Watering outdoor surfaces can lower temperature by up to 10 degrees Celsius, says architect

Architect Julia Ayuso suggests that a simple act, watering outdoor surfaces like patios and sidewalks, can significantly reduce their temperature, offering a practical way to combat urban heat. This traditional practice, known in Japan as 'uchimizu,' involves wetting the ground, particularly in the early morning or at sunset. Ayuso, who specialized in sustainability and lived in Tokyo for two years, became interested in this custom after observing its prevalence.

Her research into available studies revealed compelling data: "The ground can cool down by 6 to 10 ºC, while the air temperature drops considerably less, around 1.5 ºC," she stated in an interview with La Vanguardia. While the difference in air temperature might seem small, Ayuso explains its significance lies in how the human body perceives heat. We feel not only the air temperature but also the radiant heat from surrounding surfaces like pavement and buildings. Cooling these surfaces can therefore make a noticeable difference in perceived temperature.

However, Ayuso cautions that watering is not a universal solution for high temperatures. Its effectiveness depends heavily on environmental conditions. It works best in dry climates, on surfaces that can retain moisture, and when the pavement is no longer exposed to direct sunlight. In a country like Spain, water consumption is also a critical factor. Ayuso emphasizes that this technique should be seen as a supplementary measure, not the primary response to heat. "Watering is the last gesture, not the first," she summarized.

Explaining the science, Ayuso noted that when water hits a hot surface, it evaporates. This process requires heat, which is drawn primarily from the surface itself and secondarily from the air. This explains the significant temperature drop on the ground compared to the air. She referenced measurements in Arizona where, with an air temperature of 48 ºC, the radiant temperature reached 76 ºC, a nearly 30-degree difference in what the body perceives. The ground, being a major emitter of this radiant heat, benefits greatly from cooling. One limitation, however, is the duration of the effect, which was measured over an entire summer in Kobe, showing a noticeable impact on sunny days.