Assam is built almost entirely around the Brahmaputra valley, one of the widest and most flood- and erosion-prone river systems in the world, with the smaller Barak valley in the south forming a distinct secondary region. For live interactive rainfall, wind and satellite maps over Assam, open VayuMet.
The Brahmaputra, one of the world's largest rivers by discharge, runs the length of the state through a broad, flat valley hemmed in by the Himalayan foothills to the north and the Meghalaya plateau, Karbi Anglong hills and Patkai range to the south. The smaller Barak valley in the south of the state, closer to Mizoram and Meghalaya, forms a distinct, hillier secondary region.
Winter (December-February) is mild with valley-floor morning fog. Pre-monsoon summer (March-May) is marked by severe nor'wester thunderstorm squalls, locally called Bordoisila. The monsoon (June-September) arrives comparatively early — Assam is typically among the first mainland regions after West Bengal to receive it — and is both the state's main rain source and its main flood season, as the Brahmaputra rises with local rainfall plus runoff from Arunachal Pradesh and the eastern Himalaya. Post-monsoon (October-November) is a short, dry transition.
Assam's own name for the severe pre-monsoon thunderstorm squalls common across eastern India — sudden, damaging winds, hail and lightning, especially in April.
The Brahmaputra's high sediment load, shifting channels and huge upstream catchment (including heavy rainfall over Arunachal Pradesh and the eastern Himalaya) mean some degree of flooding across the valley is expected almost every monsoon, with severity varying by year.
Assam typically receives the monsoon earlier than most of mainland India, via the Bay of Bengal branch moving up through West Bengal.
Frequencies cited from published IMD/IITM climatology where a source is named inline; day-count figures in the table below are computed directly from the 1951–2025 IMD daily archive.
The table below is a state-wide average across Assam's 34 districts, built from IMD's real 1991-2020 gridded rainfall/temperature normals and 1951-2025 daily archives (not a forecast). The Brahmaputra valley and the hillier Barak valley in the south can differ from this average — use the district climatology tool linked below for any single district's own figures.
| Season | Rainfall (mm) | Rain days | Mean Max / Min (°C) | Typical Hottest / Coldest (°C) | Sky (Sunny·Partly·Overcast days) |
|---|---|---|---|---|---|
| Winter Jan–Feb | 36.1 | 12.2 | 23.8 / 11.8 | 26.6 / 9.7 | 30.8·28.2·0.0 |
| Pre-Monsoon (Summer) Mar–May | 483.5 | 57.0 | 29.2 / 19.3 | 31.2 / 14.5 | 14.2·72.6·5.2 |
| Monsoon (SW) Jun–Sep | 1207.0 | 105.5 | 31.2 / 24.3 | 32.4 / 22.9 | 1.6·96.8·23.6 |
| Post-Monsoon Oct–Dec | 134.9 | 21.9 | 27.1 / 16.5 | 31.1 / 10.1 | 37.4·54.1·0.6 |
State figures are a simple average across 34 member districts (IMD Pune 1991–2020 normal; day-count and sky figures from real 1951–2025 daily archives — see District Climatology tool for any single district's own numbers, which can differ substantially from this state average).
Computed directly from the real IMD 1951–2025 daily gridded archive — not a forecast, and not the same as a single-station record. IMD's gridded temperature is 1°×1° (~111 km/cell) and rainfall is 0.25°×0.25° (~25 km/cell); both are spatial analyses of many station observations; a famous single-station record (e.g. a specific town's all-time hottest day) will typically read higher than the grid-cell value shown here, since one station reading isn't smoothed over a ~25–111 km box. "Place" is the nearest district to the grid cell where the extreme occurred, not necessarily the exact spot.
Monsoon (Jun-Sep). The Brahmaputra floods to some degree almost every monsoon, driven by local rainfall combined with runoff from its vast Himalayan catchment.
Monsoon and beyond. The Brahmaputra's shifting channel causes chronic, severe erosion of farmland and settlements along its banks, a distinct and ongoing hazard beyond the flood season itself.
Pre-monsoon (Mar-May). Damaging winds, hail and lightning from nor'wester squalls are a recurring structural and agricultural hazard.
Monsoon (Jun-Sep). The hill districts around Karbi Anglong and the Barak valley face landslide risk on steep, heavily-rained-on terrain.
Click a district on the District Climatology tool or search it on the 10-day forecast page for its own normals — the grid below is the full current list (India's Local Government Directory codes, cross-checked against the latest state reorganisation).