During all the recent extreme heat, you may have found yourself feeling really hot even while in the shade of a parasol or beach umbrella. Well, it isn鈥檛 just you 鈥� calculations suggest that parasols reduce the effective temperature only by around 1掳C (1.8掳F).
鈥淭he difference is not as big as one might think,鈥� says at the University of Innsbruck in Austria.
The findings could have some practical implications, he says. Many cities are now taking steps to try to help people stay cool as summers get warmer, such as installing water sprays and increasing shading. Standard fabric shades are much less effective than trees, Wohlfahrt鈥檚 findings confirm. But wetting fabric parasols or making parasols from materials that radiate less infrared light would boost their cooling effect.
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Wohlfahrt became interested in the issue while sitting with colleagues under a parasol in a beer garden. 鈥淚t was freaking hot, and there was one guy who had this smartphone with an infrared camera. He pointed it up, and we realised, wow, the parasol is 70掳C [158掳F].鈥�
The problem, the researchers realised, is that while the parasol was blocking shortwave radiation from the sun, the hot fabric was emitting longwave radiation 鈥� infrared 鈥� down at them, thus counteracting the cooling effect of blocking the shortwave radiation. So what difference does it make overall?
To find out, Wohlfahrt and his colleague , also at the University of Innsbruck, created a mathematical model that estimates how much shortwave is blocked by a parasol and how much longwave the parasol emits downwards. It also takes into account how much longwave radiation is emitted upwards by hot sand that is either in the sun or in the shade of the parasol.
The model uses this to calculate the universal thermal climate index (UTCI), a measure of how hot a person would feel, which takes into account radiant heat, humidity and wind speed, as well as air temperatures.
Wohlfahrt and Hammerle then compared the UTCI of a person standing in the sun with that of someone under a parasol on a sandy beach in various conditions, such as at different times of day or varying cloudiness. According to the model, the median difference is just 1掳C cooler under the parasol, and when the sun is low, it can be up to 1掳C hotter under the parasol.
鈥淚 cannot say if those are the correct numbers, but definitely it鈥檚 less that one might expect if thinking only of the shading of the shortwave solar radiation,鈥� says at the Finnish Meteorological Institute in Helsinki.
Rinne is developing a similar model to estimate carbon emissions from the soil at a former peat extraction site now covered in solar panels. 鈥淭he parasol case is similar to a solar photovoltaic panel, which shades the ground below from the shortwave solar radiation 鈥� converting some of that to electricity 鈥� but heats up and emits increased amounts of longwave radiation due to the warming of the panel,鈥� he says.
Wohlfahrt says he plans to do actual measurements to confirm the model is correct. He presented his and Hammerle鈥檚 results at a meeting of the European Geosciences Union in May.
A 1998 study found that people could become sunburnt even in shade, because of dust particles in the air reflecting UV light sideways. The study didn鈥檛 include beach umbrellas, but suggests they provide partial protection from sunburn at best.
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