Modelling of sample surface temperature in an outdoor weathering test

A heat transfer model is constructed to derive the surface temperature of specimens in outdoor weathering. Data on thermal properties of materials and routinely collected weather data are used as inputs for the model. The model is validated against surface temperature data measured on samples of 11 different materials exposed to natural outdoor weathering in Jokioinen, Finland, over a period of 12 months. The agreement between the model and the surface measurements is more than satisfactory with a mean difference less than 0.5 K and a root-mean-square difference ranging from 0.4 to 1.4 K (0.1-0.5%), depending on material. The mean differences between the surface and ambient temperatures range from −0.1 to 1.3 K. Use of ambient temperature instead of sample surface temperature is shown to lead to severe misjudgements in estimations of reaction rates.