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Kiryū, nestled in the southeastern portion of Gunma Prefecture, Japan, occupies a geographically significant position within the Kantō region. Situated at approximately 36.4052° N, 139.4973° E, the city’s terrain is characterized by a gradual transition from the foothills of the Japanese Alps to the Kanto Plain. The elevation ranges from roughly 60 to 250 meters above sea level, influencing local weather patterns and air circulation. Kiryū is bisected by the Akagane River, a vital waterway that contributes to the region’s agricultural productivity and provides a degree of humidity that can impact pollutant dispersion. The surrounding landscape is a mosaic of rice paddies, terraced fields, and forested slopes, reflecting a strong agricultural heritage. To the west, the mountainous terrain acts as a partial barrier, potentially trapping pollutants under certain meteorological conditions. Eastward, the urban–rural gradient gradually blends into the broader Kanto Plain, an area with significant industrial and population density. While Kiryū itself isn't a major industrial hub, its proximity to larger cities like Tokyo and Utsunomiya means it can be affected by transboundary pollution. Historically, the region’s textile industry contributed to localized air quality concerns, though modern regulations have mitigated some of these impacts. The prevailing winds, generally from the west and southwest, play a crucial role in transporting pollutants, either dispersing them or concentrating them within the Kiryū valley. The combination of topography, riverine influence, and regional connectivity shapes Kiryū’s unique air quality profile.

Kiryū’s air quality experiences a distinct seasonal cycle heavily influenced by Japan’s monsoon climate. Spring (March-May) often brings a period of relatively clean air, aided by increased rainfall and stronger winds that effectively flush out accumulated pollutants. However, the arrival of the ‘yellow dust’ – particulate matter blown from the Gobi Desert – can significantly degrade air quality during this season, particularly in April and May. Summer (June-August) is typically the cleanest period, with frequent rainfall and high temperatures promoting atmospheric stability and pollutant dispersion. The humid subtropical climate also reduces the likelihood of temperature inversions. Autumn (September-November) marks a shift, with decreasing rainfall and a gradual increase in temperature inversions, especially during calmer periods. These inversions trap pollutants near the ground, leading to potential air quality deterioration. The most challenging period is winter (December-February), when cold, stable air masses and frequent fog events exacerbate the effects of temperature inversions. Reduced sunlight and limited wind speeds further contribute to pollutant accumulation. Sensitive groups, including children, the elderly, and individuals with respiratory conditions, should limit prolonged outdoor exposure during these months, particularly on days with dense fog or calm winds. Maintaining indoor air quality through ventilation and air purification can also be beneficial. While summer generally offers the most favourable conditions, even then, monitoring local weather forecasts for dust events remains prudent.