Monsoon Depressions Explained

A monsoon depression is a rain-bearing low-pressure system that forms over the head of the Bay of Bengal during the southwest monsoon and moves west-north-westward across central and northern India, bringing widespread heavy rain. Monsoon depressions are weaker than cyclones, with surface winds generally below about 60 km/h, but they travel over land for days and deliver a large share of the rain that central India receives between June and September.

Every monsoon season produces a handful of these systems, and farmers, dam managers and disaster agencies watch each one closely. A monsoon depression can refill reservoirs and rescue a struggling crop, and it can also flood rivers, towns and villages within hours. Knowing how a monsoon depression forms and moves helps explain why the heartland of India gets its rain in bursts rather than evenly.

Quick Facts

Key Takeaways

• A monsoon depression is a low-pressure system with sustained surface winds of roughly 31 to 49 km/h that forms over the north Bay of Bengal during the southwest monsoon and tracks west-north-west across India.
• Monsoon depressions bring a large share of the heavy rain that central and northern India receive in July and August, often producing rain over areas hundreds of kilometres wide.
• The heaviest rainfall from a monsoon depression usually falls in its south-western sector, so the districts south-west of the centre are most at risk of flooding.
• Monsoon depressions travel along the monsoon trough, and their passage typically ends a dry phase and starts an active spell of the monsoon.
• The India Meteorological Department classifies systems by wind speed as low-pressure area, depression, deep depression and cyclonic storm, and only cyclonic storms receive names.
• Monsoon depressions rarely grow into cyclones because they form close to land and in a region of strong vertical wind shear, so their main danger is rain and flooding rather than wind.

What is a monsoon depression?

A monsoon depression is a large, slow-moving, rain-bearing cyclonic circulation with a closed area of low pressure that develops within the southwest monsoon flow. The system is a few notches stronger than the more common monsoon low-pressure area, and it is organised enough to be tracked as a distinct weather system.

In the official classification of the India Meteorological Department (IMD), a depression is a low-pressure system whose surface winds reach 17 to 27 knots, roughly 31 to 49 km/h, and a deep depression reaches 28 to 33 knots, roughly 50 to 61 km/h. A monsoon depression is simply a depression or deep depression that forms during the monsoon season and is embedded in the monsoon circulation. Such systems are large, often more than a thousand kilometres across, though their winds are modest.

How is a monsoon depression classified?

A monsoon depression is classified by the strength of its sustained surface winds, forming one step in the IMD scale that runs from a low-pressure area to a cyclonic storm and beyond. Wind speed is measured at the surface and averaged over a few minutes.

Systems that fall below the depression threshold are called low-pressure areas or well-marked low-pressure areas. These are far more numerous than depressions during the monsoon and also bring significant rain. Once a depression crosses the coast and moves inland, it is often described as a land depression as it begins to weaken.

Where and when do monsoon depressions form?

Monsoon depressions form mainly over the northern Bay of Bengal, close to the coasts of West Bengal, Bangladesh and Odisha, and they are most frequent in July and August. Their formation region is the eastern end of the monsoon trough, where the sea is warm and the monsoon winds converge.

A smaller number develop over land or over the north Arabian Sea, but the head of the Bay is by far the main cradle. Some begin as a cyclonic circulation drifting in from the South China Sea and the western Pacific, which crosses the Indochina peninsula and re-forms over the Bay. Systems are fewer in June, when the monsoon is still advancing, and in September, when it begins to weaken.

How does a monsoon depression form?

A monsoon depression forms when a pocket of cyclonic circulation over the warm north Bay of Bengal draws in moist air, which rises, condenses and releases heat, deepening the low-pressure area until it becomes a closed system. The process starts from a pre-existing disturbance in the monsoon trough.

Several conditions favour the process:

• Warm sea surface: the Bay of Bengal in summer is warm enough to supply large amounts of water vapour.
• Monsoon trough alignment: a trough extending to the head of the Bay provides cyclonic spin and low-level convergence.
• Seeding disturbances: westward-moving cyclonic circulations or low-pressure areas from the east provide the initial vortex.
• Supportive winds aloft: a favourable upper-level flow allows the system to organise instead of being sheared apart.

Unlike a tropical cyclone, which is a warm-core vortex powered mainly by heat from the ocean surface, a monsoon depression draws strength from the large-scale monsoon flow and from the release of latent heat in its rain bands. This is one reason it is bigger and slower than a cyclone, with less intense winds.

What path does a monsoon depression take?

A monsoon depression moves west-north-westward along the monsoon trough, crossing the Odisha or West Bengal coast and then travelling across Chhattisgarh, Madhya Pradesh and Gujarat or Rajasthan, sometimes as far as Pakistan. This track reflects the general westerly steering of the monsoon flow.

The movement is generally steady, covering a few hundred kilometres per day. Over land the system gradually weakens because it loses its supply of warm ocean moisture and meets friction, though the remains can keep bringing rain for days. If the monsoon trough lies to the south, the track shifts southward through the peninsula, and if it lies north, the track hugs the Gangetic plain.

The path matters greatly, because places directly in the track, and particularly those in its southern half, bear the heaviest rain. Forecasters spend much effort predicting where the track will go, since a deviation of a hundred kilometres can turn a mild rain into a flood for a particular district.

Why does a monsoon depression bring such heavy rain?

A monsoon depression brings heavy rain because strong low-level winds carry large volumes of moist air from the Bay of Bengal and the Arabian Sea into its circulation, where the air converges, rises and condenses over a wide area. Rising motion is strongest in the south-western sector of the system, where the incoming moist flow is most vigorous.

The rain pattern is usually asymmetric. Intense bands of cloud form in the south and south-west of the centre, with a zone of lighter rain north of the centre. The widespread cloud shield can cover several states at once. As the system moves, the zone of heavy rain moves with it, giving areas day after day of showers.

The rain is also boosted when the system meets high ground. Along the eastern fringe of the Western Ghats or the hills of Chhattisgarh, Madhya Pradesh and Odisha, orographic lifting can enhance the downpour. When a depression stalls or moves slowly, rain accumulations can be extreme.

How many monsoon depressions occur each season?

A typical monsoon season produces roughly half a dozen monsoon depressions, though the number varies from season to season. Some years have only two or three, while others see more than eight.

Several studies have reported that the number of full monsoon depressions has declined in recent decades, while weaker low-pressure areas, sometimes called monsoon lows, have remained common. Researchers link the trend to changes in the large-scale atmosphere and ocean, though the explanation is still debated. The decline in depressions is one of several factors scientists consider when examining shifts in the character of monsoon rainfall in central India.

Seasons with many depressions tend to have active monsoon spells and better rainfall over central India, whereas seasons with few often show longer breaks. Because of this connection, the count of depressions is watched as an indicator of how the monsoon is faring.

How is a monsoon depression different from a cyclone?

A monsoon depression differs from a cyclone in being weaker, larger, slower and driven by the monsoon flow, whereas a tropical cyclone is a compact, intense, warm-core storm powered by the heat of the ocean. A cyclone has sustained winds of at least 62 km/h by the IMD definition, while a depression stays below that.

• Wind: a depression has winds of about 31 to 61 km/h; a cyclone exceeds 62 km/h and can reach much higher.
• Size: a monsoon depression is large and diffuse, while a cyclone has a compact core and eye in its mature stage.
• Season: monsoon depressions occur in the monsoon season, whereas the severe cyclones of the Bay of Bengal and Arabian Sea are concentrated in the pre-monsoon (April to June) and post-monsoon (October to December) periods.
• Hazard: the main hazard of a depression is rain and flooding, while cyclones add destructive winds and storm surge.
• Naming: only cyclonic storms receive names from the regional list, so depressions are identified by number and date.

During the monsoon, strong wind shear and the disorganising effect of the monsoon flow usually prevent depressions from intensifying into cyclones, though there have been exceptional cases.

What damage can a monsoon depression cause?

A monsoon depression can cause river floods, flash floods, waterlogging, landslides and crop damage, mainly through heavy and prolonged rain, although its winds seldom cause major destruction. Because the rain falls over a wide area for several days, rivers in the path can rise together.

• River floods: basins such as the Mahanadi, Brahmani, Godavari, Narmada, Tapi and the Ganga system can swell, forcing dam authorities to release water.
• Urban flooding: towns and cities in the track can experience waterlogging, traffic disruption and damage to low-lying neighbourhoods.
• Landslides and flash floods: hilly districts in the Western Ghats, central highlands and the Himalayan foothills face slope failures.
• Agriculture: standing crops can suffer waterlogging and damage, even though the overall rain is welcome.

The IMD issues colour-coded warnings and works with state disaster management authorities and the National Disaster Management Authority (NDMA), and administrations use these forecasts to evacuate people from flood-prone areas and to manage reservoir releases.

How does the IMD forecast monsoon depressions?

The IMD forecasts monsoon depressions using satellite images, Doppler weather radars, ocean buoys, ship and surface observations, upper-air data and numerical weather prediction models. The forecasts are published in bulletins that include the expected track, intensity and rainfall.

Forecasters first watch for a cyclonic circulation or a low-pressure area over the Bay of Bengal and then assess its chance of intensifying. Once a depression has formed, the IMD issues regular updates on its position and expected path, along with district-level rainfall warnings. Extended-range forecasts also attempt to predict the likelihood of a low-pressure system forming one to two weeks ahead.

Track forecasts have improved over time, while the exact rainfall amounts at a specific place remain harder to forecast. Rain from monsoon depressions is highly uneven, so forecasts are expressed as categories of expected rainfall rather than single numbers.

Why do monsoon depressions matter for water and farming?

Monsoon depressions matter because they supply a large part of the heavy monsoon rainfall over central and northern India, replenishing reservoirs, recharging groundwater and reviving crops during dry spells. Much of the rain that falls over the Indian heartland comes in bursts linked to these systems.

For rain-fed agriculture in states such as Madhya Pradesh, Chhattisgarh, Maharashtra and Gujarat, a depression arriving at the right time can end a dry spell and save the kharif crop. Reservoirs and rivers in central India depend heavily on such events for their annual filling. A season with few depressions often means long breaks in the monsoon and lower storage.

Their positive and negative effects come together: the same system that fills a reservoir can also flood a town downstream. This dual character is why monsoon depressions are among the most closely monitored features of the Indian monsoon.

Bottom Line

A monsoon depression is the workhorse of the Indian monsoon: a large, moderate-wind, low-pressure system that forms over the Bay of Bengal and delivers heavy rain to central and northern India. It is welcome for water and crops and dangerous for floods, which is why forecasting its track matters so much. Last updated: 30 October 2026.

Frequently Asked Questions

What is a monsoon depression?

A monsoon depression is a low-pressure weather system that forms over the Bay of Bengal during the monsoon and moves west-north-west across India. Its surface winds are about 31 to 61 km/h. It brings widespread heavy rain over central and northern India.

Where do monsoon depressions form?

Most monsoon depressions form over the northern Bay of Bengal, near the coasts of West Bengal and Odisha. They develop along the eastern end of the monsoon trough. A few form over land or the Arabian Sea.

Is a monsoon depression the same as a cyclone?

No, a monsoon depression is weaker and larger than a cyclone. Its winds are below 62 km/h, while a cyclonic storm has winds of 62 km/h or more. Depressions rarely become cyclones during the monsoon because of strong wind shear.

How many monsoon depressions form in a season?

About half a dozen monsoon depressions form in a typical season, though the number varies from year to year. Most occur in July and August. Some studies report fewer full depressions in recent decades.

Which areas get the most rain from a monsoon depression?

The areas to the south and south-west of the depression centre get the heaviest rain. This usually means Odisha, Chhattisgarh, Madhya Pradesh, Maharashtra and Gujarat as the system moves west-north-west. Rivers in these regions can flood during and after the event.

Are monsoon depressions good or bad for India?

Monsoon depressions are mostly beneficial because they supply a large share of the monsoon rain, filling reservoirs and reviving crops. They can also be harmful, since heavy rain over several days can cause floods and landslides. The India Meteorological Department tracks them and issues warnings to limit damage.

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