Calculating the Cosine of the Solar Zenith Angle for Thermal Comfort Indices

The cosine of the solar zenith angle (cossza) is a key component in Mean Radiant Temperature (MRT). Mean Radiant Temperature is used in the calculation of the Universal Thermal Climate Index (UTCI) and can be used in the calculation of Globe Temperature a component of the Wet Bulb Globe Temperature (WBGT), both of which are important thermal comfort and heat stress indices. It has previously been demonstrated that in numerical weather prediction services cossza should be integrated over a time step for the most accurate results. Here, we present the comparison of the operational cossza being used to create ERA5-HEAT, an instantaneous approach and a Gauss-Legendre Integration cossza. We further calculate MRT and UTCI for the ERA5-HEAT method and the methodology in the thermofeel library and see discrepancies in the approaches of on average -1.5K for MRT and -0.42K for UTCI. We suggest that the methodology in the thermofeel library supersedes the operational c code and is published alongside the existing ERA5-HEAT dataset in addition to forecast data being published, for users to make their own comparisons and extend this data’s usefulness. We also suggest that a sensitivity analysis of the UTCI is carried out to aid better understanding of this thermal comfort index.