Cool la neighborhoods. Image: los angeles department of public works
To lower temperatures in urban heat islands, you can paint streetlights and rooftops to reflect sunlight, but there’s a side effect that hasn’t been noticed yet
Cities have microclimates because of the many sealed surfaces that store heat; they are urban heat islands (uhis) compared to the surrounding countryside. Differences of up to two meters in height can be as much as 10 degrees celsius, as in beijing, which will lead or has already led to health problems as the climate warms up. At night the warmth is released again and prevents cooling down. This is not only the result of the fact that there is less green or buildings blocking air corridors, but that the sealed surfaces are made of materials with different thermal properties, which already because of their dark color, like roofing or asphalt, absorb more heat than vegetation and radiate it especially at the height of 2m, where people stay.
It is obvious that the temperatures in the residential areas of the richer are usually lower than in those of the poorer. The richer people like to live in neighborhoods with many single houses, gardens and parks, in the poorer neighborhoods the buildings are usually denser and there are fewer trees or gardens, so that it is not only warmer here, but it also cools down less during the night. Dealing with the urban climate is therefore also a social ie.
New study shows only slight cooling effect
A remedy could be roofs, facades or pavements, which reflect the sunlight similar to snow or ice. You could simply make them female instead of gray or red, which would increase the albedo, or green roofs and facades. The streets could be made of cobblestones as in the past, which at least allowed them to absorb rainwater. Or they cover streets and squares with white paint or a coating that reflects sunlight.
Sounds plausible and effective at first, even if the reduction in air temperature at a height of two meters above the ground seems to be only slight, namely between 0.5 and 1 degree, if all urban surfaces reflect sunlight, as urban climate researcher ariane middel and colleagues from arizona state university in tempe have calculated for a suburb of phoenix on the basis of meteorological quantities with a numerical flow mechanics. Here, 19 percent of the land is used for roads and 15 percent for buildings, mostly single-family homes. This is not typical for inner cities, so other scientists came up with much higher cow effects up to 5 degrees. Even a white city with its buildings, streets and squares is sealed and not green, which can be changed only by greening the roofs and facades and demolishing the sealed areas.
Reflected heat is absorbed by people
Middel, together with the urban planner v. Kelly turner (ucla) also found a side effect with amounts of unshaded streets with normal and reflective asphalt in pacoima and sun valley, as citylab reports. Air temperature, humidity, wind speed and radiation were measured from morning to evening on a day in summer. Although reflective asphalt like coolseal lowers the temperature of the city, the people walking on the apparently cooler sealed surfaces do not experience a cooler environment, but it becomes warmer for them because they absorb the reflected warmth. While this doesn’t play a gross role overall, it does make life in the city a bit far less enjoyable.
The mean radiant temperature, which is used to measure the comfort of humans in an environment based on the warmth/heat emanating from surfaces, was 7 degrees higher at noon on the sunny day above the cool pavement of the street. Later in the day, the temperature went down, but even in the afternoon, the strawberries felt 3 degrees warmer. Normally, asphalt absorbs heat and slowly releases it into the air; with the reflective pavement, heat is radiated at an average rate of 130 watts per square meter. This is comparable to a 10 percent increase in solar radiation.
Many factors come into play here, so the observation cannot be generalized, but what is clear is that even such reflective surfaces, which are climatically positive and reduce the uti effect, can be not only uncomfortable but also hazardous to the health of residents traveling on the roads. Add to that the fact that there could be racial consequences, because people with darker skin were more heat absorbing. That it would be better to wear light clothes than dark ones is beside the point.
What can have a negative impact on people who spend time in the public space and move around on foot or by bicycle, which is what you want, is of course different for reflective roofs, unless they are roof terraces. However, this could be less effective for the city in terms of advertising, and whether the homeowners will do this if there are no demand inspections is questionable or not. Rather unlikely. In any case, to change the microclimate of a city, this had to be done in a crude way.
What could be done? You could limit the street space and replace it with a flat surface or simply plant more trees, even at the expense of parking spaces or traffic lanes. The latter was actually reflective asphalt uberflussig, however, it takes time and care until the trees are coarse enough to provide sufficient shade.
Painting the streets with a reflective coating is probably more politically interesting because the effects of the decrease are more visible in the short term, as is the case in los angeles with the program "cool la neighborhoods" happens. The strains are coated with coolseal, which can be used to treat 40.The project, which costs us$ 000 per mile and will last for seven years, will. The street net is 35.000 km long and covers more than 500 square kilometers of park area. However, there was also one in the spring "tree summit", where they discussed how to plant more trees. In the long run, this would not only be more favorable, but it would also improve the climate of the city in a more sustainable way, and not just lower the temperature.