Why sitting under a parasol is barely cooler than being in the sun

Recent extreme heat events have left many people feeling hotter even when under the shade of a parasol or beach umbrella, according to calculations. While parasols block shortwave radiation from the sun, they emit longwave radiation or infrared heat back down, negating their cooling effect. This means that the effective temperature reduction by parasols only amounts to around 1°C (1.8°F), according to Georg Wohlfahrt, a researcher at the University of Innsbruck in Austria.

The findings could have practical implications, as cities are seeking ways to help people stay cool in the face of increasingly extreme heat. While standard fabric shades are less effective than trees, Wohlfahrt's findings confirm that wetting fabric parasols or using materials that emit less infrared light could improve their cooling impact.

The researchers' interest in the issue arose during a hot day in a beer garden, where an infrared camera revealed the parasol's surface temperature at 70°C (158°F). To determine the overall impact of parasols, Wohlfahrt and his colleague Albin Hammerle developed a mathematical model that measures how much shortwave radiation is blocked and how much longwave radiation the parasol emits downwards.

This model also considers the longwave radiation emitted upwards by hot sand, either in the sun or beneath the parasol, and calculates the universal thermal climate index (UTCI), which measures how hot a person feels. The model accounts for radiant heat, humidity, wind speed, and air temperatures. The results show that the median difference in UTCI between standing in the sun and under a parasol is just 1°C cooler, and in some conditions, it can even be up to 1°C hotter under the parasol.

Experts caution that these numbers may not be entirely accurate, but the study highlights the limited effectiveness of parasols in providing relief from extreme heat.