Comparison of different methods of estimating the mean radiant temperature in outdoor thermal comfort studies
Correlations between outdoor thermal indices and the calculated or measured mean radiant temperature Tmrt are in general of high importance because of the combined effect on human energy balance in outdoor spaces. The most accurate way to determine Tmrt is by means of integral radiation measurements, i.e. measuring the short- and long-wave radiation from six directions using pyranometers and pyrgeometers, an expensive and not always an easily available procedure. Some studies use globe thermometers combined with air temperature and wind speed sensors. An alternative way to determine Tmrt is based on output from the RayMan model from measured data of incoming global radiation and morphological features of the monitoring site in particular sky view factor (SVF) data. The purpose of this paper is to compare different methods to assess the mean radiant temperature Tmrt in terms of differences to a reference condition (Tmrt calculated from field measurements) and to resulting outdoor comfort levels expressed as PET and UTCI values. The Tmrt obtained from field measurements is a combination of air temperature, wind speed and globe temperature data according to the forced ventilation formula of ISO 7726 for data collected in Glasgow, UK. Four different methods were used in the RayMan model for Tmrt calculations: input data consisting exclusively of data measured at urban sites; urban data excluding solar radiation, estimated SVF data and solar radiation data measured at a rural site; urban data excluding solar radiation with SVF data for each site; urban data excluding solar radiation and including solar radiation at the rural site taking no account of SVF information. Results show that all methods overestimate Tmrt when compared to ISO calculations. Correlations were found to be significant for the first method and lower for the other three. Results in terms of comfort (PET, UTCI) suggest that reasonable estimates could be made based on global radiation data measured at the urban site or as a surrogate of missing SR data or globe temperature data recorded at the urban area on global radiation data measured at a rural location.
KeywordsThermal indices Urban microclimate monitoring Mean radiant temperature RayMan
- Bird RE, Hulstrom RL (1991). A simplified clear sky model for direct and diffuse insolation on horizontal surfaces. SERI Technical Report SERI/TR-642-761, Solar Energy Research Institute, Golden, CO.Google Scholar
- ISO 7726 (1998) Ergonomics of the Thermal Environment Instruments of measuring physical quantities. ISO, SwitzerlandGoogle Scholar
- ISO 7730 (2005) Ergonomics of the Thermal Environment: Analytical determination and interpretation of thermal comfort using calculation of the PMV and PPD indices and local thermal comfort criteria. ISO, SwitzerlandGoogle Scholar
- ISO 8996 (2004) Ergonomics of the thermal environment: determination of metabolic rate. ISO, SwitzerlandGoogle Scholar
- Johansson E, Emmanuel R, Thorsson S, Krüger E (2013). Instruments and methods in outdoor thermal comfort studies. The need for standardization. Paper submitted to Urban Climate (in press).Google Scholar
- Krüger E, Drach P, Emmanuel R, Corbella O (2012) Assessment of daytime outdoor comfort levels in and outside the urban area of Glasgow, UK. Int J Biometeorol 57:1–17Google Scholar
- LUCID (2013). The development of a local urban climate model and its application to the intelligent design of cities. http://www.homepages.ucl.ac.uk/~ucftiha/overview.html. Accessed 1 Sept 2013.
- Matzarakis A (2013) Stadtklima vor dem Hintergrund des Klimawandels. Gefahrstoffe - Reinhaltung der Luft 73:115–118Google Scholar
- Thorsson S, Lindberg F, Eliasson I, Holmer B (2007) Different methods for estimating the mean radiant temperature in an outdoor urban setting. Int J Biometeorol 27:1983–1993Google Scholar
- UK MET OFFICE (2011). Western Scotland: climate. <http://www.metoffice.gov.uk/climate/uk/ws/print.html. Accessed 16 Jun 2013.
- VDI (1998) Methods for the human-biometeorological assessment of climate and air hygiene for urban and regional planning, Part I: Climate, VDI guideline 3787. Part 2. Beuth, BerlinGoogle Scholar