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Mothīhāri, situated in Bihar's East Champaran district at coordinates 26.6500°N, 84.9167°E, occupies a strategic position in the fertile Gangetic Plain of northern India. This urban centre of 426,158 residents lies approximately 150 kilometres northwest of Patna and 50 kilometres from the Nepal border, placing it within a dynamic transboundary region where agricultural, urban, and rural influences converge. The city's terrain is predominantly flat, with an elevation around 60 metres above sea level, characteristic of the alluvial plains formed by the Gandak River system that flows nearby. This low-lying topography, combined with proximity to the Himalayan foothills to the north, creates a natural basin effect that traps airborne pollutants. Mothīhāri functions as a regional hub surrounded by intensive agricultural zones where rice, wheat, and sugarcane cultivation dominate, contributing seasonal biomass burning emissions. The urban-rural gradient shows dense settlement patterns in the city centre transitioning to scattered villages and farmland, with limited industrial activity beyond small-scale manufacturing and brick kilns. The city's location in the Indo-Gangetic Plain—one of the world's most polluted airsheds—means it experiences regional pollution transport from upwind sources, while local emissions from transportation, residential cooking, and waste burning compound air quality challenges. The absence of significant water bodies or elevation variations means pollutants disperse slowly, particularly during calm wind conditions common in this interior continental location.

Mothīhāri's air quality follows a distinct seasonal rhythm shaped by the monsoon-dominated climate of eastern India. During winter (December-February), cold temperatures and frequent temperature inversions trap pollutants near the surface, creating persistent haze and fog that mix with emissions from heating sources and agricultural residue burning. This period typically sees the worst air quality, with sensitive groups like children, elderly, and those with respiratory conditions advised to limit outdoor exposure, especially during morning hours when inversion layers are strongest. Spring (March-May) brings rising temperatures and increased wind activity that improves dispersion, though occasional dust storms from the Thar Desert region can temporarily elevate particulate levels. The summer monsoon (June-September) dramatically cleanses the atmosphere through heavy rainfall and strong southwesterly winds, making this the best season for outdoor activities with generally good air quality. Post-monsoon (October-November) sees a rapid deterioration as winds weaken, temperatures drop, and agricultural burning peaks after the kharif harvest, creating hazardous smoke plumes that blanket the region. Residents should monitor local air quality reports and plan strenuous activities for monsoon months when pollution is lowest, while winter months require precautions like masks and indoor air purifiers for vulnerable populations.