Bihar lies between humid West bengal (in the East) and sub-humid Uttar Pradesh (in the West).Hence, the climate of Bihar shows a transition between humid to sub-humid or dry climate. Basically the Eastern part of Bihar is more or less humid in nature and the Western part of Bihar is sub-humid or dry in nature. Bihar lies in the Sub Tropical region of Temperate zone, according to standard classification its climatic type is Humid Sub Tropical (Cwa).
Factors Affecting the Climate of Bihar
Factors influencing the climate of Bihar.
The climate of Bihar is influenced mainly by following five factors:-
- Nearness to Himalayas.
- Nearness to Tropic of Cancer.
- Nearness to Bay of Bengal.
- Activities of South-West monsoon.
- Nor-westers and Summer Winds.
Seasons in Bihar
There are three distinct seasons are in Bihar because Bihar is situated in the Tropical region.
- Summer Season:- March to June
- Rainy Season:- July to October
- Winter Season:- November to February
Also Read: Physiographic Divisions of Bihar
Summer Season
It starts in March with the slowly and steadily shifting Inter Tropical Convergence zone towards the Tropic of Cancer and reaches upto Ganga Plains. Which lead to rise in temperature and fall in the air pressure in hte region. The highest temperature reach upto 45° Celsius in Bihar especially in Gaya, which is the hottest district of Bihar.
In the month of April, May and early June, a prominent summer wind blows in the plains of Bihar called as 'Loo'. It is dry and hot air, which causes many health problems and even deaths in Bihar.
In the month of April and May Bihar witness thunderstorms (carrying dusts and strong wind) almost every weeks in afternoon and evening, which often causes light drizzle of rain but some times it also accompanied with hail storms which causes great damage to Rabi Crops especially wheat.
In the month of May-June Norwesters (Kal Baisakhi) also occurs, which influences the climate of Bihar(especially Eastern Bihar). It causes harms to crops as well as human life.
Rainy Season
In the mid-June the Inter Tropical Convergence Zone (ITCZ) shifts completely over Ganga Plain which attracts South-West monsoon from Arabian Ocean and Bay of Bengal. Bihar receives monsoon from the both the branches of the Monsoon:-
- Arabian Ocean Branch of S-W Monsoon.
- Bay of Bengal Branch of S-W Monsoon.
However, early Rain in Bihar happens generally by the Bay of Bengal Branch of the Monsoon.
Bihar receives almost 91% of it's annual rainfall in the rainy season and that too in three months i.e. June, July and August.
| Months | Rainfall (in mm) |
|---|---|
| June | 185.5 |
| July | 340.0 |
| August | 259.0 |
This is the season of Paddy cultivation in Bihar, which is heavily dependent on Monsoon. but Monsoon in Bihar especially Southern Bihar is very erratic and almost after every 2-3 years drought occurs in this part. But Northern Part of Bihar receives good rainfall almost every year and almost every year it also witnesses flood in the region, not due to heavy rain but due to excess water released by Nepal through various streams (especially Koshi river). Floods in Northern Bihar causes large scale damages every year.
Besides floods and drought, Bihar also witnesses large number of diseases like malaria, Typhoid and Dengue etc in the rainy season.
Retreating Monsoon
After August the S-W Monsoon became slow and starts retreating. It is accompanied with very little or no rain. Retreat in Monsoon ends upto early November.Little drizzle of rain due to retreating Monsoon in the month of September October and in early November slowly invites winter season in the state.
The average temperature decreases from 34° Celsius July-August to (25° Celsius - 30° Celsius) in the end of October.
Also Read: Geological Structure of Bihar
Winter Season
With the end of Rainy Season the Solar radiation also reduced which leads to less heating of land surface. Hence, temperature starts dropping and brings the winter.
The day time reduces tremendously and nights become long and cold. December and January witness maximum drop in the temperature. The temperature decreases from 19.4° - 22.2° Celsius in November to 10.5° - 7° Celsius in the month of January.
North-Western Disturbances bring light rainfall (about 5% - 10/% of the total rainfall of the year) in the month of January and February, it helps in the growth of Rabi Crops.
In January, North-Western Bihar (Shiwalik Ranges) witnesses frost in the region, which causes much harm to crops (especially Potato).