On a global scale, most large cities are urban wet islands. Photo: AFP
City dwellers often complain about bad weather. An American study published in the scientific journal PNAS proves them right. Its authors claim that many cities around the world are more prone to precipitation than the surrounding countryside.
Researchers from the Universities of Texas and Georgia came to this conclusion after analysing rainfall data from 1,056 cities worldwide between 2001 and 2020, as well as from the surrounding rural areas.
They found that 63% of these metropolises are more exposed to rainfall than the rural areas surrounding them.
In some cases, the difference is striking. For example, an average of 127mm more rain falls in Houston (US) than in the surrounding countryside. The same phenomenon can be observed in Ho Chi Minh City (Vietnam) and Sydney (Australia), where more than 100mm more rain falls per year than in the surrounding area. This makes these two cities “urban wet islands”.
The opposite pattern can be observed in other global metropolises. In Seattle (US) and Rio de Janeiro (Brazil), for example, it rains much less than in the surrounding area, making them “dry islands”. Generally speaking, scientists have found that “urban dry islands” are cities located in plains or valleys. The surrounding mountains would expose them less to precipitation.
But on a global scale, most large cities are urban wet islands. Zong-Liang Yang, professor of geology at the University of Texas and co-author of the study, argues that the design of urban space has a significant impact on this meteorological phenomenon.
“The buildings further enhance (the convergence of air toward city centres) by slowing the winds, resulting in a stronger upward motion of air. This upward motion promotes the condensation of water vapour and cloud formation, which are critical conditions for producing rainfall and precipitation,” he explains in a press release.
Population density also influences the amount of precipitation recorded in cities. This is because large populations generally create denser, more extensive urban areas, which produce more greenhouse gas emissions. This, in turn, disrupts the water cycle and increases the potential for heavy precipitation.
“If the local climate is hotter, if it’s wetter, then it may have a larger rainfall anomaly compared to the cities in cooler, drier places,” points out Xinxin Sui, co-author of the study, in the same press release.
Climatologists have long known that cities can concentrate precipitation. But this study is the first to show that this is a global phenomenon. In future, urban planners could use the findings of this research to design cities differently, and better prepare them for the consequences of climate change. – AFP Relaxnews
