Bay of Bengal Low-Pressure Systems

Bay of Bengal low-pressure systems are cyclonic atmospheric disturbances that draw moisture-laden winds towards a region of relatively low pressure. Their convergence, ascent and condensation produce organised clouds and heavy rainfall, especially along the eastern coast and over adjoining inland areas.

Formation and moisture supply

During the southwest monsoon, strong evaporation over the warm Bay provides abundant water vapour. Disturbances forming near the monsoon trough intensify when converging surface winds rise, release latent heat through condensation and sustain deep convection.

• The Bay's broad moisture supply allows rain-bearing clouds to remain active after the system approaches the eastern coast.
• Cyclonic circulation continuously transports moist air into the disturbance, producing widespread rather than purely local rainfall.

How rainfall becomes intense

Air spiralling towards the low-pressure centre is forced upward. As it rises, it expands, cools and condenses into deep convective clouds, producing prolonged and sometimes very heavy rain.

• Slow movement or temporary stagnation can repeatedly expose the same area to rain bands, creating cumulative rainfall.
• Interaction with the monsoon trough can strengthen moisture convergence across a wide region.
• Uplift over coastal and inland relief can enhance rainfall through the orographic effect.

Why eastern India is especially affected

Many Bay systems move westward or northwestward, cross the coast and continue along the monsoon trough. Consequently, Odisha, West Bengal, Jharkhand, Bihar and adjoining central India can receive heavy rainfall even after landfall.

• Repeated systems can saturate soils and keep rivers and reservoirs at high levels.
• Intense short-duration rain causes urban flooding, while sustained basin-wide rain can generate river floods and landslides in hilly areas.
• Rainfall generally weakens inland as the system loses its oceanic moisture source and encounters greater surface friction.