Bihar is lying approximately between 21o58'10'' and 27o31'15''N latitudes and 82o 19'50'' and 88o17'40''E longitudes in the lower and middle Gangetic region extending 483 Km from west to east. This state embraces some of the most fertile lands of India. Bihar, squeezed in between West Bengal, Jharkhand and Uttar Pradesh, reaches up to the Himalayas in the north and is completely land locked. Bihar is bounded on the north by Nepal, on the south by Jharkhand, on the east by West Bengal and on the west by Uttar Pradesh.

Bihar gets the worst of the cold and the worst of the heat and plenty of floods. Northern portion of Bihar is almost entirely a level tract, while the south is wooded and hilly. Bihar has a number of rivers, the most important of which is the Ganga. The river Ganga flows right across it from west to east. North Bihar is extremely fertile, the land being watered by the rivers Sarayu, Gandak and Ganga. The other rivers are the Sone, Poonpoon, Falgu, Karmanasa, Durgawati, Kosi, Ghaghara etc.

Natural Divisions

Bihar is traditionally divided into

1) The North Ganga plain

2) The South Ganga plain

The North Ganga Plain

It extends from the base of the Terai in the north to the Ganga in the south, covering an area of about 56,980 Sq Km. It spreads over the whole of Tirhut, Saran, Darbhanga and Kosi divisions and has a gentle slope towards the south. The Ganga flows from west to east near the southern margin of the plain. Towards the north and north-west in the east and west Champaran districts, the country begins to undulate and the alluvial plain gives place to broken hilly region known as the Dun or Ramnagar Dun. This consists of a range of low hills. Below these hills, large grassy prairies watered by numerous hill streams extend southwards and eastwards. The soil even at the foot of the hills has no rocky formation and whenever water can be impounded, rich growth of crop is possible.

The South Ganga Plain

The alluvial filling south of the Ganga is shallow, a mere veneer and the Peninsular edge is very rugged. Many groups of small craggy hills rise up to 488 meters from islands of bare rock or scrub. In the west, where the stream Sone makes a great deltaic reentrance into the older rocks, this alluvial strip is some 137 Km wide. But in the east where the Rajmahal hills lies on the extreme north-east point of the Peninsula, it goes almost directly on to the Ganga. The river bank itself lies high, except in Bohjpur district and at high water the tributaries are flooded and pushed back. The Punpun valley, parallel to the stream Sone on the east, is thus annually flooded.

Both in the north and the south of Ganga, the construction of railways across the drainage causes local but sometimes disastrous water logging and flooding. Some of these temporary inundations are agriculturally useful, either rabi crops are grown on them when they dry out or they are bunted for producing dry weather rice.

The cold weather commences early in November and comes to an end in the middle of March. The hot weather then sets in and lasts till the middle of June. Soon after this the rainy season commences and continues till the end of September, the beginning of this season occurs when a storm from the Bay of Bengal passes over Bihar. The commencement of monsoon may be as early as the last week of May or as the first or second week of July. The climate in the cold weather is pleasant. The days are bright and warm and the sun is not too hot. As soon as the sun sets the temperature falls and the heat of the day yields place to a sharp bracing cold. The mean temperature in November all over Bihar varies from 19.6oC to 22.2oC. The mean temperature in Gaya and Patna in December is 17oC and 18.2oC respectively. January is the coldest month in Bihar. The mean minimum temperature varies from 7.5oC to 10.5oC though some places like Netarhat record much lower temperatures than 7.5oC.

The highest temperature is often registered in May which is the hottest month in the state. Like the rest of the northern India, Bihar also experiences dust-storms, thunder-storms and dust raising winds during the hot season. Dust storms having a velocity of 48-64 Km/Hour are most frequent in May and with second maximum in April and June. The hot winds (loo) of Bihar plains blow during April and May with an average velocity of 8-16 Km/hour. This hot winds greatly affects human comfort during this season. The rainy season begins in June. The rainiest months are July and August. The rains are the gifts of the south west monsoon. There are in Bihar three distinct areas where rainfall exceeds 1800mm. Two of them lie on northern and north-western wings of the state and the third lies in the Netarhat pat. The south-west monsoon normally withdraws from Bihar in the first week of October. An important feature of the retreating monsoon season in Bihar is the invasion of tropical cyclones originating in the Bay of Bengal at about 12oN latitude. Bihar is also influenced by the typhoons originating in the south China sea. The maximum frequency of the tropical cyclones in Bihar is during September-November especially during the asterism called hathiya. These cyclones are essential for the maturing of paddy, and are required for the moistening of the soil for the cultivation of rabi crops.

One of the most striking feature of the river system of Bihar is the dominant role of Ganga. The important rivers that join the Ganga from the north are, from west to east, Ghaghra, the Gandak, the Burhi Gandak, the Kosi, the Mahananda and its tributaries. Sonpur, which is situated along Ganga's bank is famous for the great bathing festival which is the occasion for the greatest cattle and elephant fair in the world. The Karmanasa, the Sone, the Punpun, the Phalgu, the Sakri and the Kiul are the principal streams that joins the Ganga from the south.