A Variable Sky-View Platform for the Measurement of Ultraviolet Radiation

One of the more difficult tasks confronting atmospheric researchers today is the acquisition of long-term radiometric measurements that encapsulate variability in the sky hemisphere as well as time. High quality spatial measurements would allow for a greater understanding of atmospheric processes, both specific to the site and between locations. This information would lead to further development of better spatial models for predicting future trends. This study develops a timer-controlled variable sky-view platform (VSP), designed to encapsulate both the spatial and temporal variation of radiation in the sky hemisphere for Hobart, Australia (42.90°S, 147.33°E). The VSP essentially allows for a sensor to move up and down inside a fixed cylinder, therefore causing the skyview factor to vary. A full description of the VSP design, electronics, and construction is given. Emphasis is on high-precision long-term operation under all weather conditions. A modeling scheme and output results illustrate the advantages of the VSP over current sky-observing instruments. It can accurately parameterize the direct and diffuse erythemal irradiance as well as the azimuthally integrated erythemal sky radiance distribution for every 2-min duration (at the current setting). In addition, all data can be obtained for clear-sky and cloudy conditions. The VSP is currently in use with the erythemal ultraviolet (UV) biometer 501-A manufactured by Solar Light Company, Inc. (SLC). As such, it provides an additional source of UV sky distribution data to the Hobart UV-climatology database.