Climate of Bihar: Factors, Agro-Climatic Zones, Seasons and Agricultural Impact

1. Introduction: Understanding the Climate of Bihar

The climate of Bihar is a typical tropical monsoon type climate, strongly influenced by the southwest monsoon winds. Located between about 24°N to 27°N latitude, Bihar lies in the humid subtropical belt of northern India. The state experiences three major seasons — summer, monsoon and winter — with significant seasonal variation in temperature and rainfall.

Average annual rainfall in Bihar ranges between 1,000 mm to 1,200 mm, with nearly 80–85% of rainfall received during the monsoon months (June to September). Northern districts generally receive higher rainfall compared to southern districts.

Temperature also shows wide variation. In peak summer (May–June), maximum temperatures often cross 40°C, especially in southern Bihar. During winter (December–January), minimum temperatures can fall below 8°C in several districts.

The climate of Bihar plays a crucial role in shaping agriculture, water resources, disaster patterns and overall economic activities. Frequent floods in North Bihar and occasional drought-like conditions in South Bihar highlight how deeply climate influences development patterns in the state.

2. Factors Affecting Climate of Bihar

Latitudinal Location

The climate of Bihar is influenced by its location between approximately 24°N and 27°N latitude. This places the state in the tropical to subtropical transition zone. As a result, Bihar experiences high summer temperatures, often crossing 40°C in May and June. Being located north of the Tropic of Cancer but close to it, Bihar receives strong solar radiation in summer, leading to intense heat waves, especially in districts like Gaya and Aurangabad.

Distance from the Sea (Continentality)

Bihar is a landlocked state, situated far from maritime influence. Unlike coastal states such as West Bengal or Odisha, it does not experience moderating effects of sea breezes. This continental nature causes extreme seasonal temperature variation. The difference between peak summer temperatures (above 40°C) and winter minimum temperatures (around 5–8°C in some districts) clearly reflects this continental climatic control.

Himalayan Influence

The Himalayan range to the north plays a critical role in shaping the climate of Bihar. During winter, cold winds descending from the Himalayas lower temperatures significantly. At the same time, the Himalayas block monsoon winds, forcing them to rise and release heavy rainfall over North Bihar. This is why northern districts like Sitamarhi and Supaul receive higher rainfall compared to southern plateau regions.

Southwest Monsoon Winds

The southwest monsoon is the most important climatic factor. Nearly 80–85% of Bihar’s annual rainfall occurs between June and September. Average annual rainfall ranges between 1,000 mm and 1,200 mm, though northern districts often receive more than 1,300 mm. Variability in monsoon timing and intensity directly affects agriculture, floods and drought conditions, making monsoon performance central to the climate of Bihar.

Relief and Topography

Bihar has distinct physiographic divisions — the northern alluvial plains and the southern plateau region. The flat alluvial plains allow rivers like Kosi, Gandak and Bagmati to change course frequently, increasing flood vulnerability. In contrast, the southern plateau region receives relatively lower rainfall and experiences higher summer temperatures due to rocky terrain and lower moisture retention capacity.

Western Disturbances

During winter months, western disturbances — cyclonic systems originating in the Mediterranean region — influence the climate of Bihar. These disturbances bring light to moderate rainfall in January and February, which is beneficial for rabi crops like wheat. Although winter rainfall is limited compared to monsoon rainfall, it plays an important role in agricultural productivity and seasonal temperature moderation.

3. Effect of Climate of Bihar on Agriculture

The climate of Bihar directly shapes cropping patterns, productivity levels and rural livelihoods. Since nearly half of the state’s workforce depends on agriculture, variations in temperature and rainfall immediately affect income, food security and migration patterns. The monsoon-dominated nature of the climate makes agriculture highly sensitive to seasonal performance.

Monsoon Dependence and Crop Output

Around 80–85% of annual rainfall in Bihar is received between June and September. This period coincides with the kharif cropping season, especially paddy cultivation. Bihar produces over 90 lakh metric tonnes of rice annually, and timely monsoon rainfall is critical for transplantation and growth. A delayed or weak monsoon reduces sowing area, directly impacting output and farmer income. Thus, the climate of Bihar creates a strong monsoon-agriculture linkage.

Flood Impact on North Bihar

North Bihar is one of the most flood-prone regions in India. Approximately 68–73% of the state’s geographical area is considered flood-prone, mainly in districts adjoining Nepal. Rivers like Kosi and Gandak carry heavy sediment loads, frequently changing course. Floods damage standing crops, particularly paddy and maize, affecting lakhs of hectares annually. Recurrent floods increase agricultural uncertainty and reinforce rural poverty.

Drought and Rainfall Variability in South Bihar

While North Bihar struggles with floods, parts of South Bihar face rainfall deficiency in weak monsoon years. Annual rainfall declines as one moves southwards, creating regional imbalance in agricultural productivity. Rainfall variability affects maize, pulses and oilseed crops. When monsoon rainfall falls significantly below normal, irrigation dependency increases, raising cultivation costs and reducing profit margins.

Temperature Extremes and Crop Stress

The climate of Bihar includes extreme summer temperatures often exceeding 40°C. High temperatures during grain-filling stages can reduce wheat productivity. Similarly, heat waves in April–May affect vegetable and fruit cultivation. On the other hand, cold waves during winter sometimes damage potato and horticultural crops. Thus, temperature variability influences both rabi and horticulture output.

Winter Rainfall and Rabi Crops

Light winter rainfall from western disturbances supports wheat and pulses cultivation. Bihar produces around 70–80 lakh metric tonnes of wheat annually, and timely winter showers improve soil moisture and yield. However, excessive winter rain can damage mature crops. Therefore, even small seasonal climatic variations significantly impact agricultural outcomes.

Climate Change and Emerging Risks

Recent years show increasing climate variability, including irregular rainfall distribution and intense rainfall events. Short-duration heavy rainfall leads to waterlogging rather than gradual soil absorption. Such changes affect cropping calendars and increase input risks. Climate resilience, crop diversification and irrigation expansion are becoming essential for sustaining agriculture under the evolving climate of Bihar.

4. Agro-Climatic Zones in Bihar

The climate of Bihar is not uniform across the state. Differences in rainfall, soil type, temperature and flood patterns have led to the classification of Bihar into distinct agro-climatic zones. These zones help in scientific crop planning, irrigation management and agricultural diversification. Bihar is broadly divided into three agro-climatic zones, based on rainfall pattern, soil characteristics and physiography.

Zone I: North Alluvial Plain (North-West Bihar)

This zone includes districts such as West Champaran, East Champaran, Gopalganj, Siwan and Saran. The region receives relatively high annual rainfall, often ranging between 1,200–1,400 mm. Soils are fertile alluvial types deposited by Himalayan rivers. Paddy, wheat, maize and sugarcane are major crops. However, frequent floods and waterlogging affect productivity. The climate of Bihar in this zone supports high cropping intensity but also increases flood vulnerability.

Zone II: North-East Alluvial Plain

This zone covers districts like Darbhanga, Madhubani, Supaul, Saharsa, Purnea and Katihar. Rainfall here is slightly higher than Zone I in some districts, often exceeding 1,300 mm annually. The area is highly flood-prone due to rivers like Kosi and Bagmati. Maize, paddy, jute and makhana are important crops. High humidity and fertile soils favour crop growth, but recurring floods create instability in farm income.

Zone III: South Bihar Alluvial and Plateau Region

This zone includes districts such as Patna, Nalanda, Gaya, Aurangabad and Rohtas. Rainfall here is comparatively lower, generally between 900–1,100 mm annually. The region experiences higher summer temperatures, often crossing 42°C. Irrigation plays a larger role due to relatively lower rainfall. Wheat, pulses, oilseeds and maize are significant crops. Compared to North Bihar, flood incidence is lower, but drought-like conditions may occur in weak monsoon years.

Significance of Agro-Climatic Zonation

The classification of agro-climatic zones helps in crop diversification, targeted irrigation planning and climate-resilient agriculture. Since the climate of Bihar varies across regions, uniform agricultural policies may not be effective. Zone-specific strategies, such as flood-resilient crops in North Bihar and drought-resistant varieties in South Bihar, are essential for improving productivity and reducing agricultural risk.

5. Seasons in Bihar: Temperature, Rainfall and Wind Pattern Analysis

The climate of Bihar is characterised by clear seasonal variation. Based on temperature, rainfall distribution and wind systems, Bihar experiences three major seasons — summer, monsoon and winter. Each season has distinct climatic features that directly influence agriculture, water availability and disaster patterns.

Summer Season (March to Mid-June)

Summer in Bihar is marked by rising temperatures and dry conditions. From April onwards, maximum temperatures frequently cross 38°C, and during May–June they often exceed 40–42°C, especially in districts like Gaya and Aurangabad.

Hot, dry winds locally known as loo blow from the west and northwest, increasing heat stress. Relative humidity remains low before the arrival of monsoon. Occasional pre-monsoon thunderstorms, locally called Kalbaisakhi in eastern districts, bring short-duration rainfall and temporary relief.

High evapotranspiration during this season reduces soil moisture, affecting early sowing activities.

Southwest Monsoon Season (Mid-June to September)

The monsoon is the most crucial season in the climate of Bihar. It contributes around 80–85% of the state’s annual rainfall, with total annual rainfall averaging 1,000–1,200 mm. Northern districts often receive higher rainfall compared to southern districts due to proximity to the Himalayas.

Moisture-laden winds from the Bay of Bengal branch of the southwest monsoon dominate this season. Orographic lifting near the Himalayan foothills enhances rainfall in North Bihar. However, uneven spatial distribution often causes floods in the north and rainfall deficiency in parts of South Bihar.

Heavy rainfall events within short durations increasingly lead to waterlogging and river overflow.

Retreating Monsoon and Post-Monsoon (October to November)

After September, monsoon winds gradually withdraw. Rainfall sharply declines, and humidity levels reduce. Day temperatures remain moderately high in October but begin to fall in November.

This period is important for harvesting kharif crops and preparing fields for rabi sowing. The atmosphere becomes clearer, and wind patterns shift gradually from moist monsoon flow to dry continental air masses.

Winter Season (December to February)

Winter in Bihar is influenced by continental air masses and western disturbances. Minimum temperatures often fall to 6–8°C, and occasionally below 5°C in some districts. Day temperatures range between 15–22°C.

Western disturbances bring light rainfall in January and February. Though winter rainfall is limited compared to monsoon rainfall, it is beneficial for wheat and pulses. Cold waves occasionally affect northern districts, influencing horticulture and vegetable crops.

The large difference between summer and winter temperatures reflects the continental nature of the climate of Bihar.

BPSC Mains Practice Questions (Climate of Bihar)

Discuss the major factors affecting the climate of Bihar. Analyse the role of monsoon, Himalayan influence and continental location in shaping seasonal variation.
Examine the impact of the climate of Bihar on agricultural productivity and regional imbalance. Suggest climate-resilient strategies for sustainable farming.
Describe the agro-climatic zones of Bihar and explain how seasonal variations influence cropping patterns.

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Climate of Bihar

1. Introduction: Understanding the Climate of Bihar