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Sagamihara, situated in Kanagawa Prefecture within Japan's Kanto region, occupies a distinctive geographic position that significantly influences its air quality patterns. Nestled between the densely urbanized Tokyo-Yokohama metropolitan area to the northeast and the forested Tanzawa Mountains to the northwest, the city straddles an urban-rural gradient that creates complex pollution dynamics. With coordinates at 35.5667°N, 139.3667°E, Sagamihara lies approximately 40 kilometers southwest of central Tokyo, placing it within the Greater Tokyo Area's industrial and transportation corridors. The city's terrain transitions from relatively flat eastern sections near the Sagami River basin to gently rolling hills and foothills in the west, with elevations ranging from about 20 to 200 meters above sea level. This topography, combined with proximity to Sagami Bay and the Pacific Ocean to the south, facilitates both marine air penetration and occasional stagnation when winds are light. The Sagami River flows through the city, providing some natural ventilation corridors, while surrounding agricultural zones in western areas contribute minimal particulate matter compared to urban emissions. Sagamihara's position within the Keihin Industrial Zone's extended influence means it receives transported pollutants from Tokyo Bay industries via prevailing northwesterly winds in winter, while summer southerlies bring cleaner oceanic air. The city's substantial population of over 720,000 generates significant local emissions from transportation and residential sources, yet its geographic setting allows for better dispersion than more densely packed central Tokyo wards, resulting in generally moderate air quality that reflects both regional transport and local urban characteristics.

Sagamihara experiences distinct seasonal air quality variations driven by Japan's temperate climate and regional meteorological patterns. During winter (December-February), cold, stable conditions frequently create temperature inversions that trap pollutants near the surface, particularly when high-pressure systems dominate. These inversions, combined with increased heating emissions and transported industrial pollutants from the Keihin coastal zone, typically make late January and February the period of poorest air quality. Sensitive groups should limit prolonged outdoor exertion during calm, cold mornings when particulate matter concentrations peak. Spring (March-May) brings gradual improvement as increasing solar radiation breaks up inversions and southerly winds begin, though March can still experience poor dispersion during transitional weather. The rainy season (June-July) provides natural cleansing through precipitation, resulting in generally good air quality despite high humidity. Summer (August-September) offers the cleanest conditions as persistent southerly and easterly winds from the Pacific Ocean bring fresh marine air, though occasional heat waves can elevate ozone levels on hot, sunny afternoons. Autumn (October-November) sees a gradual deterioration as winds shift and temperature inversions return, with November often marking the transition to winter stagnation patterns. Throughout the year, Sagamihara benefits from its distance from central Tokyo's densest pollution sources, but remains vulnerable to regional transport during unfavorable wind patterns. Residents with respiratory conditions should monitor daily forecasts particularly during winter stagnation episodes and summer ozone alerts, adjusting outdoor activity timing accordingly.