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Rouen sits nestled within the deeply incised valley of the Seine, serving as a critical nexus between Paris and the English Channel. This specific topography—a steep-sided valley—plays a decisive role in the city's atmospheric dynamics. The urban core is characterized by a dense historical center that transitions rapidly into industrial zones and strategic logistical hubs along the riverbanks. To the north and south, the terrain rises into the plateau of the Pays de Caux and the plateau of the Seine, creating a natural basin. This bowl-like configuration is problematic for air quality; pollutants from the heavy petrochemical clusters and port activities often become trapped near the valley floor. The proximity to the coast brings moist Atlantic air, but the valley walls can obstruct the dispersing power of prevailing westerly winds. Surrounding the urban periphery, a mix of intensive agriculture and fragmented forests and varied undulating landscapes creates a complex urban–rural gradient. The interaction between the river's moisture and the industrial emissions frequently produces a dense haze. Because the city is positioned at a low elevation relative to its surrounding plateaus, cold air often settles in the valley bottom, suppressing the vertical mixing of air masses. This geographic confinement ensures that emissions from road transport and industry remain concentrated, making the city's air quality highly sensitive to subtle changes in wind speed and direction, effectively turning the Seine corridor into a localized pollution trap that requires constant environmental vigilance.

In Rouen, air quality follows a distinct seasonal rhythm dictated by the oceanic climate and valley topography. Winter is the most challenging period, as frequent temperature inversions occur when cold air settles in the Seine valley, trapping nitrogen dioxide and particulate matter and industrial effluent from heating and traffic. During these stagnant months and atmospheric stability, pollutants accumulate near the ground, making January and February the highest-risk periods for sensitive groups, such as asthmatics, who should limit strenuous outdoor exercise. Spring brings a transition, where increasing rainfall helps scrub the atmosphere, though early spring can still see spikes in ozone precursors as temperatures rise. Summer often presents a different challenge; higher solar radiation triggers the formation of ground-level ozone, particularly during heatwaves when air remains stagnant. July and August can see poor air quality alerts, urging residents to avoid midday sun. Autumn is generally the most favorable season, characterized by stronger westerly winds that flush the valley and clear the air. However, late autumn fogs can occasionally trap pollutants in a damp, low-lying layer. For the general population, the autumn months offer the best window for outdoor activities. Health guidance emphasizes monitoring local alerts during winter inversions and summer ozone peaks. By understanding these meteorological drivers, residents can better manage their exposure, favoring the breezy autumns and rainy springs while remaining cautious during the oppressive stillness of winter mornings and the scorching heat of mid-summer afternoons to protect their respiratory health.