The thermal cost of sitting under a parasol: a biometeorological essay

For many people, beaches are a place they long for and emblematic for summer vacation vibes, even though environmental conditions may be actually physiologically stressful. In order to reduce radiation load, and thereby also exposure to UV radiation, the use of parasols for shading is thus common practise. The parasol, depending on the optical properties of the used fabric, attenuates part of the solar (shortwave) radiation, however at the expense of additional longwave radiation radiated in the downward direction in proportion to the surface temperature of the parasol. Here we ask the question whether sitting under a parasol may actually increase thermal discomfort as the reduction in transmitted shortwave radiation may be compensated by an increase in downward longwave radiation. To this end we have developed a model which allows simulating human thermal comfort in the open (without parasol) compared to below a parasol on a beach. Human thermal comfort is quantified with the Universal Thermal Comfort Index (UTCI). Environmental model inputs are air temperature and relative humidity, mean horizontal wind speed and incident short- and longwave radiation at some reference height above the ground surface. The attenuation of shortwave radiation by the parasol, the upward longwave radiation flux from the sand and the downward longwave radiation flux from the parasol are calculated by solving the radiative and energy balance of the parasol and the sand surface. The radiation calculations below the parasol take the modification of upper and lower hemispheric view factors into account and separately solve for the temperature of the sunlit and shaded sand surface. Our calculations show that the UTCI is generally lower under the parasol (and thus human thermal comfort higher), but differences are often small. Moreover, under certain combinations of conditions, sitting under a parasol feels hotter and we discuss which conditions favour this outcome. Finally, we demonstrate our findings for summertime conditions at some of the globally most well-know beach destinations.