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Climate of Bihar
Climate of Bihar
Climate of Bihar
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Climate of Bihar
Climate chart (explanation)
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Located in eastern India, Bihar is the twelfth-largest Indian state, with an area of 94,163 km2 (36,357 mi2) and an average elevation of about 150 metres above mean sea level. The landlocked state shares a boundary with Nepal to the north, the state of West Bengal to the east, Jharkhand to the south, and Uttar Pradesh to the west.[2] Bihar lies in a subtropical temperate zone and its climatic type is humid subtropical.[3]

Classification

[edit]

According to the Köppen climate classification, Bihar's climate mainly falls under subtropical monsoon, mild and dry winter, and hot summer (Cwa), except for southeastern parts of the state, such as Jamui, Banka, Munger, Lakhisarai, Khagaria, Shekhpura, some parts of Bhagalpur, Saharsa, and Begusarai. The southeastern part of the state is located in an extreme that falls under tropical savanna, hot, and seasonally dry (usually winter) (Aw).[4]

Seasons

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Winter

[edit]

Cold weather commences early in November and comes to an end in the middle of March.[5] The climate in October and November is pleasant. The days are bright and warm. As soon as the sun sets, the temperature falls and the heat of the day yields to a sharp bracing cold. Cold waves, locally known as Sheet-lahar, bring in the sharpness in winter and drastically drop the temperature in Bihar, disrupting lives of millions of poor people and also causing several deaths.[6][failed verification] The temperature in winter all over Bihar varies from 0–10 °C.[5] On 7 January 2013, in early morning, the mercury dipped to a record low of -2 °C in Forbesganj, 0 °C in Gopalganj, 0.2 °C in Jehanabad, 0.7 °C in Vaishali, -1 °C in Patna and Muzaffarpur, as well as other cities. December and January are the coldest months in Bihar.

Summer

[edit]

Hot weather arrives in March and lasts until the middle of June. The highest temperature is often registered in May. Like the rest of northern India, Bihar also experiences dust storms, thunderstorms, and dust-raising winds during the hot season. Dust storms with a velocity of 48–64 km/hour are most frequent in May, followed by April and June. The hot winds (loo) of the Bihar plains blow during April and May, with an average velocity of 8–16 km/hour. The hot wind greatly affects human comfort during this season.

Rain (monsoon)

[edit]
variation in rainfall across India

Monsoon season in Bihar is usually unpredictable and erratic. It begins in mid-June and continues until the end of September.[7]

Autumn (post-monsoon)

[edit]

An important feature of the retreating monsoon season in Bihar is the invasion of tropical cyclones originating in the Bay of Bengal at about 12° N latitude. Bihar is also influenced by typhoons originating in the South China Sea. The maximum frequency of the tropical cyclones occurs in September–November,[7] especially during the asterism called hathiya. These cyclones are essential for the maturing of rice paddies and are required for the moistening of the soil for the cultivation of rabi crops.

Statistics

[edit]

Temperature

[edit]
Average temperatures in various cities of Bihar (°C)[8][9][10][11]
— Winter
(Jan – Feb) 		Summer
(Mar – May) 		Monsoon
(Jun – Sep) 		Post-monsoon
(Oct – Dec) 		Year-round
City Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Avg
Patna 16 19 25 30 31 31 29 29 28 26 22 17 26
Arrah 16 18 24 30 31 30 29 28 29 26 21 17 25
Darbhanga 16 18 23 28 29 29 29 28 28 26 21 17 25

Precipitation

[edit]
Average precipitation in various cities of Bihar (mm)[8][9][10][11]
— Winter
(Jan – Feb) 		Summer
(Mar – May) 		Monsoon
(Jun – Sep) 		Post-monsoon
(Oct – Dec) 		Year-round
City Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Total
Patna 10 --- 10 --- 40 120 220 260 170 70 10 --- 990
Gaya 20 19 12 7 21 137 314 328 206 53 10 4 1130
Arrah 10 10 10 --- 30 180 290 330 210 50 --- --- 1180
Darbhanga 10 10 10 10 60 190 300 340 230 50 --- --- 1260

Climate data

[edit]
Climate data for Patna (Köppen Cwa)
Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Year
Record high °C (°F) 30.0
(86.0) 		35.1
(95.2) 		41.4
(106.5) 		44.6
(112.3) 		45.6
(114.1) 		46.6
(115.9) 		41.2
(106.2) 		39.7
(103.5) 		37.5
(99.5) 		37.2
(99.0) 		34.1
(93.4) 		30.5
(86.9) 		46.6
(115.9)
Mean daily maximum °C (°F) 22.4
(72.3) 		26.0
(78.8) 		32.2
(90.0) 		37.0
(98.6) 		37.4
(99.3) 		36.4
(97.5) 		33.0
(91.4) 		32.9
(91.2) 		32.5
(90.5) 		31.9
(89.4) 		29.0
(84.2) 		24.5
(76.1) 		31.3
(88.3)
Mean daily minimum °C (°F) 9.3
(48.7) 		12.1
(53.8) 		16.7
(62.1) 		22.1
(71.8) 		25.1
(77.2) 		26.7
(80.1) 		26.3
(79.3) 		26.3
(79.3) 		25.5
(77.9) 		21.5
(70.7) 		15.1
(59.2) 		10.5
(50.9) 		19.8
(67.6)
Record low °C (°F) 1.1
(34.0) 		3.4
(38.1) 		8.2
(46.8) 		13.3
(55.9) 		17.7
(63.9) 		19.3
(66.7) 		21.1
(70.0) 		20.5
(68.9) 		19.0
(66.2) 		12.0
(53.6) 		7.7
(45.9) 		2.2
(36.0) 		1.1
(34.0)
Average rainfall mm (inches) 11.1
(0.44) 		14.0
(0.55) 		9.5
(0.37) 		12.7
(0.50) 		43.7
(1.72) 		162.5
(6.40) 		354.4
(13.95) 		277.8
(10.94) 		197.7
(7.78) 		49.1
(1.93) 		7.2
(0.28) 		6.1
(0.24) 		1,145.8
(45.11)
Average rainy days 1.2 1.3 0.7 1.0 2.9 6.9 14.3 12.5 9.5 2.9 0.5 0.6 54.2
Average relative humidity (%) (at 17:30 IST) 65 52 37 32 42 59 75 75 76 68 67 69 60
Mean monthly sunshine hours 207.7 228.8 260.4 264.0 272.8 192.0 130.2 151.9 162.0 238.7 240.0 201.5 2,550
Mean daily sunshine hours 6.7 8.1 8.4 8.8 8.8 6.4 4.2 4.9 5.4 7.7 8.0 6.5 7.0
Average ultraviolet index 6 7 9 11 12 12 12 12 11 8 6 5 9
Source 1: India Meteorological Department (sun 1971–2000)[12][13][14]
Source 2: Weather Atlas[15]
Climate data for Gaya (Köppen Cwa)
Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Year
Record high °C (°F) 31.7
(89.1) 		36.1
(97.0) 		42.1
(107.8) 		45.0
(113.0) 		47.1
(116.8) 		47.9
(118.2) 		43.7
(110.7) 		42.3
(108.1) 		42.3
(108.1) 		37.2
(99.0) 		35.0
(95.0) 		31.1
(88.0) 		47.9
(118.2)
Mean daily maximum °C (°F) 23.0
(73.4) 		26.7
(80.1) 		32.8
(91.0) 		38.7
(101.7) 		39.8
(103.6) 		37.8
(100.0) 		33.4
(92.1) 		32.9
(91.2) 		32.6
(90.7) 		31.6
(88.9) 		28.8
(83.8) 		24.9
(76.8) 		31.9
(89.4)
Mean daily minimum °C (°F) 8.7
(47.7) 		11.6
(52.9) 		16.1
(61.0) 		21.8
(71.2) 		25.4
(77.7) 		26.7
(80.1) 		25.6
(78.1) 		25.4
(77.7) 		24.6
(76.3) 		20.5
(68.9) 		14.2
(57.6) 		9.7
(49.5) 		19.2
(66.6)
Record low °C (°F) 1.5
(34.7) 		2.7
(36.9) 		7.8
(46.0) 		12.9
(55.2) 		14.1
(57.4) 		18.3
(64.9) 		16.7
(62.1) 		18.5
(65.3) 		17.5
(63.5) 		12.2
(54.0) 		6.1
(43.0) 		1.4
(34.5) 		1.4
(34.5)
Average rainfall mm (inches) 13.9
(0.55) 		16.0
(0.63) 		10.7
(0.42) 		11.3
(0.44) 		32.5
(1.28) 		157.6
(6.20) 		306.0
(12.05) 		266.0
(10.47) 		177.3
(6.98) 		56.0
(2.20) 		9.5
(0.37) 		4.9
(0.19) 		1,061.6
(41.80)
Average rainy days 1.2 1.4 1.0 1.0 2.5 7.1 14.2 13.6 8.8 2.5 0.6 0.5 54.4
Average relative humidity (%) (at 17:30 IST) 55 47 31 25 35 54 76 77 76 66 56 58 54
Source: India Meteorological Department[16][17]
Climate data for Bhagalpur (Köppen Cwa)
Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Year
Record high °C (°F) 31.9
(89.4) 		35.8
(96.4) 		42.6
(108.7) 		45.3
(113.5) 		46.4
(115.5) 		46.0
(114.8) 		42.3
(108.1) 		39.7
(103.5) 		38.6
(101.5) 		40.0
(104.0) 		37.4
(99.3) 		32.2
(90.0) 		46.4
(115.5)
Mean daily maximum °C (°F) 23.0
(73.4) 		27.1
(80.8) 		33.4
(92.1) 		37.6
(99.7) 		37.5
(99.5) 		36.4
(97.5) 		33.3
(91.9) 		33.4
(92.1) 		33.2
(91.8) 		32.7
(90.9) 		29.6
(85.3) 		24.9
(76.8) 		31.9
(89.4)
Mean daily minimum °C (°F) 12.4
(54.3) 		15.4
(59.7) 		20.6
(69.1) 		24.2
(75.6) 		26.0
(78.8) 		27.2
(81.0) 		26.9
(80.4) 		26.9
(80.4) 		26.4
(79.5) 		23.9
(75.0) 		19.1
(66.4) 		14.2
(57.6) 		21.9
(71.4)
Record low °C (°F) 4.2
(39.6) 		5.0
(41.0) 		10.8
(51.4) 		13.1
(55.6) 		14.5
(58.1) 		19.5
(67.1) 		22.4
(72.3) 		20.1
(68.2) 		21.5
(70.7) 		15.4
(59.7) 		11.1
(52.0) 		3.9
(39.0) 		3.9
(39.0)
Average rainfall mm (inches) 13.6
(0.54) 		9.4
(0.37) 		9.4
(0.37) 		23.9
(0.94) 		75.6
(2.98) 		201.7
(7.94) 		300.8
(11.84) 		256.5
(10.10) 		217.2
(8.55) 		85.5
(3.37) 		5.2
(0.20) 		7.5
(0.30) 		1,206.3
(47.49)
Average rainy days 1.1 1.2 0.9 1.8 4.7 8.9 14.0 11.7 9.6 3.4 0.5 0.7 58.5
Average relative humidity (%) (at 17:30 IST) 70 58 47 45 55 68 78 77 77 71 67 70 65
Source: India Meteorological Department[18][19]

Disasters

[edit]

Floods

[edit]
A view of the 2008 Bihar flood
Main article: Floods in Bihar
See also: 2008 Bihar flood and 2007 Bihar flood

The total numbers of deaths due to flooding in Bihar between 1979 and 2006 were: human deaths 5,874; animal deaths 19,044.[20][21]

Bihar is India's most flood-prone state, with 76% of the population in northern Bihar living under the recurring threat of flood devastation.[22] According to some historical data, 16.5% of the total flood-affected area in India is located in Bihar, while 22.1% of the flood-affected population in India lives in Bihar.[23] About 68,800 square kilometres (26,600 sq mi) out of a total geographical area of 94,160 square kilometres (36,360 sq mi), comprising 73.06% of the state, is flood-affected.

See also

[edit]

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Climate of Bihar

View on Grokipedia
from Grokipedia
The of is predominantly humid subtropical, featuring hot summers with temperatures often exceeding 40°C, a season that delivers the bulk of annual , and mild winters with occasional frost, resulting in marked seasonal contrasts and high variability in . experiences four primary seasons: a cold weather period from late to with mean minimum temperatures of 7.8–11.9°C, a pre- hot season from to mid- peaking in May at mean maxima of 34–40.5°C, the southwest from mid- to characterized by high and frequent thunderstorms, and a post- transition from to . Annual rainfall averages 1164 mm across the state, with 82–90% concentrated in the months of to , exhibiting high interannual variability that ranges from 39% to 253% of normal in some periods, contributing to both and risks. The state's flat alluvial topography, intersected by numerous rivers including the and the flood-prone Kosi—known as the "Sorrow of "—amplifies deluges into recurrent flooding, affecting vast areas and underscoring the 's influence on and disaster vulnerability.

Geographical Influences

Topography and Regional Variations

Bihar's topography is characterized by vast alluvial plains formed by the River and its tributaries, with an average elevation of approximately 53 meters above , though varying from near in river valleys to over 300 meters in the southern districts adjacent to the Chotanagpur Plateau. The state is divided by the into the Plain, which lies north of the river and extends toward the Himalayan foothills, and the South Bihar Plain to the south, featuring slightly undulating terrain with residual hills in areas like Kaimur and Rohtas districts. This bifurcation influences local microclimates, as the northern plains benefit from orographic uplift of moist air from Himalayan-fed rivers, while the southern regions experience reduced moisture convergence. Regional climatic variations stem primarily from these topographic distinctions and proximity to moisture sources. , encompassing districts like and , receives higher annual rainfall averaging 1,200 to 1,700 mm, driven by intensified activity from the Gandak, Kosi, and other transboundary rivers that enhance convective . In contrast, South Bihar districts such as and Gaya record lower of 900 to 1,200 mm annually, attributable to greater distance from Himalayan orographic barriers and exposure to drier westerly winds, leading to higher vulnerability. These patterns result in frequent flooding in the north due to flat impeding drainage, whereas the south's subtle elevations facilitate quicker runoff but limit during deficient monsoons. Spatial rainfall gradients further highlight topographic modulation, with northeastern Bihar exhibiting the highest variability and intensity from depressions tracking parallel to the plains, while southwestern areas show subdued influenced by the rain shadow-like effects near the Vindhyan escarpment. Empirical data from the indicate that elevation differences, though modest, correlate with a 10-20% rainfall decrement per 100 km southward, underscoring causal links between terrain gradients and atmospheric moisture distribution.

Proximity to Himalayas and River Systems

Bihar's northern regions lie in close proximity to the Himalayan foothills, approximately 100-150 km south across the Nepal border, acting as a climatic barrier that obstructs the influx of cold, dry continental air masses from Central Asia during winter months. This shielding effect results in milder winters compared to regions at similar latitudes without such a barrier, with mean January minimum temperatures typically ranging from 8°C to 12°C across the state, though western disturbances originating over the Himalayas can occasionally trigger cold waves, dropping temperatures to as low as 0°C in northern districts like Darbhanga or 1.4°C in Patna. The further influence Bihar's precipitation patterns through orographic enhancement of the southwest , as moisture-laden winds from the are forced to ascend the southern slopes, leading to heavier rainfall in northern Bihar districts nearer the . Annual rainfall in these areas can exceed 1,700 mm, with contributions accounting for 82-90% of totals— for instance, Purnea records 1,732.8 mm annually, while receives 2,215 mm, compared to drier southern districts like Rohtas at 994.4 mm. This gradient underscores the ' role in redistributing moisture southward into the Indo-Gangetic plains. Bihar's extensive river systems, dominated by the Ganga and its trans-Himalayan tributaries such as the Kosi, Gandak, and Bagmati—which originate in the Himalayas and drain vast catchments spanning Nepal—exert a profound effect on local humidity and thermal moderation. These rivers, augmented by snowmelt and heavy upstream monsoon precipitation, maintain high water volumes that elevate relative humidity to 75-85% during the monsoon (June-September) and 60-80% in the post-monsoon period, fostering uncomfortable mugginess even as temperatures decline nocturnally. Evaporation from river floodplains and wetlands moderates extreme heat, reducing diurnal temperature swings, but also contributes to persistent winter fog, with the Kosi's silt-laden flows exacerbating flood-prone conditions that temporarily saturate soils and amplify local moisture. The Himalayan sourcing of these rivers ensures flow, with discharges peaking due to synchronized rainfall and glacial melt, leading to recurrent flooding that covers up to 73% of Bihar's area in severe years and influences microclimatic variability through increased and altered land-atmosphere feedbacks. Districts in , such as those along the Gandak, experience amplified and cooler effective temperatures from these dynamics, contrasting with the relatively lower in peninsular-fed southern rivers like the .

Climatic Classification

Köppen-Geiger Framework

The Köppen-Geiger climate classification system delineates global climates into five main groups (A through E) based on empirical thresholds of monthly temperature and precipitation, reflecting vegetation suitability and seasonal regimes. Bihar predominantly aligns with the Cwa subtype, denoting a humid subtropical monsoon climate characterized by mild, dry winters; hot summers; and precipitation maxima during the warm season. This classification applies to the majority of the state's Gangetic plains, where the coldest month (typically January) records mean temperatures between 10°C and 18°C—meeting the 'C' mesothermal criterion of at least one month below 18°C but above 0°C—while the warmest month (May) exceeds 22°C, satisfying the 'a' hot summer designation. The 'w' suffix reflects a dry winter, defined by winter precipitation below 60 mm or less than one-tenth of the wettest summer month, contrasting with the monsoon-driven rainfall exceeding 1,000 mm annually, concentrated from June to September (accounting for 82-90% of totals). Regional variations within Bihar arise from topographic and latitudinal gradients, with southeastern districts such as , Banka, , , , , , Saharsa, and exhibiting Aw characteristics—tropical with hot, seasonally dry conditions—due to marginally warmer winters (all months averaging above 18°C in some locales) and more pronounced wet-dry contrasts, with annual rainfall around 900-1,100 mm but sharper seasonal deficits. Northern districts like , Purnea, and reinforce Cwa traits through higher intensity (up to 2,200 mm annually) and cooler winters influenced by Himalayan proximity, though occasional western disturbances introduce minor winter precipitation. Central areas, including and Gaya, exemplify core Cwa parameters, with means of 15-16°C and May maxima often surpassing 40°C. These delineations, derived from long-term observational data (e.g., 1901-2020 averages), underscore Bihar's transitional position between tropical and subtropical zones, without significant shifts to arid (B) or polar (E) categories. Discrepancies in subtype labeling, such as occasional references to Cwg (emphasizing monsoon influence with dry winters), stem from adaptations in regional analyses but align fundamentally with Cwa under standardized global thresholds, as validated by temperature profiles (winter minima 7-12°C, summer 35-40°C) and precipitation seasonality. No substantial areas fall into other C subgroups like Cfa (no dry season) due to the state's pronounced winter .

Monsoonal and Subtropical Characteristics

Bihar's climate is characterized by humid subtropical conditions overlaid with strong al dynamics, featuring hot summers, mild winters, and a pronounced seasonal reversal in wind patterns driven by the (ITCZ). Under the Köppen-Geiger classification, the state predominantly aligns with Cwa (humid subtropical with dry winter and hot summer), where the dry winter period receives less than 3% of annual , while summers exhibit mean temperatures exceeding 18°C in the coolest month and above 22°C in the warmest, with significant rainfall interrupting the dry season. This classification reflects the region's latitude (24°-27°N), which places it in the subtropical belt, moderated by continental influences from its inland position and the Gangetic Plain's flat topography, leading to extreme temperature swings of up to 25°C between day and night in non-monsoon periods. The monsoonal regime is dominated by the southwest (June-September), which delivers 80-85% of the state's annual rainfall, averaging 1,000-1,200 mm statewide, with higher amounts (up to 1,500 mm) in the northwest due to orographic enhancement from the and lower in the southwest. This influx stems from moisture-laden from the , funneled by the region's low relief, resulting in high (often >80%) and frequent convective thunderstorms, though variability remains moderate at 10-15% for both annual and monsoon totals. Subtropical traits manifest in the extended pre-monsoon heat (March-May), with temperatures routinely surpassing 40°C and heat indices amplified by rising , fostering conditions for loo —hot, dry gusts that exacerbate before the 's arrival. Post-monsoon influences include occasional tropical cyclones from the , introducing erratic heavy rains in October-November and underscoring the monsoon's transitional role between wet and dry subtropical phases. Winters (December-February) embody the dry subtropical winter, with low rainfall (<50 mm total) and fog-prone mornings due to radiation cooling over the plains, maintaining mean temperatures of 15-20°C despite occasional cold waves dipping to 5-10°C. These characteristics collectively highlight Bihar's hybrid profile: subtropical thermal extremes tempered by monsoonal , which sustains but amplifies risks through uneven spatial distribution.

Seasonal Dynamics

Winter (December-February)

During the winter months of to , Bihar experiences cool and dry conditions influenced by the influx of cold continental air from the northwest, moderated by its location in the Gangetic plain. Daytime maximum temperatures typically range from 22.4°C to 24.8°C across the state, while nighttime minima average 7.8°C to 11.9°C, with January being the coldest month. These values reflect data from multiple observatories, such as (maxima 23.3–24.5°C, minima 9.1–9.8°C) and Gaya (maxima 23.5°C, minima 8.9°C). Precipitation remains negligible, accounting for approximately 2% of Bihar's annual rainfall, with seasonal totals around 23.8 mm and 0.5 to 1.8 rainy days per month. District-level data indicate low volumes, such as 10.7 mm in and 12.2 mm in over the season, often associated with western disturbances bringing light rain or snow to higher latitudes. Variability in rainfall is high, with coefficients of variation reaching 45% to 320%, particularly elevated in southwestern and eastern districts. Cold waves, triggered by these western disturbances, periodically intensify conditions, dropping minima to 1.3–4°C in affected areas, as recorded in (1.4°C on January 21, 1984) and Purnea (1.3°C on January 31, 1971). is a common feature, persisting for 10–15 days in northwestern during and , with observatories like reporting 6.3 days in and 7.2 days in , reducing visibility and impacting transportation. Relative humidity averages 60–80%, contributing to the misty atmosphere. Regional variations arise from topography and proximity to the : northern Bihar (e.g., , minima around 9.6°C) tends to be milder with slightly higher (up to 28.5 mm seasonally) and more foggy days, while southern districts (e.g., Gaya, Sabour, minima 7.8–8.9°C) are cooler with lower rainfall (e.g., 4.4 mm in Arwal). These patterns underscore the continental climate's dominance, with minimal maritime influence during this period.

Summer (March-May)

Summer in Bihar marks the onset of the hot season, with temperatures rising steadily from onward due to the region's subtropical and lack of moderating oceanic influences. Average maximum temperatures in typically range from 30°C to 35°C across the state, escalating to 38–42°C in and peaking at 40–45°C in May, particularly in southern districts like Gaya and . Minimum temperatures follow suit, averaging 20–22°C in and climbing to 26–29°C by May. The period is predominantly dry, with scant rainfall—often less than 20–30 mm monthly—interrupted occasionally by pre-monsoonal thunderstorms known as "Kal Baisakhi" in late May, which bring brief relief but can cause localized gusty winds and dust storms. Hot, dry westerly winds called Loo prevail during and May, blowing at speeds of 8–16 km/h, exacerbating heat stress by lowering relative humidity to 20–30% and drying out vegetation and soils. Heatwaves are frequent and intensify toward May, defined by the (IMD) as maximum temperatures exceeding 40°C in the plains accompanied by a departure of at least 4.5°C above normal. Historical records show extremes like 47.4°C in Gaya on May 29, 2024, surpassing previous benchmarks, with multiple districts routinely surpassing 42°C during prolonged events. These conditions strain agricultural cycles, particularly for rabi crops nearing harvest, and elevate health risks from and heatstroke, though official mortality data often underreports due to inconsistent attribution.

Monsoon (June-September)

The southwest monsoon advances into during the second week of , typically covering the eastern districts by around June 15 and the entire state by the end of the second week, driven by the branching of currents from the . This period, spanning to , delivers approximately 1,164 mm of rainfall statewide, constituting 86% of 's annual average of 1,191 mm, with and accounting for the heaviest downpours at 28% and 24% of the yearly total, respectively. Precipitation exhibits marked spatial variation, with northeastern districts such as receiving up to 1,839 mm during the season—83% of their annual 2,215 mm—while southwestern areas like Arwal record lower totals closer to 740 mm (86% of annual 858 mm). The for monsoon rainfall ranges from 10% to 85% across the state, lowest (around 15%) in northeastern and northwestern districts and highest (up to 75%) in western regions, reflecting influences from topographic features and proximity to moisture-laden winds. Heavy convective activity, often triggered by low-pressure systems from the , results in frequent thunderstorms and intense daily events, with recorded 24-hour extremes exceeding 500 mm in districts like (580 mm on August 11, 1987) and (510.5 mm on June 23, 1911). Temperatures moderate with the onset, yielding mean maximums of 31.9–33.5°C in and mean minimums of 23–26.3°C, though early maxima can reach 34–38°C before full establishes. Relative humidity rises to 70–90%, peaking at 80–86% from through , contributing to muggy conditions despite the diurnal cooling from rainfall. Nights remain warm, often above 25°C, due to persistent cloudiness and southerly winds.
District ExampleMonsoon Rainfall (mm)% of Annual Total
(NE)1,83983%
West Champaran (NW)1,23286%
Begusarai (Central)~95586%
Rohtas (SW)~88689%
The season's withdrawal commences in northern during the first week of , marking a transition to drier conditions, though intermittent rains may persist into mid-. Overall, the monsoon's reliability supports , but its variability—evident in district-specific ranges from 36–177% of normals in places like Purnea—underlines the challenges of uneven distribution.

Post-Monsoon (October-November)

The post-monsoon season in Bihar marks the transition from the retreating southwest to winter, featuring generally clear skies, diminishing , and a gradual cooling trend, with daytime temperatures remaining warm while nights become progressively cooler. This period typically spans to , during which the influence of residual moisture from the leads to occasional depressions or cyclones, particularly in early , contributing to sporadic heavy rainfall events. levels range from 60% to 80%, supporting comfortable daytime conditions but fostering initial formation toward . Average maximum temperatures in hover around 31–32°C across much of the state, dropping to 28–29°C by , while minimum temperatures decrease from approximately 21°C in to 14–15°C in , with southwest districts experiencing the lowest minima below 20°C even earlier. For instance, in , normals are 31.6°C maximum and 21.4°C minimum, shifting to 28.9°C maximum and 14.9°C minimum in ; similar patterns hold in Purnea and , reflecting the subtropical continental influence and landward heat loss post-monsoon. These temperatures support agricultural activities like harvesting but can lead to thermal discomfort if above-normal warmth persists, as observed in recent years with 2024 marking India's warmest such month on record at 1.23°C above normal. Precipitation averages 83.1 mm statewide for the , accounting for about 6% of annual totals, with around 3.5 rainy days concentrated in (e.g., 2–3 days) rather than November (0.2–0.5 days). Northeast districts like Purnea (84.9 mm) and receive higher amounts due to proximity to systems, while southwest areas like Arwal (33 mm) and Jahanabad (32.1 mm) see lower figures; spatial distribution shows a northeast-southwest , with coefficients of variation exceeding 150% in western districts indicating high year-to-year unpredictability. Rainfall variability is lower (around 75%) in core monsoon-influenced zones but can disrupt late-season farming if depressions intensify. Winds lighten to 2–3 km/h, shifting from easterly to northwesterly, with reducing to 2–2.5 , promoting clearer conditions overall. Occasional thunderstorms (e.g., 2–3 days in at ) arise from low-pressure systems, and fog days increase from 1 in to 3–4 in , particularly in riverine lowlands, signaling the onset of winter stagnation. These patterns underscore Bihar's vulnerability to post- cyclonic influences, though empirical data confirm the season's relative dryness compared to monsoon peaks.

Observational Data

Temperature Profiles

Bihar experiences a subtropical climate characterized by significant seasonal variations, with hot summers and mild winters influenced by its inland location and proximity to the . The state-wide annual mean maximum is approximately 31.2°C, while the mean minimum is 19.2°C, based on long-term observational data from key stations. marks the coldest month, with a state mean maximum of 23.5°C and minimum of 9.3°C, whereas May is the hottest, featuring a mean maximum of 37.0°C and minimum of 25.7°C. Monthly temperature profiles at , a primary meteorological station, illustrate typical patterns across much of Bihar's Gangetic plains:
MonthMean Maximum (°C)Mean Minimum (°C)
January23.39.1
February26.011.3
March32.316.2
April37.222.0
May38.024.9
June36.526.6
July32.926.0
August32.526.0
September32.325.2
October31.621.4
November28.914.9
December24.59.8
These values, derived from 1961–1990 normals, show a rapid rise from winter to pre-monsoon peaks, followed by moderation during the rainy season due to cloud cover and humidity. Extreme temperatures underscore Bihar's vulnerability to heatwaves and cold spells. The highest recorded temperature is 49.5°C at Dehri (Rohtas district) on 11 May 1988, while the lowest is -1.0°C at the same location on 18 January 1977. Other stations report highs near 47.9°C at Gaya on 9 June 1966 and lows around 0°C to 1.4°C in northern and central areas during winter cold waves. Southern Bihar, including districts like Gaya and Rohtas, tends toward higher summer maxima (up to 40.5°C means in May) and lower winter minima due to drier conditions and elevation effects, whereas northern regions like Purnea exhibit milder extremes with summer means around 35°C and winter minima rarely below 1°C, moderated by higher humidity and proximity to Nepal's terai.

Precipitation Distributions

Bihar experiences an average annual precipitation of 1164 mm, with the southwest monsoon season (June to September) accounting for approximately 86% of the total. July typically records the highest monthly rainfall statewide, often exceeding 300 mm in many districts, while non-monsoon periods contribute minimally, with winter (December to February) and pre-monsoon (March to May) seasons together providing less than 10% of the annual total. This temporal concentration results in about 50 rainy days per year on average, defined as days with at least 2.5 mm of rainfall. Spatially, exhibits marked variation across , increasing from southwest to northeast and northwest due to orographic enhancement from the Himalayan foothills influencing northern districts. Northeastern districts like receive up to 2215 mm annually, while southwestern areas such as Arwal and Jahanabad average around 858 mm. This gradient contributes to higher variability in northern (22-24% ), exacerbating localized flood risks, compared to relatively more uniform patterns in the south. The following table summarizes mean annual rainfall for select districts based on 1951-2000 data from the :
DistrictAnnual Rainfall (mm)
2215.0
Purnea1732.8
West Champaran1434.1
1387.8
1373.0
Arwal857.9
Jahanabad858.2
District-specific monsoon contributions range from 82% to 90% of annual totals, with post- (October-November) rainfall adding 5-10% and showing upward trends in some analyses (0.068 mm/year over 1901-2002). Year-to-year fluctuations are pronounced, with annual rainfall deviating 36-177% from normals in districts like Purnea and , underscoring the state's susceptibility to intra-annual variability driven by monsoon dynamics.

Recorded Extremes

The highest temperature ever recorded in Bihar was 49.5 °C at Dehri in Rohtas district on 11 May 1988. Other notable maxima include 47.9 °C at Gaya on 9 June 1966 and 46.6 °C at Patna and Chapra on 9 June 1966. The lowest minimum temperature was -1.0 °C, also at Dehri on 18 January 1977. Additional record lows include 0.0 °C at Darbhanga on 31 January and 3 February 1971, and at Motihari on 3 February 1905.
Station/DistrictExtreme TypeValueDate
Dehri (Rohtas)Highest maximum 49.5 °C11 May 1988
GayaHighest maximum 47.9 °C9 June 1966
Dehri (Rohtas)Lowest minimum -1.0 °C18 1977
Lowest minimum 0.0 °C31 1971
The heaviest 24-hour rainfall recorded was 580 mm at Majarganj in on 11 August 1987. Other significant single-day totals include 550 mm at Tharary in Bhojpur district on 13 September 1987 and 520 mm at in East on 25 August 2005. These extremes, primarily during the monsoon season, underscore Bihar's vulnerability to intense convective activity influenced by low-pressure systems and orographic enhancement from nearby terrain. No verified post-2012 events have surpassed these temperature thresholds, though localized maxima above 45 °C occurred in districts like and during prolonged heatwaves in 2023.
Location24-Hour Rainfall (mm)Date
Majarganj (Sitamarhi)58011 August 1987
Tharary (Bhojpur)55013 September 1987
(East Champaran)52025 August 2005
Katihar ()510.523 June 1911

Long-Term Temperature Changes (1901-Present)

Analysis of observational data from the Indian Meteorological Department indicates that Bihar's annual mean land surface air exhibited no significant trend over the period from 1901 to 2021. This contrasts with the national average for , where the annual mean increased by 0.62 °C per 100 years during 1901–2020. The lack of a detectable long-term rise in Bihar's mean may reflect regional variability in factors such as changes, aerosol loading from agricultural burning, and local patterns, which can modulate broader warming signals observed elsewhere in . Disaggregating by daily extremes reveals divergent patterns: maximum temperatures in Bihar increased at a rate of +0.54 °C per 100 years from 1901 to 2021, while minimum temperatures decreased by -0.53 °C per 100 years over the same interval. These opposing shifts imply an expansion of the diurnal temperature range, potentially linked to enhanced solar insolation during daylight hours and greater nocturnal radiative cooling, influenced by Bihar's agrarian landscape and seasonal fog cover. Studies of southern Bihar sub-regions corroborate a decreasing trend in mean temperatures during certain periods, such as 1901–2010, particularly in monsoon and post-monsoon seasons. Recent decadal assessments, including data up to 2021, show annual means hovering around 24.0 °C, with 2021 registering a slight positive anomaly of +0.1 °C relative to the 1981–2010 baseline. Spatial heterogeneity persists, with higher maximum temperatures in southwestern districts like Kaimur and Rohtas, and lower minima in western and northeastern areas. Short-term fluctuations, such as cooler-than-average maxima in May 2021 (-3.3 °C anomaly, the fourth coolest since 1901), underscore the influence of interannual variability, including El Niño-Southern Oscillation effects, on local trends. Overall, Bihar's temperature record from 1901 onward emphasizes stability in mean values amid changes in extremes, warranting continued monitoring to distinguish natural oscillations from potential emerging anthropogenic signals. Bihar receives an average annual precipitation of approximately 1200 mm, with the majority—around 84%—occurring during the southwest season from to . Rainfall exhibits a pronounced spatial gradient, decreasing from northeast to southwest across the state; northeastern districts such as record over 2000 mm annually, while southwestern areas like average below 1000 mm. This pattern aligns with orographic influences from the and varying moisture influx from the . Precipitation variability is high, particularly in northern , where coefficients of variation range from 22% to 28%, contributing to frequent floods and droughts despite overall adequacy. Statewide, annual coefficients of variation span 15% to 70% across districts, with non-monsoon seasons showing even greater irregularity—up to 320% in winter. Non-monsoon contributions remain minor: pre-monsoon (March-May) accounts for about 6%, post-monsoon (October-November) another 6%, and winter (December-February) roughly 2%. Long-term trends from 1901 to 2021 indicate a general decline in annual rainfall, with significant decreasing patterns detected in 21 of 's districts using Mann-Kendall tests, at rates of -2 to -4 mm per year in areas like Arwal and . Only Purnea district shows a statistically significant increase of +2 mm per year. rainfall, the dominant component, also exhibits declines in most districts, such as -3.27 mm per year in Arwal, alongside rising extreme events in northern regions. Decadal analyses reveal fluctuations but an overarching reduction, particularly in central , based on gridded data.

Hazards and Risks

Flood Events: Natural and Recurrent Drivers

Bihar's recurrent flood events stem primarily from its location in the lower Gangetic plain, where intense precipitation interacts with the region's and . Approximately 73.63% of north 's area is flood-prone, affecting 28 out of 38 districts annually during the monsoon season from June to September. This period accounts for the majority of the region's rainfall, with river discharges increasing up to 50 times normal levels due to concentrated downpours in the catchment areas. The heavy orographic rainfall in the Himalayan foothills, particularly in , exacerbates runoff into Bihar's river systems. Major transboundary rivers originating from the , such as the Koshi, Gandak, Bagmati, Burhi Gandak, and Mahananda, serve as primary conduits for floodwaters. These rivers experience steep gradients in their upper reaches, facilitating rapid and high loads that naturally deposit in Bihar's flatter , elevating riverbeds and reducing over time. The Indo-Gangetic plain's gentle slope and extensive alluvial soils impede swift drainage toward the , leading to widespread inundation and waterlogging when river volumes peak. Historical records indicate floods occur with high frequency; for instance, between 1987 and 2013, many districts faced inundation in over 20 of those 27 years, underscoring the recurrent nature driven by the predictable cycle. Natural variability in intensity further amplifies these drivers, with events like the 2008 Koshi River breach flooding over 3 million people due to exceptional upstream rainfall, though such extremes align with long-term hydrological patterns rather than unprecedented shifts. The rivers' braided channels and meandering courses in the plain contribute to unpredictable shifts, compounding overflow risks during peak flows that can reach magnitudes far exceeding dry-season capacities. These factors collectively render floods a seasonal norm, with empirical data showing no upward trend in frequency over decades, consistent with stable climatic forcings.

Droughts, Heatwaves, and Other Extremes

Bihar experiences periodic droughts primarily in its southern and southwestern districts, where annual rainfall often falls below 750 mm, rendering approximately 33% of the state vulnerable to moisture deficits during the season. Districts such as , , Rohtas, Bhojpur, , and Gaya are classified as drought-prone due to erratic precipitation patterns and reliance on . In August 2024, eleven districts—including , , Banka, , , , , , Gaya, , and Arwal—faced drought-like conditions from 12% to 40% rainfall deficits, leading to reduced paddy cultivation areas from an average of 5 acres to 2-5 acres per farmer compared to prior years. The 2022 kharif season marked Bihar's sixth-driest agricultural period on record, exacerbating crop stress from deficiencies at critical growth stages. Of Bihar's 38 districts, 28 are annually impacted by either droughts or floods, highlighting the state's dual exposure to water extremes driven by variability. Heatwaves in Bihar typically occur from to , with moderate and severe episodes showing increasing frequency trends based on long-term daily data from the Indian Meteorological Department (IMD). The state's average has risen by 1.3°C over the past 50 years, per IMD records, intensifying heat stress in agrarian regions. In May 2024, over 100 heat-related deaths were reported amid prolonged high temperatures exceeding IMD heatwave thresholds (e.g., maximum ≥40°C with a departure of ≥4.5°C from normal). Heatwaves contribute to agricultural losses by accelerating and reducing crop yields, particularly for kharif crops like and , as documented in trend analyses of extreme events. Other extremes include cold waves, which predominantly affect northern and central Bihar during winter months (December-February), with moderate and severe events linked to risks and disruptions in rural livelihoods. frequency exhibits variable trends, with some districts showing increases in impact severity from prolonged sub-zero departures (e.g., minimum temperature ≤4°C with ≥3°C below normal). Additional hazards encompass thunderstorms, strikes, and dust storms, which occur sporadically and amplify vulnerabilities in exposed populations, though quantitative trends indicate less consistent patterns compared to extremes. These events underscore Bihar's susceptibility to non-monsoonal extremes, compounded by topographic and land-use factors.

Human Contributions to Amplification

Deforestation in upstream catchment areas, particularly in , has contributed to increased in Bihar's rivers, reducing and exacerbating severity by elevating river beds. Silt accumulation, driven by land clearance for and , has narrowed river cross-sections, with reports indicating that this human-induced factor accounts for heightened peaks in rivers like the Kosi and Gandak. Embankment construction and maintenance practices have amplified risks through frequent breaches, often resulting from inadequate , poor quality materials, and insufficient upkeep. In , over 125 embankment failures were recorded in 1998 alone, attributed to factors including rat burrowing, wave undercutting, and human encroachment on floodplains, which intensify inundation in populated areas. These structural interventions, intended to contain rivers, have instead promoted waterlogging in adjacent farmlands by confining waters into narrower zones. Urban expansion in cities like has intensified heatwaves via the effect, where conversion of vegetated land to impervious surfaces raises local temperatures. Land use changes from 1988 to 2022 in increased built-up areas, correlating with elevated land surface temperatures and prolonged heat events, as impervious cover traps heat and reduces evaporative cooling. Agricultural practices, including over-reliance on groundwater for irrigating water-intensive crops like paddy, have depleted aquifers and heightened vulnerability. In , unregulated pumping has led to declining water tables, with inefficient flood irrigation exacerbating depletion during dry spells and reducing resilience to shortfalls. This over-extraction, driven by subsidized electricity and crop patterns, has amplified socio-economic impacts in rain-fed regions.

Attribution and Future Projections

Natural Variability Versus Anthropogenic Forcing

Bihar's patterns, which account for approximately 75% of annual rainfall during the summer season, exhibit substantial interannual variability primarily driven by large-scale natural climate oscillations such as the El Niño-Southern Oscillation (ENSO) and the (IOD). ENSO events, particularly El Niño phases, have been correlated with reduced rainfall over eastern , including , through altered and suppressed , while positive IOD phases often enhance rainfall by strengthening winds and moisture convergence. These teleconnections explain much of the observed year-to-year fluctuations in Bihar's rainfall, with correlations between Niño 3.4 indices and regional reaching -0.4 to -0.6 in seasons from 1901-2020. Long-term empirical analyses of gridded data from 1901-2020 reveal no consistent monotonic trend in Bihar's annual or rainfall, with trends often trendless or non-monotonic when accounting for variability, underscoring the dominance of internal modes over any emerging anthropogenic signal. For instance, decadal shifts in strength, such as the relative abundance of strong years in the early followed by weaker phases post-1950, align more closely with multi-decadal ENSO and IOD modulations than with from gases. Attribution efforts using detection and attribution methods indicate that variability accounts for up to one-third of recent deficit trends in , with the remainder potentially linked to anthropogenic warming, though regional attribution remains inconclusive due to model discrepancies in simulating dynamics. Anthropogenic forcing, primarily through elevated atmospheric CO2 concentrations, has contributed to observed surface temperature rises in , with warming rates of 0.1-0.2°C per decade from 1980-2020 partially attributable to human-induced forcings after removing natural variability signals like and volcanic aerosols. However, for extremes, while some model-based studies suggest anthropogenic warming may intensify heavy rainfall events via increased atmospheric moisture capacity (Clausius-Clapeyron relation), from shows increasing extreme events but attributes their frequency more to shifts in circulation influenced by ENSO-IOD interactions than to direct effects. Formal attribution analyses for Indian freshwater declines (1980-2020) implicate anthropogenic in exacerbating reductions, yet these rely on climate models that often overestimate sensitivity to forcings and underestimate natural internal variability. Overall, while global-scale warming is empirically linked to human activities, 's regional climate response highlights natural variability as the primary driver of hydrological extremes, with anthropogenic influences detectable but secondary in modulating .

Empirical Uncertainties and Modeling Limitations

Empirical uncertainties in Bihar's climate data stem primarily from limitations in observational networks and methods. Precipitation records, reliant on networks that exhibit significant spatial variability across Bihar's districts, suffer from inconsistencies due to uneven station density, particularly in rural and flood-prone northern areas. Gridded datasets, such as those from the Indian Meteorological Department (IMD) at 0.25° resolution using Shepard's interpolation, introduce deterministic biases and fail to fully capture localized extremes, with ensemble approaches revealing uncertainties in daily estimates exceeding 10-20% in periods. Temperature observations from 1901-2022 show upward trends of approximately 0.1-0.2°C per decade, but district-level analyses highlight gaps in high-elevation Tarai regions, where data sparsity amplifies errors in trend attribution. These observational challenges are compounded by natural variability in the Indian summer , which dominates Bihar's climate and introduces decadal fluctuations that obscure long-term signals; for instance, annual rainfall trends from 1901-2021 indicate a declining of -3.5 mm/year statewide, yet sub-regional analyses reveal non-significant trends in pre- and post- seasons due to high interannual variance. Entropy-based assessments of monitoring networks underscore suboptimal coverage, leading to underestimation of extremes in southern drought-prone districts like those in the Gangetic plain. Such empirical gaps hinder robust detection of anthropogenic influences, as short-term records (post-1950) are insufficient to disentangle monsoon oscillations from forced changes without assuming model-derived forcings. Climate modeling limitations for Bihar arise from the coarse spatial resolution of global circulation models (GCMs) in CMIP5 and CMIP6 ensembles, which typically operate at 100-200 km scales and inadequately resolve regional and land-atmosphere interactions critical to 's north-south hydroclimatic divide. Projections for increases of 3-5°C and rises of 13-30% by 2100 under high-emission scenarios exhibit wide inter-model spreads, with biases in simulation—such as systematic dry biases over northern (15-30% underprediction) and excessive equatorial light rain—propagating errors into downscaled outputs. Bias correction techniques, while applied in some South Asia-focused studies, rely on historical analogs that may not hold under future and land-use changes specific to Bihar's intensive . Regional models, including those for heatwaves and , reveal cold biases of 2-3°C in Bihar-adjacent simulations and uncertainties in low-intensity rainfall trends, limiting reliable futures for flood-drought amplification. These deficiencies underscore the need for customized, high-resolution projections tailored to India's dynamics, as global ensembles like those in IPCC reports often conflate equatorial biases with subcontinental signals, yielding inconsistent outcomes for vulnerable states like . Peer-reviewed critiques emphasize that without addressing convective parameterization flaws and ocean-atmosphere coupling errors, model-derived attributions risk overconfidence in anthropogenic dominance over natural forcings.

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