Climate, Monsoon and Floods in Assam
Assam has one of the most distinctive climates in India. It is warm, it is humid, and it receives some of the heaviest rainfall in the country. This same generosity of rain, however, is also the reason why the state floods almost every year. The monsoon that makes the fields green also swells the Brahmaputra and its tributaries until they break their banks. Understanding this single relationship - between the monsoon and the floods - explains a great deal about Assam's agriculture, its economy and even its politics. It is also a topic that appears regularly in the geography and general knowledge sections of state examinations.
A humid subtropical climate
Assam has a humid subtropical climate. The average annual rainfall is high, generally between about 2,500 and 3,000 millimetres, and it is concentrated in the monsoon months. Humidity stays high all year, roughly between 70 and 90 per cent, which is why the air feels heavy even when the temperature is not very high. Summer temperatures usually range from about 25 to 32 degrees Celsius, while winter temperatures fall to about 10 to 15 degrees Celsius. The hill district of Dima Hasao, because of its elevation, is the coolest part of the state. Places in the northern districts such as Lakhimpur and Dhemaji receive very heavy rain, and together with the neighbouring Meghalaya hills they form one of the wettest regions on earth.
The four seasons
Assam's year is commonly divided into four seasons. Winter runs from about December to February and is cool and dry. The pre-monsoon season of March to May becomes hot and humid and is marked by thunderstorms. The monsoon from June to September brings the main rainfall. Finally, the retreating monsoon of October and November brings lighter rain and a gradual cooling that leads back into winter. Each season has its own effect on farming: the pre-monsoon showers help prepare the fields, the monsoon waters the paddy, and the dry winter is the season for harvesting and for many festivals.
How the monsoon reaches Assam
Most of Assam's rain comes from the south-west monsoon. Moisture-laden winds blow in from the Bay of Bengal and, to a lesser extent, from the Arabian Sea. As these winds travel into the north-east and meet the hills, they are forced to rise, cool and release their moisture as heavy rain. The funnel shape of the region, with hills on almost all sides, traps the monsoon and squeezes out an enormous amount of rain. This is why the same monsoon that gives moderate rain to much of India gives Assam a downpour. The retreating monsoon, which blows back towards the sea in October and November, brings only light rain and marks the change of season.
Kalbaisakhi, the pre-monsoon storm
In April and May, Assam experiences violent local thunderstorms known as Kalbaisakhi, and in Assamese they are often called Bordoisila. These storms arrive suddenly with strong winds, thunder and heavy but short showers. They can damage standing crops and uproot trees, yet they are also welcomed because they cool the air and prepare the ground for the monsoon. The word Bordoisila is also linked in Assamese culture to a daughter returning to her parents' home, and the storm is remembered in folk songs and stories. Questions on this storm are common because it is a purely local seasonal feature.
Why Assam floods
The floods of Assam have several causes working together. The first is simply the huge volume of monsoon rain that falls in the catchment of the Brahmaputra and the Barak. The second is siltation: the rivers carry so much sand and silt that their beds rise, and once the bed is high the river needs less water to overflow. The third is that the Brahmaputra is a braided and shifting river, so its channels move and its banks erode, sometimes breaching the embankments built to hold it. Hills and deforestation in the upper catchment make the runoff faster, and the narrow valley of the Barak traps water in the south. As a result, about 40 per cent of Assam's area is flood-prone, and the state usually sees two to three waves of floods in a monsoon season. Districts such as Morigaon, Nagaon, Barpeta and Dhubri are among the worst affected, and the Barak valley has its own separate flood problem.
Managing the floods
Flood management in Assam is a large and difficult task. The Brahmaputra Board, set up in 1980, is the statutory body responsible for planning flood control and the development of the Brahmaputra valley. The Assam State Disaster Management Authority, or ASDMA, coordinates relief, rescue and preparedness during floods. Embankments have been built along many rivers to keep the water in, though they are frequently breached and often only shift the problem from one place to another. Experts increasingly argue for a mix of measures - better embankments, dredging and river training, watershed management in the hills, and improved early warning - rather than relying on embankments alone.
Why this topic matters for competitive examinations
Climate and floods are asked both as direct facts and as applied questions. Direct facts include the type of climate, the average rainfall, the months of heaviest rain, the name of the pre-monsoon storm and the meaning of Bordoisila. Applied questions ask about the causes of floods, the agencies responsible for flood control and the percentage of flood-prone area. Because this topic connects geography with current affairs and disaster management, it is useful not only for the geography paper but also for general studies and interview preparation.
How to prepare this topic
- Learn the type of climate and the rainfall range as fixed facts.
- Fix the four seasons and the months of each in order.
- Understand the south-west monsoon and why the hills of the region cause such heavy rain.
- Note the two local names for the pre-monsoon storm - Kalbaisakhi and Bordoisila.
- List the causes of floods and the two main agencies, the Brahmaputra Board and ASDMA.
Common exam traps
- Assam's climate is humid subtropical, not tropical or temperate.
- The heaviest rain comes from the south-west monsoon, not the retreating monsoon.
- Bordoisila and Kalbaisakhi refer to the same pre-monsoon storm.
- Floods are not caused only by heavy rain; siltation, braiding and bank erosion are just as important.
Frequently Asked Questions
What type of climate does Assam have and how much rain does it receive?
Assam has a humid subtropical climate. Its average annual rainfall is generally between about 2,500 and 3,000 millimetres, most of it falling during the south-west monsoon from June to September. Humidity remains high throughout the year, usually between 70 and 90 per cent, and the northern districts receive some of the heaviest rain in the region.
What is Kalbaisakhi or Bordoisila?
Kalbaisakhi is the violent pre-monsoon thunderstorm that occurs in Assam in April and May, and in Assamese it is popularly called Bordoisila. These storms come suddenly with strong winds, thunder and heavy short showers. They can damage crops and trees, but they also cool the air and prepare the soil for the coming monsoon, which is why they are both feared and welcomed.
What are the main causes of floods in Assam?
The main causes are the extremely heavy monsoon rainfall in the catchment, the siltation that raises river beds, and the braided, shifting nature of the Brahmaputra which causes bank erosion and breaches embankments. Deforestation and rapid runoff in the hills, and the hill-locked Barak valley in the south, add to the problem. Together these factors make about 40 per cent of the state flood-prone.
Which agencies handle flood control and disaster management in Assam?
The Brahmaputra Board, set up in 1980, is the statutory body for planning flood control and the development of the Brahmaputra valley. The Assam State Disaster Management Authority, ASDMA, is responsible for disaster preparedness, relief and rescue during floods. Together with the state water resources department, these agencies carry out embankment work, early warning and relief operations.
How many waves of floods does Assam usually face in a year?
Assam generally experiences about two to three waves of floods during a single monsoon season. The first wave usually comes with the early heavy rain, and later waves follow as the monsoon continues and the rivers remain full. The Barak valley also floods, but its floods are shorter and more local because the valley is smaller and surrounded by hills.