Climate of Assam: Seasons, Temperature, Rainfall, Monsoon & Climatic Regions
The climate of Assam is predominantly warm, humid and monsoonal, shaped by the state's location in Northeast India, the Southwest Monsoon, surrounding mountain ranges, extensive river valleys and varied relief. Heavy rainfall, high atmospheric humidity, comparatively mild winters and warm summers are the most important climatic characteristics of the state.
Assam receives rainfall during almost every season, but the greatest share arrives during the Southwest Monsoon from June to September. Pre-monsoon thunderstorms during March, April and May are another distinctive climatic feature. These storms are popularly associated with Bordoisila or Bordoichila and often bring strong winds, thunder, lightning and intense rainfall.
The state's climate is closely related to its physiography of Assam. The broad Brahmaputra Valley, wetter Barak Valley, Himalayan foothills, Karbi Anglong Plateau and Dima Hasao hills experience different rainfall, temperature and humidity conditions because of differences in altitude, relief and exposure to moisture-bearing winds.
According to the latest Assam Economic Survey, the state receives an average annual rainfall of approximately 2,297.4 mm. Rainfall is not distributed uniformly, however. Official agro-climatic descriptions indicate annual rainfall broadly ranging from about 1,500 mm in relatively less-wet regions to around 3,500 mm in parts of the Barak Valley.
This detailed guide examines the seasons, temperature, rainfall, Southwest Monsoon, pre-monsoon thunderstorms, humidity, climatic controls, regional climatic variations, agro-climatic zones, floods, agriculture, biodiversity and climate-change trends of Assam.
Table of Contents
- Climate of Assam at a Glance
- What Type of Climate Does Assam Have?
- Factors Controlling the Climate of Assam
- Major Seasons of Assam
- Winter Season
- Pre-Monsoon / Spring-Summer Season
- Bordoisila and Pre-Monsoon Thunderstorms
- Southwest Monsoon Season
- Post-Monsoon and Autumn Season
- Temperature of Assam
- Rainfall in Assam
- Seasonal Distribution of Rainfall
- Latest Rainfall Snapshot
- Regional Distribution of Rainfall
- Climatic and Agro-Climatic Regions of Assam
- Upper Brahmaputra Valley
- North Bank Plains
- Central Brahmaputra Valley
- Lower Brahmaputra Valley
- Barak Valley
- Hill Region
- Humidity, Fog and Cloudiness
- Climate and Agriculture
- Climate and Tea Cultivation
- Climate, Floods and Riverbank Erosion
- Climate, Forests and Biodiversity
- Climate and Soil
- Extreme Weather in Assam
- Climate Change in Assam
- Importance of Climate to Assam
- Important Facts for APSC & ADRE
- Frequently Asked Questions
- Conclusion
- References
Climate of Assam at a Glance
| Climatic Feature | Main Characteristics |
|---|---|
| General Climate | Humid tropical to subtropical monsoon climate |
| Main Rain-Bearing System | Southwest Monsoon, particularly its Bay of Bengal moisture flow |
| Average Annual Rainfall | About 2,297.4 mm according to Assam Economic Survey 2025–26 |
| Main Rainy Season | June to September |
| Normal Monsoon Share | About 65% of annual rainfall |
| Pre-Monsoon Rainfall Share | About 25% during March–May |
| Summer Conditions | Warm to hot and highly humid; maximum temperatures can reach the mid to upper 30s°C during hot spells |
| Winter Conditions | Cooler and comparatively dry; colder spells may bring minimum temperatures into the 5–8°C range in some areas |
| Pre-Monsoon Storm | Bordoisila / Bordoichila thunderstorms |
| Major Climatic Hazards | Floods, thunderstorms, lightning, intense rainfall, erosion, waterlogging, occasional drought-like conditions and heat stress |
| Broad Regional Contrast | Wet Barak Valley and foothill belts; humid Brahmaputra Valley; cooler hill districts |
What Type of Climate Does Assam Have?
Assam has a predominantly humid monsoonal climate. Government climate documentation describes it as warm and humid and also uses the expression “Tropical Monsoon Rainforest Climate”.
However, a single climatic label does not describe every part of Assam equally well.
The state extends across:
- low alluvial plains;
- river valleys;
- foothills;
- plateaus;
- forested hills; and
- higher uplands.
Consequently, the Brahmaputra and Barak plains have predominantly humid tropical to subtropical conditions, while higher areas of Karbi Anglong and Dima Hasao are comparatively cooler.
This climatic diversity forms part of the wider climate of Northeast India, where mountains, plateaus and valleys create major local differences in rainfall and temperature.
Factors Controlling the Climate of Assam
The climate of Assam results from the interaction of several geographical and atmospheric factors rather than from latitude alone.
1. Geographical Location
Assam lies roughly between 24°N and 28°N latitude in the northeastern part of India.
Its relatively low latitude gives the state a generally warm climatic base, while its inland position and surrounding hills produce strong seasonal and regional variations.
2. Southwest Monsoon
The Southwest Monsoon is the dominant control over Assam's rainfall.
Moist air from the Bay of Bengal moves toward Northeast India and is forced to rise when it encounters hills and mountains.
Cooling of this moisture-laden air produces widespread monsoonal rainfall.
3. Surrounding Mountains and Plateaus
Assam is surrounded or bordered by major elevated regions including:
- the Eastern Himalaya to the north;
- the Patkai-Naga hill systems toward the east and southeast;
- the Meghalaya Plateau to the south-west;
- the Barail Range; and
- the hill systems of Karbi Anglong and Dima Hasao.
These highlands influence the direction, uplift and concentration of moisture-bearing winds.
4. Brahmaputra and Barak Valleys
The broad river valleys provide relatively low-lying corridors through which warm, moist air circulates.
The two major valley systems also contain extensive rivers, wetlands, floodplains and moist agricultural landscapes that contribute to high atmospheric humidity.
5. Altitude
Temperature generally decreases with increasing altitude.
Therefore, hill districts such as Dima Hasao and elevated parts of Karbi Anglong generally experience milder summer temperatures than the low plains.
6. Pre-Monsoon Atmospheric Disturbances
During spring and early summer, atmospheric instability, strong surface heating and moisture combine to generate intense thunderstorms.
These storms provide an important source of rainfall before the regular Southwest Monsoon becomes established.
Major Seasons of Assam
The seasonal cycle of Assam can be understood in two ways.
For rainfall statistics, official meteorological and economic publications commonly use:
- Winter: January–February;
- Pre-Monsoon / Summer: March–May;
- Southwest Monsoon: June–September; and
- Post-Monsoon: October–December.
In everyday geographical description, the cool period often extends from November through February, while spring and autumn act as transitional seasons.
| Season | Approximate Period | Major Characteristics |
|---|---|---|
| Winter / Cold Season | November–February; January–February in rainfall statistics | Cool, comparatively dry, foggy mornings, low rainfall |
| Pre-Monsoon / Spring-Summer | March–May | Rising temperature, high humidity, thunderstorms and Bordoisila |
| Southwest Monsoon | June–September | Heavy rainfall, high humidity, cloudy weather, floods and waterlogging |
| Post-Monsoon / Autumn | October–November, extending into December in rainfall statistics | Declining rainfall and humidity, clearer skies and falling temperature |
Winter Season in Assam
Winter is the coolest and driest period of the year.
The broader cool season generally begins after the withdrawal of the monsoon during October and November and becomes well established by December.
January is normally among the coldest months.
Temperature
Winter minimum temperatures in the plains commonly remain around or above the upper single digits, but colder spells may bring temperatures to approximately 5–8°C in some locations.
Higher hill regions may experience still cooler local conditions.
Rainfall
Winter is the driest meteorological season.
The Assam Economic Survey indicates that only around 2% of normal annual rainfall occurs during January and February.
Occasional rainfall may occur when western disturbances or other weather systems influence the region.
Fog
Fog and mist frequently form during winter mornings, especially in:
- river valleys;
- wetland areas;
- low-lying plains; and
- areas where night-time cooling is strong.
Visibility can therefore fall considerably during some winter mornings.
Pre-Monsoon / Spring-Summer Season
March to May marks an important transitional period between the cool winter and the Southwest Monsoon.
Temperature rises progressively, but Assam receives much more pre-monsoon rainfall than many parts of northern and western India.
The latest Economic Survey indicates that approximately 25% of normal annual rainfall occurs during March–May.
This rainfall is particularly important because it:
- reduces extreme daytime heating;
- provides moisture before the main monsoon;
- supports tea growth;
- benefits pre-kharif agriculture;
- replenishes ponds and wetlands; and
- increases river discharge before June.
However, the season also experiences severe thunderstorms, lightning and strong winds.
Bordoisila and Pre-Monsoon Thunderstorms
One of the best-known features of Assam's weather is the occurrence of powerful pre-monsoon thunderstorms commonly called Bordoisila, Bordoichila or related local spellings.
These storms occur particularly during April and May.
They may be accompanied by:
- strong squally winds;
- intense rainfall;
- thunder and lightning;
- rapid cloud development;
- occasional hail; and
- abrupt falls in temperature after storm passage.
Importance of Bordoisila Rainfall
The storms contribute significantly to Assam's pre-monsoon rainfall.
They help maintain soil moisture and are particularly important to tea-growing areas before the regular monsoon becomes firmly established.
Hazards
Severe thunderstorms may damage:
- houses;
- trees;
- power infrastructure;
- standing crops;
- tea gardens; and
- transport and communication networks.
Lightning associated with convective storms also represents an important weather hazard.
Southwest Monsoon in Assam
The Southwest Monsoon is the most important climatic system affecting Assam.
The principal rainy season extends from June to September.
During this period:
- humidity becomes very high;
- cloud cover increases;
- rainfall becomes frequent and widespread;
- rivers rise rapidly;
- wetlands expand;
- flooding becomes common; and
- agriculture becomes strongly dependent on rainfall distribution.
Bay of Bengal Moisture
A large part of Assam's monsoonal moisture reaches Northeast India from the Bay of Bengal.
Moist air moving inland is channelled and uplifted by the surrounding hills and mountains, increasing rainfall.
Orographic Rainfall
Orographic uplift plays a major role in the spatial distribution of rainfall.
When humid air encounters elevated terrain, it is forced upward. The rising air cools, condensation occurs and rainfall develops.
This process helps explain the high rainfall of:
- Himalayan foothill areas;
- parts of the North Bank;
- Barak Valley and its surrounding hills; and
- other windward hill-margin regions.
Monsoon Withdrawal
The Southwest Monsoon normally begins withdrawing toward the end of September and through October.
As rainfall decreases, skies become clearer, atmospheric humidity gradually falls and temperatures begin declining.
Post-Monsoon and Autumn Season
October and November form one of the most pleasant climatic periods in Assam.
After the withdrawal of the main monsoon:
- rainfall decreases;
- river levels generally begin falling;
- humidity becomes less oppressive;
- night temperatures decline; and
- clearer weather becomes more common.
The Economic Survey indicates that roughly 7% of normal annual rainfall occurs during the October–December post-monsoon period.
Rainfall can still occur when depressions, cyclonic systems or local atmospheric disturbances affect eastern and northeastern India.
Temperature of Assam
Temperature varies considerably across Assam because of altitude, cloud cover, rainfall, humidity and regional relief.
The Assam Climate Change Knowledge Portal describes the overall climate as warm and humid and reports that summer maximum temperatures can reach approximately 35–39°C, while winter minima may fall to around 5–8°C.
These figures represent broad state-level temperature limits rather than uniform averages for every district.
Temperature in the Plains
The Brahmaputra and Barak plains experience relatively warm conditions.
Summer daytime temperatures commonly rise above 30°C, but heavy cloud cover, thunderstorms and rainfall often prevent temperatures from reaching the extreme levels common across the hot interior plains of northern India.
Temperature in the Hills
Karbi Anglong and Dima Hasao generally experience cooler conditions because of their higher elevation.
Local temperature differences can be considerable between:
- hilltops;
- valley bottoms;
- forested slopes; and
- urban settlements.
Seasonal Temperature Cycle
| Period | Temperature Character |
|---|---|
| December–February | Cool; coldest nights of the year |
| March–April | Rapid warming moderated periodically by thunderstorms |
| May | Warm and very humid with frequent convective storms |
| June–September | Warm and humid; extensive cloud and rainfall reduce daytime heating |
| October–November | Gradual cooling after monsoon withdrawal |
Rainfall in Assam
Rainfall is the most important element of Assam's climate because it controls river discharge, floods, agriculture, wetlands, vegetation, groundwater and soil erosion.
The Assam Economic Survey 2025–26 reports a statewide average annual rainfall of approximately 2,297.4 mm.
However, rainfall varies considerably across the state.
The official agro-climatic descriptions indicate broad annual rainfall ranges from around 1,500–2,500 mm in some central and hill regions to approximately 2,500–3,500 mm in the Barak Valley.
Why Does Assam Receive Heavy Rainfall?
The major reasons are:
- large supplies of moisture from the Bay of Bengal;
- strong Southwest Monsoon circulation;
- orographic uplift along surrounding mountains and hills;
- pre-monsoon thunderstorms;
- frequent monsoon depressions and low-pressure systems;
- high atmospheric humidity; and
- the state's location within one of India's major monsoonal rainfall regions.
Seasonal Distribution of Rainfall in Assam
Rainfall is strongly seasonal despite precipitation occurring throughout much of the year.
| Season | Months | Approximate Share of Normal Annual Rainfall |
|---|---|---|
| Winter | January–February | About 2% |
| Pre-Monsoon / Summer | March–May | About 25% |
| Southwest Monsoon | June–September | About 65% |
| Post-Monsoon | October–December | About 7% |
Note: The percentages are rounded values from the Assam Economic Survey and therefore may not sum to exactly 100%.
Latest Rainfall Snapshot for Assam
The Statistical Handbook, Assam 2025 provides a useful recent comparison between actual and normal rainfall.
| Rainfall Indicator | Actual | Normal |
|---|---|---|
| Average Monthly Rainfall, 2024 | 185.4 mm | 217.6 mm |
| Winter Rainfall, 2025 | 37.8 mm | 45.0 mm |
| Summer / Pre-Monsoon Rainfall, 2025 | 529.2 mm | 537.9 mm |
| Southwest Monsoon Rainfall, 2025 | 993.0 mm | 1,461.2 mm |
Regional Distribution of Rainfall in Assam
Rainfall is not evenly distributed across Assam.
Differences arise because of:
- distance from moisture-bearing wind paths;
- altitude;
- orientation of hills;
- local rain-shadow effects;
- valley geometry; and
- proximity to Himalayan and Meghalaya hill systems.
Upper Assam
Upper Assam generally receives abundant rainfall and high humidity.
Official agro-climatic information gives a broad annual rainfall range of approximately 2,000–3,000 mm.
The warm, moist environment is particularly favourable to tea cultivation.
North Bank
The North Bank Plain, lying close to the Eastern Himalayan foothills, also receives high rainfall, commonly within the broad range of approximately 2,000–3,000 mm.
Heavy rain in Himalayan catchments can rapidly raise the discharge of north-bank tributaries.
Central Assam
The Central Brahmaputra Valley generally receives around 1,500–2,500 mm of annual rainfall according to official agro-climatic descriptions.
Lower Assam
The Lower Brahmaputra Valley receives approximately 1,500–2,600 mm annually across the broad agro-climatic zone.
Barak Valley
The Barak Valley is among the wettest broad climatic regions of Assam.
Official agro-climatic information gives annual rainfall of approximately 2,500–3,500 mm.
The surrounding hills and enclosed valley setting contribute to:
- high humidity;
- heavy monsoon rainfall;
- waterlogging; and
- frequent flooding.
The relationship between rainfall and drainage is explained further in the guide to the Barak River Basin of Assam.
Hill Districts
Karbi Anglong, West Karbi Anglong and Dima Hasao show strong local variation according to altitude and slope orientation.
The broad agro-climatic rainfall range is approximately 1,500–2,500 mm, although individual locations may differ considerably.
Climatic and Agro-Climatic Regions of Assam
There is no single universally used set of purely meteorological “climate regions” for Assam.
For geographical, agricultural and competitive-examination study, the most useful official regional framework is Assam's six agro-climatic zones.
These zones are based on a combination of:
- rainfall;
- temperature;
- terrain;
- soil;
- drainage; and
- agricultural conditions.
They should therefore be understood as agro-climatic regions rather than strict Köppen climatic zones.
| Agro-Climatic Region | Broad Location | Approx. Annual Rainfall | Main Climatic Character |
|---|---|---|---|
| Upper Brahmaputra Valley | Upper Assam | 2,000–3,000 mm | Highly humid, wet, flood-prone; excellent tea climate |
| North Bank Plains | Northern Brahmaputra Valley | 2,000–3,000 mm | Heavy rainfall, Himalayan runoff and frequent flash flooding |
| Central Brahmaputra Valley | Nagaon–Morigaon–Hojai region | 1,500–2,500 mm | Humid monsoonal with extensive alluvial agriculture |
| Lower Brahmaputra Valley | Western Assam plains | 1,500–2,600 mm | Humid monsoonal, warm summers and recurrent floods |
| Barak Valley | Southern Assam | 2,500–3,500 mm | Very humid, high rainfall and strong waterlogging tendency |
| Hill Zone | Karbi Anglong, West Karbi Anglong and Dima Hasao | 1,500–2,500 mm | Altitude-modified climate, cooler uplands and strong local rainfall variation |
Administrative note: Official agro-climatic zone descriptions may retain older district groupings even after district boundaries are reorganized. The climatic regions themselves are geographical rather than strictly administrative units.
Upper Brahmaputra Valley Climate
The Upper Brahmaputra Valley includes much of Assam's major tea-growing and petroleum-producing landscape.
Its climate is characterized by:
- high humidity;
- abundant rainfall;
- warm summers;
- cool winters;
- frequent pre-monsoon thunderstorms; and
- recurrent monsoon flooding.
The region receives approximately 2,000–3,000 mm of annual rainfall in the official agro-climatic classification.
Dibrugarh, Tinsukia, Jorhat and adjoining areas experience particularly favourable conditions for tea because of the combination of rainfall, warmth, humidity and suitable soils.
North Bank Plains Climate
The North Bank Plains lie between the Brahmaputra and the Eastern Himalayan foothills.
This location strongly affects rainfall and flood behaviour.
Moist monsoon winds interacting with the Himalayan foothills produce heavy rainfall, while tributaries descending from Arunachal Pradesh respond rapidly to intense precipitation.
The region therefore experiences:
- high annual rainfall;
- flash floods;
- riverbank erosion;
- sediment deposition;
- high humidity; and
- strong monsoon seasonality.
Central Brahmaputra Valley Climate
Central Assam, including the Nagaon-Morigaon-Hojai region in the official agro-climatic description, receives approximately 1,500–2,500 mm of rainfall annually.
The climate remains humid and monsoonal but rainfall is generally lower than in the wettest foothill and Barak Valley regions.
The interaction between rainfall, wetlands, the Kopili system and extensive alluvial plains strongly influences agriculture and flooding.
Lower Brahmaputra Valley Climate
Lower Assam experiences a warm, humid monsoonal climate.
The broad region receives approximately 1,500–2,600 mm of rainfall annually.
Temperature and humidity can become uncomfortable before and during the monsoon, particularly in densely populated urban and lowland environments.
Heavy rainfall across Assam and upstream catchments contributes to high discharge in the Brahmaputra and its tributaries.
Climate of the Barak Valley
The Barak Valley has a distinctive warm and highly humid climatic environment.
Official agro-climatic information gives the valley approximately 2,500–3,500 mm of annual rainfall, making it one of Assam's wettest broad regions.
The valley is surrounded by hills on several sides. This physical setting contributes to:
- orographic rainfall;
- high humidity;
- reduced ventilation during some periods;
- frequent waterlogging; and
- heavy monsoon runoff.
Rainfall and runoff from surrounding hills rapidly enter the Barak and its tributaries, increasing flood risk.
Climate of Karbi Anglong and Dima Hasao
The hill districts experience greater climatic variation than the plains because altitude and slope exposure change over relatively short distances.
Higher elevations generally have:
- cooler temperatures;
- lower summer heat stress;
- greater local cloudiness;
- frequent mist; and
- strong slope-controlled rainfall variation.
The official agro-climatic classification gives the Hill Zone an annual rainfall range of approximately 1,500–2,500 mm.
The distinctive plateau and hill environment of Karbi Anglong creates substantial local differences in temperature, soil moisture, forest cover and agricultural potential.
Humidity, Fog and Cloudiness in Assam
High humidity is one of the defining characteristics of Assam's climate.
Humidity is particularly high from the pre-monsoon period through the monsoon and into the early post-monsoon season.
Reasons for High Humidity
Important factors include:
- abundant monsoon moisture;
- large river systems;
- wetlands and beels;
- waterlogged floodplains;
- dense vegetation; and
- frequent rainfall.
Winter Fog
During winter, clear nights allow the land surface to cool.
Where moist air occurs close to the ground, condensation may produce fog, especially over:
- low-lying valleys;
- riverbanks;
- wetlands;
- agricultural fields; and
- urban-peripheral lowlands.
Influence of Climate on Agriculture in Assam
Climate exerts a major influence on Assam's agricultural calendar.
The state's major crops depend on the timing and distribution of rainfall rather than simply the annual total.
The relationship between rainfall and cropping seasons can be explored in detail in Assam's agricultural seasons and major crops.
Ahu Rice
Ahu rice is generally sown during the spring period and benefits from pre-monsoon rainfall.
Sali Rice
Sali rice is the principal monsoon-season rice crop.
Its cultivation depends strongly on:
- monsoon onset;
- seasonal rainfall;
- field water levels; and
- absence of destructive prolonged flooding.
Boro Rice
Boro rice is grown during the relatively dry season, particularly in low-lying areas where irrigation or retained moisture is available.
Mustard and Pulses
Lower winter rainfall benefits crops that require comparatively drier conditions during maturation and harvesting.
Climate and Tea Cultivation in Assam
Assam's climate is one of the principal reasons for the development of its globally important tea industry.
Tea thrives under:
- warm temperatures;
- high humidity;
- abundant but well-distributed rainfall;
- long growing seasons; and
- well-drained acidic soils.
Upper Assam is particularly favourable because frequent rainfall and warm, humid conditions support repeated tea-flush cycles.
Pre-monsoon rainfall is especially important because it helps initiate active plant growth before the main rainy season.
Excessive rainfall, prolonged waterlogging, hail, strong winds and extreme heat, however, may damage tea plantations.
Climate, Floods and Riverbank Erosion in Assam
Flooding is one of the most important consequences of Assam's monsoon climate.
Heavy rainfall occurs not only within Assam but also throughout the vast upstream catchments of the Brahmaputra and its tributaries.
The interaction between climate and drainage is examined in detail in the Brahmaputra River Basin of Assam.
Why Monsoon Rainfall Causes Flooding
Flooding results from the interaction of:
- heavy monsoon rainfall;
- intense rain over Himalayan catchments;
- rapid tributary runoff;
- high sediment load;
- low-lying floodplains;
- braided river channels;
- drainage congestion; and
- simultaneous flood peaks in tributaries and the Brahmaputra.
Flash Floods
Flash floods are especially important along foothill tributaries, where steep upstream gradients allow rainfall to reach the plains rapidly.
Barak Valley Flooding
The Barak Valley is also vulnerable because heavy rainfall from its surrounding hills feeds numerous tributaries into a relatively low-lying valley.
Climate, Forests and Biodiversity of Assam
Warm temperatures and abundant rainfall support Assam's exceptional biological diversity.
The ecological connection between rainfall, forests, wetlands and wildlife is examined in the detailed guide to the biodiversity of Assam.
Variations in temperature, moisture, drainage and altitude help support:
- tropical wet evergreen forests;
- semi-evergreen forests;
- moist deciduous forests;
- subtropical hill vegetation;
- riverine grasslands;
- swamps;
- wetlands; and
- bamboo-dominated vegetation.
Heavy monsoon rainfall is therefore both an ecological resource and a source of environmental stress.
Climate and Soil Formation in Assam
Climate plays a major role in the development and modification of Assam's soils.
High rainfall encourages:
- chemical weathering;
- leaching;
- soil acidity;
- erosion on steep slopes;
- movement of nutrients through soil profiles; and
- rapid decomposition of organic matter under warm conditions.
At the same time, seasonal floods deposit fresh sediments across parts of the Brahmaputra and Barak valleys.
This produces extensive alluvial soil landscapes.
The relationship between rainfall, relief and soil development is discussed further in Soils of Assam: Types, Distribution and Characteristics.
Extreme Weather in Assam
Although Assam's climate is generally humid and rainfall-rich, the state experiences considerable weather variability.
Heavy Rainfall
Short-duration intense rainfall may cause:
- urban flooding;
- flash floods;
- landslides;
- erosion;
- crop damage; and
- transport disruption.
Thunderstorms and Lightning
Pre-monsoon thunderstorms can be particularly severe and are associated with strong winds, lightning and occasional hail.
Floods
Large-scale monsoon floods remain the state's most widespread climatic and hydrological hazard.
Drought-Like Conditions
A high-rainfall climate does not prevent drought.
If monsoon rainfall is substantially below normal or poorly distributed, agricultural drought-like conditions may occur.
Crop stress may therefore arise even in a state with high long-term average rainfall.
Heat and Humidity
High humidity reduces the effectiveness of evaporative cooling from the human body.
As a result, conditions can feel significantly hotter than the measured air temperature during humid summer periods.
Climate Change in Assam
Climate change adds another dimension to Assam's already variable monsoon environment.
The Assam Climate Change Knowledge Portal reports an IMD analysis based on weather observations from 1951 to 2010.
For the Assam stations used in that historical analysis:
- mean temperature showed an increasing trend of approximately 0.01°C per year; and
- annual rainfall showed a decreasing trend of approximately 2.96 mm per year.
Potential Climate Concerns
For Assam, important climate-related concerns include:
- greater rainfall variability;
- intense rainfall events;
- changes in monsoon timing;
- heat stress;
- agricultural drought;
- flood risk;
- landslides in hill areas;
- changes in tea productivity;
- effects on wetlands; and
- changes in species distribution.
Why Rainfall Variability Matters
For agriculture and water management, the timing of rainfall may be as important as the annual total.
A year may receive substantial total precipitation but still experience agricultural stress if:
- rain arrives too late;
- long dry spells occur between storms;
- rain falls in a small number of extreme events; or
- heavy rainfall causes prolonged flooding rather than useful soil moisture.
Importance of Climate in the Geography of Assam
Climate influences almost every major geographical process and economic activity in Assam.
Agriculture
Rainfall controls rice cultivation, tea production, horticulture, jute, oilseeds and numerous other crops.
River Systems
The monsoon controls the seasonal discharge of the Brahmaputra, Barak and their tributaries.
Forests and Wildlife
Warm and moist conditions help sustain forests, grasslands, wetlands and diverse wildlife habitats.
Soil Formation
Heavy rainfall influences leaching, acidity, erosion and alluvial deposition.
Hydropower and Water Resources
Rainfall over Assam and adjoining hill catchments affects river flow and water availability.
Settlement and Infrastructure
Flood-prone areas, waterlogged plains, erosion zones and landslide-prone hills strongly affect settlement and transport planning.
Tourism
Seasonality influences wildlife tourism, hill tourism, river tourism and travel accessibility.
Climate of Assam: Important Facts for APSC & ADRE
For competitive examinations, the climate of Assam should be studied together with physiography, drainage, floods, soils, agriculture and natural vegetation. Students can revise these connected topics through the Geography of Assam MCQs for APSC and ADRE.
| Important Question Point | Answer |
|---|---|
| General climate of Assam | Warm, humid monsoonal climate |
| Main rain-bearing system | Southwest Monsoon |
| Main monsoon months | June to September |
| Average annual rainfall | About 2,297.4 mm in Assam Economic Survey 2025–26 |
| Share of normal rainfall during Southwest Monsoon | About 65% |
| Share of rainfall during March–May | About 25% |
| Local name associated with pre-monsoon storms | Bordoisila / Bordoichila |
| Wettest broad agro-climatic region | Barak Valley Zone – approximately 2,500–3,500 mm annually |
| Rainfall in Upper Brahmaputra Valley Zone | Approximately 2,000–3,000 mm |
| Rainfall in North Bank Plains Zone | Approximately 2,000–3,000 mm |
| Rainfall in Central Brahmaputra Valley Zone | Approximately 1,500–2,500 mm |
| Rainfall in Lower Brahmaputra Valley Zone | Approximately 1,500–2,600 mm |
| Rainfall in Hill Zone | Approximately 1,500–2,500 mm |
| Number of major agro-climatic zones | Six |
| Principal climatic cause of Assam floods | Heavy monsoon rainfall over Assam and upstream river catchments |
| Season most favourable to tea growth | Warm, humid pre-monsoon and monsoon growing period |
Frequently Asked Questions About the Climate of Assam
What type of climate does Assam have?
Assam has a predominantly warm and humid monsoonal climate. Government climate documentation also describes it as a tropical monsoon rainforest climate, although conditions vary between plains, valleys and hill districts.
What is the average annual rainfall of Assam?
The Assam Economic Survey 2025–26 reports an average annual rainfall of approximately 2,297.4 mm.
Which season brings the most rainfall to Assam?
The Southwest Monsoon season from June to September brings the largest share of rainfall, normally around 65% of the annual total.
What is Bordoisila?
Bordoisila or Bordoichila is a local name associated with powerful pre-monsoon thunderstorms that frequently affect Assam during April and May. These storms may bring strong winds, heavy rainfall, lightning and occasionally hail.
When does the monsoon occur in Assam?
The main Southwest Monsoon rainy season extends from June to September, although Assam begins receiving substantial pre-monsoon rainfall during March, April and May.
What is the hottest season in Assam?
The warmest period broadly extends from late spring through summer. Temperatures rise during April and May and remain warm during the monsoon, although cloud and rainfall moderate daytime heating.
What is the coldest season in Assam?
Winter is the coldest season, particularly December to February. January is generally among the coldest months.
How cold does Assam become in winter?
Winter conditions vary regionally. State climate documentation indicates that minimum temperatures may fall to around 5–8°C during colder periods, while hill locations can experience cooler local conditions.
How hot does Assam become in summer?
Summer temperatures in the plains commonly exceed 30°C, while state climate documentation indicates maxima can reach approximately 35–39°C during hotter conditions.
Which part of Assam receives the highest rainfall?
Rainfall varies locally, but among the six broad agro-climatic zones, the Barak Valley has the highest official range at approximately 2,500–3,500 mm annually.
How many agro-climatic zones are there in Assam?
Assam is divided into six major agro-climatic zones: Upper Brahmaputra Valley, Central Brahmaputra Valley, Lower Brahmaputra Valley, North Bank Plains, Barak Valley and Hill Zone.
Why does Assam receive heavy rainfall?
Assam receives heavy rainfall because moisture-bearing Southwest Monsoon winds interact with the Eastern Himalaya, Meghalaya Plateau, Naga-Patkai hills, Barail Range and other uplands surrounding the state's valleys.
Why is Assam highly humid?
High monsoon moisture, abundant rainfall, extensive rivers, wetlands, floodplains and dense vegetation maintain high atmospheric humidity during much of the year.
Why does Assam experience floods every year?
Heavy monsoon rainfall, rainfall over large upstream catchments, numerous tributaries, high sediment load, low-lying floodplains and rapid Himalayan runoff combine to produce recurrent flooding.
How does the climate of Assam help tea cultivation?
Tea benefits from Assam's warm temperatures, high humidity, abundant rainfall, long growing season and well-drained acidic soils, particularly in Upper Assam.
Does Assam experience drought?
Yes. Although Assam is a high-rainfall state, prolonged rainfall deficits or poorly distributed monsoon rainfall can produce agricultural drought-like conditions.
Conclusion
The climate of Assam is one of the most important forces shaping the physical geography, agriculture, forests, rivers, wetlands and economy of the state.
The overall climate is warm, humid and strongly monsoonal. The Southwest Monsoon between June and September supplies the largest share of annual rainfall, while pre-monsoon thunderstorms provide substantial additional precipitation during March, April and May.
Assam receives an average annual rainfall of approximately 2,297.4 mm, but the rainfall pattern varies greatly between different geographical regions. The Barak Valley and several foothill areas are particularly wet, while parts of central Assam and the hill zone receive comparatively lower totals.
Temperature is moderated by rainfall, cloud cover, vegetation and altitude. Plains experience warm and humid summers and cool winters, whereas the elevated landscapes of Karbi Anglong and Dima Hasao generally remain milder.
The six agro-climatic regions—Upper Brahmaputra Valley, Central Brahmaputra Valley, Lower Brahmaputra Valley, North Bank Plains, Barak Valley and Hill Zone—provide a useful framework for understanding the geographical variation of rainfall, terrain, soils and agricultural conditions across the state.
Assam's climate also explains many of its characteristic landscapes. Heavy rainfall feeds the Brahmaputra and Barak river systems, maintains extensive wetlands and forests and supports rice and tea cultivation. At the same time, intense monsoon rainfall contributes to floods, riverbank erosion, waterlogging and landslides.
Climate therefore represents both a major natural resource and environmental challenge for Assam.
Understanding changes in rainfall timing, temperature, extreme weather and monsoon variability will become increasingly important for agriculture, flood management, biodiversity conservation, urban planning and long-term development.
References
Directorate of Economics and Statistics, Government of Assam. Economic Survey, Assam 2025–26. Used for current statewide average annual rainfall and normal seasonal distribution of rainfall.
Directorate of Economics and Statistics, Government of Assam. Statistical Handbook, Assam 2025. Used for recent actual and normal rainfall figures for 2024 and 2025.
Assam Climate Change Management Society / Assam Climate Change Knowledge Portal. Official climate information used for Assam's general climatic characteristics, temperature ranges, historical climate trends and agro-climatic regional rainfall ranges.
India Meteorological Department. Climatic observations, rainfall monitoring and temperature information used through official Assam climate and government publications.
Assam State Disaster Management Authority. Flood Hazard Zonation Atlas and climatic information used for temperature, rainfall, flood and hazard context.
Water Resources Department, Government of Assam. Official documentation used for the relationship between climate, rainfall, river systems and flood hazards.
Directorate of Geology and Mining, Government of Assam. State geographical profile used for seasonal characteristics, Southwest Monsoon rainfall and Bordoisila/Bordoichila weather.
Assam Climate Change Management Society. Agro-climatic zone information used for Upper Brahmaputra Valley, Central Brahmaputra Valley, Lower Brahmaputra Valley, North Bank Plains, Barak Valley and Hill Zone.
Research Note: Rainfall and temperature values vary according to station, altitude, observation year and climatological normal period. The average annual rainfall figure of about 2,297.4 mm is taken from the Assam Economic Survey 2025–26, while recent rainfall statistics use the Statistical Handbook Assam 2025. Regional rainfall ranges are taken from the official agro-climatic classification. Historical temperature and rainfall trends refer specifically to the 1951–2010 IMD analysis cited by Assam's Climate Change Knowledge Portal and should not be treated as present-day annual rates.