A region of below-average atmospheric pressure where air rises, cools and condenses — bringing clouds, rain and stormy weather. Drives monsoon depressions and tropical cyclones in South Asia.
phenomenaWhat is a Low Pressure System?
A low pressure system — also called a “low” or cyclone — is a region where atmospheric pressure is lower than surrounding areas at the same altitude. Air rises within the low (because higher-pressure air pushes inward and has nowhere to go but up), cools as it ascends, and water vapor condenses into clouds. The result: clouds, precipitation, wind, and stormy weather.
For South Asia, low-pressure systems are the dominant rain-makers:
- Monsoon depressions deliver the bulk of the southwest monsoon’s rainfall
- Tropical cyclones in the Bay of Bengal and Arabian Sea cause the region’s most catastrophic weather
- Western Disturbances are extratropical lows delivering winter rain and snow
- Monsoon trough is a semi-permanent low-pressure feature during June-September
Without low-pressure systems, the Indian subcontinent would receive virtually no rain.
Categories of low-pressure systems
The IMD uses a graduated scale by surface wind speed:
| Stage | Sustained Wind | Pressure Drop |
|---|---|---|
| Low Pressure Area | < 31 km/h | 2-3 hPa below ambient |
| Depression | 31–50 km/h | 4-8 hPa |
| Deep Depression | 51–62 km/h | 8-12 hPa |
| Cyclonic Storm | 63–88 km/h | 12-20 hPa |
| Severe Cyclonic Storm | 89–117 km/h | 20-35 hPa |
| Very Severe Cyclonic Storm | 118–166 km/h | 35-60 hPa |
| Extremely Severe | 167–221 km/h | 60-90 hPa |
| Super Cyclonic Storm | ≥ 222 km/h | > 90 hPa |
Pressure drop is roughly proportional to wind speed (with some basin-specific variation). Super Cyclonic Storms like 1999 Odisha or 2020 Amphan had central pressures below 920 hPa — nearly 100 hPa below the standard 1013 hPa.
How low-pressure systems form
Three main mechanisms:
1. Tropical cyclogenesis (warm ocean):
- Warm SST (≥ 26.5°C) provides energy
- Low wind shear
- Sufficient Coriolis force (lat ≥ 5°)
- Pre-existing disturbance
- Result: deep tropical low intensifying to cyclone
2. Extratropical cyclogenesis (temperature contrast):
- Cold polar air meets warm subtropical air at frontal zones
- Result: mid-latitude low with fronts
- Western Disturbances are this type
3. Heat low / monsoon trough:
- Intense surface heating creates rising air
- Lower pressure forms over hot landmass
- The summer Asian monsoon trough is a giant heat-driven low
- Smaller examples: pre-monsoon “heat lows” over Rajasthan
Low-pressure systems in the Bay of Bengal
The Bay of Bengal is one of the most productive low-pressure-system basins on Earth. During monsoon:
- 12-15 monsoon depressions form per season (June-September)
- Most travel westward across India’s east coast
- Land-falling depressions deliver 100-300 mm rainfall over 3-5 days
- Many never intensify to cyclone strength — but cause widespread flooding nonetheless
During pre/post-monsoon:
- 4-6 named tropical cyclones form
- Often more intense than monsoon depressions
- Landfall over Tamil Nadu, AP, Odisha, Bengal, Bangladesh
Monsoon trough — South Asia’s defining low
The monsoon trough is a band of low pressure stretching east-west across northern India (Rajasthan to Bay of Bengal) during the southwest monsoon. It is the surface signature of the ITCZ over the subcontinent.
Properties:
- Pressure typically 1000-1005 hPa
- Anchors monsoon rainfall pattern
- Migrates north (toward Himalayas) and south (toward central India) on weekly timescales
- “Active monsoon” = trough at normal position; “Break monsoon” = trough shifted to Himalayan foothills
For Bihar, eastern UP, West Bengal, Bangladesh — the trough’s position determines daily rainfall.
Effects of low-pressure systems
Direct effects:
- Cloud cover — reduces solar heating, can cool temperatures by 5-10°C
- Rainfall — from light drizzle to torrential downpours
- Wind — counterclockwise rotation (NH); speeds vary with intensity
- Storm surge — coastal water rise from wind + pressure
- Pressure gradient — drives the winds (gradient = wind speed)
Indirect effects:
- Improved air quality when rain washes pollutants
- River level rise as runoff increases
- Reservoir filling during monsoon
- Cooling of land surface breaks heatwaves
Tracking low pressure on Mausam Online
Mausam Online displays surface atmospheric pressure on every city page. When pressure drops 5+ hPa over 6-12 hours, a low-pressure system is approaching. The hourly precipitation forecast shows expected rain timing and intensity.
For monsoon and cyclone tracking, IMD’s RSMC New Delhi publishes daily Tropical Weather Outlooks during June-November.
Frequently asked questions
Why does low pressure mean bad weather? Because low pressure forces air upward. Rising air cools and condenses water vapor into clouds. If the rising motion is strong enough, the clouds produce rain or thunderstorms. The stronger the low, the more dramatic the weather.
Are monsoon depressions and tropical cyclones the same thing? They’re related but different. Monsoon depressions are low-intensity tropical lows (winds < 63 km/h) that form within the active monsoon trough. Tropical cyclones are stronger systems (winds ≥ 63 km/h) that typically form during pre/post-monsoon seasons. Some monsoon depressions intensify into cyclones over open ocean.
Why don’t cyclones form over equator? Because the Coriolis force, which provides the rotation, is essentially zero at the equator. Cyclones need latitude ≥ 5° (north or south) to spin up. This is why Singapore and Sri Lanka rarely face direct cyclone landfall.
How low can pressure go? World record: 870 hPa in Typhoon Tip (1979 Western Pacific). Bay of Bengal records: ~912 hPa during 1999 Odisha Super Cyclone. Most monsoon depressions: 990-1005 hPa.
Where can I see live pressure for my city? Mausam Online displays surface pressure on every city page. See Mumbai, Chennai, Kolkata, Visakhapatnam, Bhubaneswar, Dhaka.