The Po River Valley, one of Italy’s most densely populated regions, is home to over 16 million people. The valley is also one of the country’s foggiest and haziest regions.
In the fall and winter months, fog and low clouds routinely blanket the basin, often preceded by haze—a mixture of airborne particles, known as aerosols, emitted by vehicles, fires, industry, and other sources. The surrounding mountain ranges limit air circulation, helping fuel temperature inversions that trap air pollution near the surface.
The MODIS (Moderate Resolution Imaging Spectroradiometer) on NASA’s Terra satellite captured this image of morning fog on November 16, 2024. “It formed when a high-pressure system over central Europe led to calm and stable conditions in the valley, ideal for the formation of radiation fog,” said Eva Pauli, a research meteorologist at the Karlsruhe Institute of Technology in Germany.
This type of fog typically occurs in the fall and winter, under clear skies, when the land surface cools at night and chills air near the surface, fueling condensation. Aerosol particles contribute to the formation of fog and other types of clouds by serving as “seeds”, or cloud condensation nuclei, that attract water vapor and develop into tiny spherical cloud drops, the building blocks of clouds.
Pauli co-authored an October 2024 study that showed hazy conditions in the valley were linked to, and often preceded, long fog events. By analyzing satellite observations collected between 2006 and 2015 and aerosol data recorded by ground-based AERONET (Aerosol Robotic Network) stations in the region, Pauli and colleagues found that fog accompanied by high loads of aerosols lasted up to three hours longer over the study period compared to fog in low aerosol conditions.
On November 16, fog lingered well into the afternoon, likely due to aerosols, according to Pauli. An AERONET station at Modena recorded what Pauli described as aerosol optical depth measurements that were “quite high,” and MODIS detected elevated concentrations of aerosols as well.
“We haven’t done a full meteorological analysis yet, but it’s clear that fog remained in the center of the valley for more than 24 hours,” Pauli said. “That matches the pattern we found in our previous analysis.”
A better understanding of the role that aerosols play in prolonging fog outbreaks could improve weather forecasts and perhaps help minimize the harmful impacts of fog, Pauli said. Fog is associated with tens of thousands of traffic accidents and hundreds of deaths in the United States each year, according to the U.S. Federal Highway Administration.
NASA Earth Observatory image by Wanmei Liang, using MODIS data from NASA EOSDIS LANCE and GIBS/Worldview. Story by Adam Voiland.
