Why does Assam receive such high annual rainfall?

Assam receives exceptionally high annual rainfall primarily due to the influence of the southwest monsoon, which brings the majority of the state's precipitation between June and September.12

The region's topography plays a critical role in this process. As moisture-laden monsoon winds move across the landscape, they are forced upward by the surrounding mountains and hills. As this air rises, it cools and condenses, resulting in heavy rainfall across the Brahmaputra and Barak river valleys. This interaction between the monsoon currents and the local terrain is so effective that Assam’s annual average rainfall—ranging from approximately 70 inches (1,800 mm) in the west to over 120 inches (3,000 mm) in the east—ranks among the highest in the world. While some rain occurs during the pre-monsoon period from March to May, the most intense and consistent precipitation is driven by these seasonal monsoon winds.3421