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Katihār, situated in Bihar's northeastern plains at coordinates 25.5300°N, 87.5800°E, occupies a strategic position in the Gangetic floodplain approximately 30 kilometres from the Nepal border. The city's elevation of just 34 metres above sea level places it within the flat, alluvial terrain characteristic of this agriculturally intensive region, where the Kosi River flows nearby, creating a humid microclimate that influences local air circulation patterns. As an urban centre within Bihar's predominantly rural Purnea division, Katihār functions as a commercial and administrative hub, exhibiting a distinct urban–rural gradient with dense settlement cores transitioning to expansive agricultural zones dominated by rice, wheat, and maize cultivation. This agricultural landscape contributes significantly to air quality through seasonal crop residue burning, while the city's general urban pollution sources—including vehicular emissions from National Highway 31 that bisects the city, small-scale industries, and domestic biomass burning—are compounded by its topography. The surrounding flat plains with minimal elevation variation create conditions conducive to pollutant accumulation, particularly during calm wind periods, while proximity to the Himalayan foothills to the north can occasionally channel regional air masses. The city's location in the monsoon trough region means it experiences distinct seasonal weather patterns that directly impact pollution dispersion, with the humid riverine environment from the Kosi basin adding moisture that can interact with particulate matter to form haze.

Katihār's air quality follows a pronounced seasonal rhythm shaped by the monsoon-dominated climate of eastern India. During winter (December–February), cold temperatures and frequent temperature inversions trap pollutants close to the ground, with calm winds and dense fog—particularly in January—creating hazardous conditions where particulate matter accumulates for days. This is the peak pollution period when sensitive groups like children, elderly, and those with respiratory conditions should limit outdoor activities, especially during morning hours when fog is thickest. Spring (March–May) brings rising temperatures and increased wind activity that gradually improves dispersion, though agricultural burning in surrounding fields during the pre-monsoon period can cause episodic spikes in pollution. The monsoon season (June–September) offers dramatic relief as heavy rainfall from the southwest monsoon washes pollutants from the atmosphere, with July and August typically having the cleanest air—ideal for outdoor activities. Post-monsoon (October–November) sees a gradual deterioration as rainfall ceases, temperatures drop, and agricultural burning resumes during the rice harvest period, creating a transition toward winter's poor conditions. Sensitive residents should monitor local air quality reports particularly during November through February, using air purifiers indoors during peak pollution months and planning outdoor exercise for monsoon months when dispersion is optimal.