The more polluted the air, the less it rains. This is the finding of a study conducted by a group of researchers from the University of Milan-Bicocca and published in Scientific Reports. The research, entitled "Variability of orographic enhancement of precipitation in the Alpine Region", examined, for the first time, rainfall data – the amount of rain that fell over a given period of time – collected from more than 3,000 stations located in various Alpine regions, calculating how the data varied over time and showing that in the mid-1980s there was a peak reduction in rainfall in the lowland regions. A peak that coincided with the high concentration of aerosols in the atmosphere.

"These results provide new insights to deepen our understanding and improve forecasts of the anthropogenic effects on mountain precipitation, which are fundamental for safety and water resource management," reads the research abstract.

Less rain in the north, pollution among the causes

The research team led by associate professor of oceanography and atmospheric physics Claudia Pasquero examined two distinct periods: from the 1950s to the 1980s, and from then to the present day. In the second phase, the greatest variations in precipitation were observed in mountainous areas. The decrease in rainfall in the north is considered a possible consequence of ongoing global warming, while the different distribution between lowlands and mountains represents a novel finding. In particular, the greatest gap between rainfall at altitude and in the valley was recorded in the 1980s, precisely when dust particles from human activities reached their peak. "Dust plays a fundamental role in cloud formation and, by obscuring the sun, causes a drop in ground-level temperature, altering atmospheric dynamics," the university explains.

Precipitation depends on climate change and pollution

"In planning and managing water resources, the regions of northern Italy will need to consider, alongside the effects of global warming, local emissions of pollutants as well," Professor Pasquero explains in a university press release. "Only in this way will it be possible to preserve mountain water resources, which to date have guaranteed the strong development of society in the Po Valley." Not only is climate change already influencing the amount and manner in which rainfall occurs across the Alpine arc and the lowlands connected to it, but we now know that rainfall can also be negatively affected by air pollution, which is present in high concentrations precisely in the Po Valley.