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Hiratsuka occupies a strategic coastal position in Kanagawa Prefecture, Japan, situated on the western edge of the Kanto Plain where it meets the foothills of the Tanzawa Mountains. This city of approximately 257,662 residents lies along the Sagami River delta, with its urban core extending just 1-2 kilometers inland from Sagami Bay. The city's geography creates a distinctive air quality profile shaped by its position between mountainous terrain and coastal influences. To the northwest, the Tanzawa Mountains rise sharply to elevations exceeding 1,500 meters, creating a natural barrier that can trap pollutants during certain meteorological conditions. Meanwhile, the city's immediate coastal location exposes it to marine breezes from Sagami Bay that typically help disperse pollutants. Hiratsuka sits within the densely populated Keihin Industrial Zone corridor that stretches from Tokyo through Yokohama, though it represents a transitional area where heavy industry gives way to more mixed residential and light industrial development. The city's terrain is predominantly flat near the coast with gentle slopes rising toward the mountains, creating microclimatic variations that affect pollution dispersion. Agricultural zones persist in the city's northern and western peripheries, particularly rice paddies and vegetable farms, though these are increasingly giving way to suburban expansion. This urban-rural gradient means Hiratsuka experiences both urban pollution from local traffic and commercial activities, plus regional transport of pollutants from the massive Tokyo-Yokohama metropolitan area to the northeast.

Hiratsuka's air quality follows distinct seasonal patterns shaped by Japan's temperate climate and regional meteorological systems. During winter (December-February), cold air drainage from the Tanzawa Mountains often creates temperature inversions that trap pollutants near the surface, particularly during calm mornings. These conditions combine with increased heating emissions to create the year's poorest air quality, with January typically seeing the highest pollution concentrations. Sensitive groups should limit strenuous outdoor activities during winter mornings when inversions are most pronounced. Spring (March-May) brings improving conditions as increasing solar radiation breaks up inversions and prevailing winds shift to southerly flows off Sagami Bay, though pollen from the region's extensive cedar forests creates additional respiratory challenges. Summer (June-August) offers generally good air quality despite high humidity, as strong sea breezes and convective mixing effectively disperse pollutants, though occasional stagnant periods during the rainy season (tsuyu) in June can temporarily degrade conditions. Autumn (September-November) represents the optimal season for outdoor activities, with crisp air, consistent northwest winds, and minimal pollution episodes, though occasional kōsa (yellow dust) events from continental Asia can affect air quality. Throughout the year, the city's coastal position provides natural ventilation that typically prevents extreme pollution episodes, but regional transport from the Tokyo metropolitan area remains a persistent background concern.