A technique for Quantifying the Reduction of Solar Radiation due to Cloud and Tree Cover

Abstract

The micro-climate of a domestic residential landscape can affect both the energy use of the dwelling and the human thermal comfort within that landscape. Radiant energy produced from, or reduced by different landscape elements such as trees, and hard and soft surfaces, directly affects the amount of heat incident on the walls of the residence or on people present in the garden. Quantifying this energy will enable the development of a relative scale of thermal performance for these elements and consequently for the landscape as a whole. This gives a measured consequence for each landscape design, allowing comparisons and hopefully improvements, between and within designs. Radiant energy is produced from direct or diffuse solar short wave and infrared radiation and longwave radiation from heated landscape elements. This paper presents a technique which has been developed for inexpensively and easily estimating the amount of incident radiation reduced by cloud and by three different tree types. The measurement surfaces of cheap temperature sensors with data logging capabilities (iButtons) were coated with either a white gloss or a matt black paint. White gloss paint has an emissivity of ~0.9 in the longwave spectrum but only ~0.3 in the short wave, whereas matt black paint has an emissivity of ~0.95 for both and can be used to detect both short and long wave radiation. The temperature difference between the two gives a measure of the amount of shortwave radiation or visible light. This enabled measurements of cloud shade and local plant shade, and an estimation of the quantity of that shade when compared with full sky exposed reference iButtons. The iButtons can be mounted concurrently at numerous points around a house envelope or in a landscape, at multiple house locations to determine the quantity of shade provided by different native and introduced plant species.