Different methods for estimating the mean radiant temperature in an outdoor urban setting

Abstract

The mean radiant temperature (T mrt) is one of the most important meteorological parameters governing human energy balance. In this paper, three different methods of obtaining the T mrt in an outdoor urban setting are compared. Method A is based on integral radiation measurements and angular factors, method B is based on measurements with a 38‐mm flat grey globe thermometer and in method C makes use of the Rayman 1.2 software is used. Measurements were performed in a large open square in a high latitude city—Göteborg, Sweden—during clear to overcast weather conditions in October 2005 and in July and August 2006. Results show that the difference between Method A and Method B was generally relatively small. Most of the discrepancy, caused by rapid changes in radiation, temperature and wind speed was smoothed out using 5 min mean values. By systematically and empirically changing the mean convection coefficient, the accuracy of Method B was improved and a new equation expressing the T mrt was obtained. With this new equation the 38 mm flat grey globe thermometer could successfully be used to estimate the T mrt in an outdoor urban setting provided that the wind speed and the air and globe temperatures are measured accurately. The study also shows that the flat grey colour of the globe thermometer slightly underestimates the level of short‐wave radiation (i.e. sunshine). Method C works very well during the middle of the day in July, i.e. at high sun elevations. However, the model considerably underestimates the T mrt in the morning and evening in July and during the whole day in October, i.e. at low sun elevations. In outdoor urban settings where thermal comfort researchers or urban planners and designers require an easy and reliable method of estimating mean radiant temperature, the 38 mm flat grey globe thermometer provides a good and cheap solution. Copyright © 2007 Royal Meteorological Society