Bihar: Climate and soils

Bihar has a continental monsoon type of climate, owing to its great distance from the sea. Its plain is covered by a thick alluvial mantle of drift origin, and its soils fall into three major types. The seasons, rainfall and post-monsoon cyclones shape farming in the state.

Soil Composition

• The Bihar plain has a thick alluvial mantle of drift origin, overlying in most parts the Siwalik and older tertiary rocks.
• The soil is mainly young loam, rejuvenated every year by the deposition of silt, clay and sand brought by different streams.
• It is deficient in phosphoric acid, nitrogen and humus.
• Potash and lime are usually present in sufficient quantity.
• Soil is described as the backbone of agricultural and industrial development; almost all economic activities depend on it directly or indirectly.

Three Major Soil Types

• Piedmont Swamp Soil: found in the northwestern part of West Champaran district.
• Terai Soil: found in the northern part of the state along the border of Nepal.
• Gangetic Alluvium: covers the plain of Bihar, both new and old alluvium.

Factors Affecting the Climate

• Latitude: the state portal says Bihar extends from 22° North to 27° North, so its location is tropical to sub-tropical.
• Himalayas: the Himalayan Mountains in the north have a significant bearing on the distribution of monsoon rainfall.
• Position: Bihar joins the Ganga delta and Assam.

Climate Overview

• Classification: Cwa.
• Average temperature: 27 °C (81 °F).
• Summer: 34 °C (93 °F); winter: 10 °C (50 °F).
• Precipitation: 975.86 mm (source: IMD).

Winter

• Cold weather begins early November and ends in mid-March.
• October and November are pleasant, with bright, warm days and sharp cold after sunset.
• Winter temperature: 0–10 °C; coldest months: December and January.
• Record lows: 0 °C in Gopalganj and -2 °C in Forbesganj.

Summer

• The hot weather sets in March and lasts until mid-June.
• May is the hottest month.
• Bihar experiences dust-storms, thunder-storms and dust-raising winds in the hot season.
• Dust storms of 48–64 km/hour are most frequent in May.
• The hot winds (loo) blow in April and May at an average 8–16 km/hour.

Monsoon

• The rainy season begins soon after mid-June and lasts till the end of September.
• It begins when a storm from the Bay of Bengal passes over Bihar.
• The rains come from the south-west monsoon; the rainiest months are July and August.
• The monsoon normally withdraws in the first week of October.

Post-Monsoon Cyclones

• Tropical cyclones from the Bay of Bengal, originating at about 12° N latitude, invade Bihar in the retreating monsoon season.
• Bihar is also influenced by typhoons from the South China Sea.
• Cyclones are most frequent in September–November, especially during the asterism called Hathiya.
• They are essential for the maturing of paddy and for moistening the soil for Rabi crops.