China's viral 'roof rain' system explained

As heatwaves pushed parts of the world toward emergency cooling measures this summer, one Chinese apartment complex found a lower-tech fix: turning its rooftops into mist machines.

The system, now nicknamed "roof rain", has drawn international attention for cooling entire streets without soaking a single resident below.

It is located on top of eight apartment buildings in the Xijian Tianmao Guobinfu community of the city of Yuncheng in the Shanxi province of China, where there are 200 high-pressure nozzles on each rooftop.

According to property manager Jia Wen, interviewed by China National Radio, the mist lowers the temperature of the surroundings by 5 to 8 degrees Celsius, which equals about 9 to 14 degrees Fahrenheit.

It has been working since August 2024 quietly in the region until this month.

Roof rain uses the same mechanism of lowering temperature as our body does when we feel hot: evaporative cooling. When the particles of mist absorb the heat in the surrounding environment, some of the particles get evaporated along with the heat, and thus they cool down the air around them.

Since the water spreads in the form of vapour and not through rainfall, the people living down there remain dry due to this side effect of rainbow formation in the fog.

The use of this technology remained mostly unknown until July 1, when a video of the technology was posted on social media by Chinese foreign ministry spokeswoman Mao Ning.

The French media had already been talking about the use of this technology amid Europe’s heatwave, citing it as an efficient means of outdoor cooling.