Many people have heard the term “urban heat island“. It refers to a meteorological phenomenon which causes urban areas to experience, among other things, higher temperatures than the surrounding areas. The difference in temperature can be as much as 15 degrees Celsius (especially at night).
New research suggests that cities can influence local weather in even more ways. Scientists reveal that many cities across the planet receive more rain than the areas surrounding them. Experts suggest that this discovery could lead to a change in the way cities of the future are built.
As Dev Niyogi of the University of Texas at Austin says: – Similar to the urban heat island effect, we are dealing with a rainfall effect in cities.
Experts have carefully analyzed satellite data regarding rainfall levels between 2001 and 2020 in 1,056 cities and nearby rural areas in different climatic regions.
Studies have revealed that more than 60 percent of cities are “wet islands” or “moist islands”. This means that the urban area received significantly more rain than the surrounding rural areas. In addition, some of cities were called “dry islands”where the opposite anomaly was documented.
DHo Chi Minh City is one of the wettest cities (formerly Saigon, a city in southern Vietnam) and Kuala Lumpur – here annual rainfall totals were more than 200 mm higher than the rural areas nearby. In turn, Lagos, Sydney, Sao Paulo, Houston, and Mexico City had annual rainfall totals more than 100 mm higher than the surrounding areas.
At the other extreme is Seattlethe cities of Kyoto-Osaka-Kobe, Jakarta and Quanzhou (China). Here, over 200 mm less rain falls annually than in rural areas. On the other hand, in Rio de Janeiro, Accra (Ghana), Lima, or Xiamen-Zhangzhou, about 100 mm less water falls annually.
Niyogi points out that The study is the first to reveal a global “urban” rainfall patternHe adds: – We need to look at rainfall and the city as a system of mutual interactions.
According to research, our country is dominated by cities which are also “wet islands”. However, the trend here is hardly noticeable. For comparison Warsawaccording to these studies, records about 25 mm more rain per year than the adjacent areas. In turn, in Łódź, Poznań and Gdańsk, about 10-15 mm more rain is recorded annually.
Scientists suggest that heat absorbed by buildings and asphalt can create updraftswhich over time can generate the formation of rain clouds. At the same time, different topography of the city (and the “roughness” of buildings) can result in rain clouds remaining in one place for a longer period of time.
Additionally air pollution in cities can both promote and limit cloud formation. At the same time, fewer green surfaces reduce evaporation and, consequently, less moisture in the air.
The above factors vary with the size and location of individual cities. It has been noted that larger and more populated cities were more likely to be “wet islands”. The largest anomalies were recorded in cities located in the temperate climate zone, in tropical and coastal regions. On the other hand, cities in mountainous areas in the vast majority of cases showed smaller disproportions.
The researchers also conclude that the average difference in annual rainfall between the “wet islands” and their surroundings increased from an initial level of 37 to 62 mm (the final study period). At the same time, the “dry anomalies” did not change. Meteorologists suggest that this may be related to rapid urbanization combined with global climate change.
The scientific article also states: “More than 60 percent of the world’s cities and their regions receive more rainfall than the surrounding rural areas. Moreover, the size of these urban “wet islands” has almost doubled in the past 20 years. Rainfall anomalies in cities show differences on different continents and in different climates, for example cities in Africa exhibit the largest urban annual anomalies and extreme precipitation.”
Current climate models ignore the impact of cities on local weather. In addition, researchers indicate that “wet cities” are more vulnerable to flooding, while “dry cities” should start building special water reservoirs.
The research results were published in a scientific journal PNAS.
Source: geekweek.interia.pl
