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Chigasaki occupies a distinctive coastal position in Kanagawa Prefecture, Japan, situated on the western shore of Sagami Bay where the Shonan coastal plain meets the Pacific Ocean. This city of approximately 242,000 residents lies at the southern edge of the densely populated Kanto Plain, approximately 50 kilometers southwest of Tokyo, placing it within the gravitational pull of the world's largest metropolitan area while maintaining its own distinct seaside character. The terrain is predominantly flat coastal plain with an average elevation of just 5-10 meters above sea level, though the western edges gently rise toward the foothills of the Tanzawa Mountains. This geographic configuration creates a unique air quality dynamic: while ocean breezes from Sagami Bay typically provide natural ventilation, the city's position between Tokyo's industrial belt to the north and agricultural zones of western Kanagawa creates a complex pollution gradient. The urban-rural transition is pronounced, with Chigasaki's eastern coastal areas featuring dense residential development while western sectors retain more agricultural land. Proximity to major transportation corridors, including the Tomei Expressway and Tokaido Shinkansen line, contributes mobile emission sources, while the shallow coastal plain can trap pollutants during specific meteorological conditions, particularly when offshore winds carry urban emissions from the greater Tokyo region southward across the flat terrain.

Chigasaki's air quality follows a distinct seasonal rhythm shaped by its coastal location and regional weather patterns. Spring (March-May) brings generally good air quality as prevailing winds from the Pacific Ocean disperse pollutants, though occasional kosa (yellow dust) events from continental Asia can temporarily elevate particulate levels in April. Summer (June-August) sees the cleanest air overall, with consistent sea breezes and frequent rainfall during the June-July rainy season washing pollutants from the atmosphere; however, high humidity can combine with vehicle emissions to create ground-level ozone on hot, stagnant afternoons in August. Autumn (September-November) begins with excellent air quality in September but transitions to more variable conditions as temperatures drop and wind patterns shift, with October-November sometimes experiencing temperature inversions that trap pollutants near the ground overnight. Winter (December-February) presents the most challenging season for air quality, particularly during calm, clear nights when radiative cooling creates persistent temperature inversions over the coastal plain, trapping vehicle emissions and heating-related pollutants. Sensitive groups should be particularly cautious during winter mornings and on days with light offshore winds from the north, which can transport additional pollutants from the Tokyo metropolitan area. The best months for outdoor activity are typically June through September, while December through February require more careful planning, especially for those with respiratory conditions.