection factors (UPF) for the shade structures ranged from 1.5 to 18 for a decreasing SZA. The data from this research is significant, because it shows that as the SZA of the sun increases so does the relative proportion of scattered UV beneath the shade structures which in turn decreases the shade structures UPF. In Australia, erythemal UV in full sun can reach levels of approximately 2.5 MED/h or more in the middle of the day during winter. Therefore, it is necessary for people that live in similar latitudes to minimise UV exposure in all climatic conditions throughout the year. Based on this research, a standard for reporting the UV protection provided by shade structures is essential for the public to make an informed decision on the efficacy of particular structures in reducing personal UV exposure

Measurement of cloud angle for enhanced UVB at the Earth's surface

This paper presents the results of a four-month study quantifying the effect of cloud and haze on enhanced UVB radiation (310 to 280nm) using a ground based sky camera system. The sky camera system is the first to measure cloud and haze properties near the sun, in contrast to whole sky assessment. images and radiation data were recorded every 6 minutes at Toowoomba, Australia (27.6° S). An image-processing algorithm estimated the amount and brightness of cloud and haze in an angular region of between 12.5 and 35 ° around the sun, as well as the amount of solar obstruction (sun not covered, partially or totally covered). It was found that UVB enhancement greater than 20 % occurred when maximum cloud area was at a 35° angle from the sun

Spectral dependency of cloud enhanced UV irradiance

[Abstract]: This paper addresses two questions of primary importance to the solar UV community, 1) “Are cloud induced UV enhancements always wavelength dependent?” and 2) “Are the enhancements greatest in the UVA or UVB wavebands?” The answer to the first question is a definite no, with the conclusion to the second question that most of the enhancements found at this southern hemisphere measurement site are in the UVB waveband. This research is based on the results from a scanning UV spectroradiometer and a colour, all-sky camera over a 19 month period. In both the UVB and UVA wavebands there were cases that showed increasing, decreasing and no spectral dependence towards the shorter and longer wavelengths respectively. This research has found that cases of spectral dependence that decreased with wavelength, tended to correspond to cloud fraction distributed in the outer field of view of the sky camera images for relatively low solar zenith angles. It is speculated that this is most likely due to an increase of scattered UV, compared to cases of increasing trends with wavelength, that would be accounted for by an increase in reflected UV from cloud surfaces in closer proximity to the sun. It also appears that wavelength dependency trends are related to the overall cloud fraction

Validation of improved sky camera algorithm for measurement of cloud around the Sun

A low cost automatic sky camera system was designed, constructed and tested for the use to study the effect of cloud on the amount of ultraviolet radiation (UV) at the ground level. This paper presents the results of a twelve-month comparison between the results obtained with the sky camera system and those of manual observation, with the major cloud type of Cumulus (36%). The sky camera system is the first to measure cloud properties near the sun, in contrast to whole-sky assessment. images were analysed using an improved image-processing algorithm estimating the cloud amount in an angular region of 37.5 around the sun for local noon at Toowoomba, Australia (27.6 oC). The state of solar disk obstruction was also determined, as it would be useful for assessing the effect of cloud on the UV level. It was found that the new algorithm compared favourably with a previous evaluation of an earlier system (85.2% for disk obstruction and 81.7% for cloud amount). The finding suggests that the camera system is suitable for the use of long-term radiation studies

Scattered UV beneath public shade structures during winter

[Abstract]: Broadband field measurements were conducted beneath three different sized public shade structures, small, medium and large, during the Southern Hemisphere winter. These measurements were compared to the diffuse UV to quantify the relationship of the UV under and around the shade structures to the diffuse UV. For the shade structures, a relationship between the diffuse UV and the UV in the shade has been provided for clear skies and solar zenith angles (SZAs) of 49o to 76o. This allows the prediction of the UV in the shade of these structures if the diffuse UV is known. The ultraviolet protection factors (UPFs) for the three shade structures ranged from 1.5 to 5.4 for decreasing SZA. For the greater SZAs of 70o to 76o, the erythemal UV in the shade was 65%, 59% and 51% of that in full sun for the small, medium and large structures respectively. For the smaller SZAs of 50o to 53o the erythemal UV in the shade was 35%, 41% and 18% for the small, medium and large shade structures respectively. From this research it can be concluded that the UV radiation levels in the shade in winter could cause erythema and other sun related disorders

Scattered and filtered solar UV measurements

Scattered and Filtered Solar UV Measurements provides comprehensive information about the scattered and filtered solar UV environment, the techniques to measure this radiation and the resulting UV exposures to humans. The incidence of skin cancer and sun-related eye disorders can be reduced by the minimization of exposures to UV radiation. For this to occur, a greater understanding of the solar UV exposure to humans for varying conditions and in different environments is necessary. Accordingly, this book aims to quantify, understand and provide information on the filtered and scattered solar UV. This book is aimed at students and new researchers in the field of solar UV and environmental measurements; however there is sufficient detail to provide a valuable reference for the experienced researcher. Researchers and students in atmospheric sciences, health sciences, environmental monitoring, photobiology and skin cancer prevention will find this monograph useful

Measured and modelled contributions to UV exposures by the albedo of surfaces in an urban environment

The increases in the erythemal UV exposures to horizontal planes and to inclined planes over three surfaces that are found in an urban environment (water, concrete and sand) due to the albedo of these surfaces have been estimated. For the cloud free case, the additional daily estimated UV exposures to a horizontal plane have a maximum value of 222 (J m-2), where the index after the unit is there to indicate that it refers to a biologically effective exposure. In comparison, the daily erythemal UV exposures over a year to a horizontal plane ranged from 425 to 8,321 (J m-2). For a vertical receiving plane that is rotating about a vertical axis, the additional erythemal daily UV exposures for the sub-tropical latitude location of this research for the ranges of solar azimuth angles encountered over the days in each season ranged from 16 to 311 (J m-2), 29 to 566 (J m-2) and 46 to 905 (J m-2) for water, concrete and sand respectively. The estimated error is ±20% and the calculations are based on clear-sky conditions. The additional erythemal UV averaged over each of the seasons was higher for the receiving plane inclined at 45 degrees below the horizontal plane. In a similar fashion, the vertical surface has the higher additional erythemal UV exposures compared to the surfaces inclined at an angle above the horizontal

The protective nature of public shade structures in Australia

[Abstract]: The specific nature of the role that solar UV radiation plays in the welfare of human beings is both good and bad, from helping bones absorb calcium more efficiently to the genesis of fatal skin cancers. As the publics understanding of the damaging effects associated with over exposure to UV radiation increases, shaded environments will be sought to reduce personal UV exposure. Local governments provide many shade structures at parks and sporting ovals for public use. However, the question remains of how effective are public shade structures at reducing biologically effective UV radiation throughout the year? In Australia, erythemal UV in full sun can reach levels of approximately 2.5 MED/h (where an MED is defined as the minimum erythemal dose) or more in the middle of the day during winter. Therefore, it is necessary for people that live in similar latitudes to minimise UV exposure in all climatic conditions throughout the year. Based on this research, a standard for reporting the UV protection provided by shade structures is essential for the public to make an informed decision on the efficacy of particular structures in reducing personal UV exposure