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Climate Science

A storm factory

Concentrated heat and pollution in the São Paulo Metropolitan Area lead to heavy rainfall during the week

EDUARDO CESARRainstorms over the Greater São Paulo have a set time and place to materialize. The heavy rains that flood some parts of the city and 39 neighboring municipal regions that are part of South America’s biggest metropolitan region tend to fall at the end of the afternoon, on Mondays or Wednesdays. In this concrete and asphalt jungle which is home to nearly 19 million people, the temperature rises progressively as the day advances and millions of automobiles invade the streets. When the temperature reaches its peak in the early afternoon, the clouds become increasingly heavier or darker until they pour an ocean of water onto the metropolis several hours later, especially in certain specific areas of this capital. It is easy to envisage the immediate impact of these rainstorms: the city’s heavy traffic, which easily reaches 150 kilometers of vehicles crawling bumper-to-bumper in the city’s main thoroughfares toward the end of the afternoon, grinds to a halt; the city’s inhabitants take hours to get home from work; and, in the underprivileged neighborhoods, people lose their few possessions in a sea of water and mud.

It is useless to blame nature only for this rain storm pattern that frequently befalls the capital city during the summer months, from December to March, say geographers Aílton Alves Filho, from FEI engineering college, and Helena Ribeiro, from the University of São Paulo school of public health. In an attempt to identify a pattern in the heavy rains that torment São Paulo, Alves studied how the rainfall was distributed over the metropolitan region’s cities, from the early eighties to the early nineties. He focused his analysis on the summer of 1991, a period which experienced the heaviest rainfall in the 20th century. Alves and Helena found that of the 17 heaviest rain storms in the period, 35% occurred on a Monday, 20% on a Tuesday and 20% on a Wednesday. That summer, there were no rainstorms on weekends, according to a recently published study in the journal InterfacEHS.

But researchers still could not understand why this happens. By comparing data from meteorological radars, records of rainfall intensity and pollution indexes, Alves and Helena concluded that one can only explain what happens in São Paulo if the influence of its inhabitants is also taken into account. The city’s location makes it susceptible to heavy rains, especially in the summer. São Paulo and several neighboring cities are in the middle of a corridor formed by the Serra da Cantareira mountain range to the north and by the Serra do Mar mountain range to the south; this facilitates the entry of damp masses of air from the Atlantic Ocean, which is just 45 kilometers away as the crow flies.

However, these geographical characteristics do not explain everything. Over the last century, the population in São Paulo City alone increased some 40 times, reaching almost 11 million inhabitants. São Paulo’s urban sprawl and that of the towns surrounding it has invaded native vegetation, now limited essentially to city and state parks. The substitution of these green areas with thousands of kilometers of asphalt and hundreds of tons of concrete have transformed the entire region into a greenhouse that stores sunlight during the day. For instance, relative to 1940, the city of São Paulo, for example, has become warmer by an average of 2 degrees Celsius. The higher temperatures provoked by this urban sprawl – an effect referred to as islands of heat – go hand in hand with the considerable heat generated by fuel from the 3,5 million cars driving around the city daily. This equals one tenth of the energy that the city gets from the sun. Years ago, geographer Tarik Azevedo, from USP, analyzed historical rainfall series during the nineties and confirmed that the heavy rains fell mostly on weekdays, rather than on weekends or holidays, when human activities generated less heat in the city. “The city does not produce the rainstorms, but helps generate them under the appropriate conditions,” says Azevedo. “This data should be used as an argument to encourage increased use of public transportation.”

By considering all these elements, Alves and Helena were able to explain the concentration of rainfall on weekday afternoons. The temperature in the Greater São Paulo rises during the day until it peaks in the early afternoon. Meanwhile, the air becomes drier and the ensuing humidity forms clouds. If the ocean breeze, which blows from sea to the land in the early afternoon, blows in time to bring more humidity, this produces the conditions for a very wet and chaotic late afternoon in the big city. “These heavy rains concentrate mostly on the central, north and east regions of São Paulo, which are more heavily populated,” says Alves. “In the last few years, the city and state governments have invested money in reducing the rain damage. But these investments will only yield palliative results if no urban zoning changes are implemented,” says the geographer from FEI.