How do cyclones form and how are they measured?

 Why in News?

• Cyclone Montha (Oct 2025) recently made landfall along the Odisha–Andhra coast, highlighting the dynamics of tropical cyclone formation, prediction, and intensity estimation in the Indian Ocean.
• Renewed focus on the accuracy of cyclone forecasting, role of wind shear, and satellite-based observation in disaster preparedness.

Relevance

• GS Paper 1 – Geography: Physical geography – climatology, tropical cyclones, atmospheric circulation.
• GS Paper 3 – Disaster Management: Early warning systems, forecasting technologies, and mitigation measures.

What is a Cyclone? — Basic Definition

• A cyclone is a large-scale, low-pressure weather system characterized by inward-spiraling winds rotating around a central core called the eye.
• Classified by region:
  • Hurricanes (Atlantic, NE Pacific)
  • Typhoons (NW Pacific)
  • Tropical Cyclones (Indian Ocean & South Pacific)

Conditions Required for Cyclone Formation

1. Warm Sea Surface Temperature (SST) — above 26.5°C to a depth of ≥50 m for sufficient latent heat.
2. Coriolis Force — needed to initiate cyclonic rotation; absent within 5° latitude of the Equator.
3. Low Vertical Wind Shear — allows organized upward convection; high shear disrupts circulation.
4. Atmospheric Instability — encourages sustained convection and rising of moist air.
5. High Humidity — in mid-troposphere (5–7 km) to sustain cloud formation.
6. Pre-existing Disturbance — e.g., a low-pressure zone or tropical wave to trigger initial rotation.

Stepwise Process of Cyclone Formation

• Stage 1: Low-pressure area develops → convergence of moist air.
• Stage 2: Rising moist air condenses → releases latent heat, intensifying convection.
• Stage 3: Warm air rises, pressure drops → inflow of more moist air.
• Stage 4: Rotation begins under Coriolis effect → organized cyclonic circulation forms.
• Stage 5: Eye formation and eyewall intensification mark a mature cyclone.

Structure of a Cyclone

Feature	Description
Eye	Central calm zone (20–50 km wide), lowest pressure, clear skies, sinking air.
Eyewall	Surrounds the eye; most intense winds & rainfall occur here. Rising convective towers dominate this zone.
Rainbands	Outer spiral bands producing intermittent rain and gusts.
Outflow	High-altitude air diverging outward, maintaining cyclone balance.

Role of Wind Shear

• Vertical Wind Shear: Difference in wind speed/direction between lower and upper atmosphere.
• Low Wind Shear: Maintains vertical alignment → cyclone strengthens.
• High Wind Shear: Tilts vortex → disrupts convection → prevents intensification or leads to dissipation.
• Example: Monsoonal shear in Bay of Bengal often limits storm strengthening near the coast.

Observational Methods

1. Satellites (Primary Source in Indian Ocean):
   1. Infrared sensors: Estimate cloud-top temperatures → proxy for intensity.
   2. Visible imagery: Identifies eye formation and structure.
   3. Microwave sensors: Reveal rainfall distribution & internal dynamics.
   4. Scatterometers: Measure surface wind speeds over oceans.
2. Ocean Buoys: Record SST, pressure, wind speed, and humidity.
3. Ground-based Observations: Weather radars, coastal stations, Doppler radars track approach and landfall.
4. Aircraft Reconnaissance (“Hurricane Hunters” – Atlantic):
   1. Fly into storms to record wind, temperature, humidity, and pressure.
   2. Deploy dropsondes—instruments that transmit vertical profiles of atmospheric data while descending.

Cyclone Classification (IMD – North Indian Ocean)

Category	Wind Speed (km/h)
Low Pressure Area	<31
Depression	31–49
Deep Depression	50–61
Cyclonic Storm	62–88
Severe Cyclonic Storm	89–117
Very Severe Cyclonic Storm (VSCS)	118–165
Extremely Severe Cyclonic Storm (ESCS)	166–220
Super Cyclonic Storm	≥221

Cyclone Forecasting & Modeling

• Forecasting Challenges:
  • Small initial data errors → large track/intensity deviations.
  • Ocean–atmosphere coupling adds complexity.
• Tools Used:
  • Numerical Weather Prediction (NWP) models assimilating global data.
  • Dynamic models (e.g., ECMWF, GFS) simulate track, intensity, and rainfall.
  • IMD’s INCOIS & MOSDAC systems integrate satellite + ocean buoy data.
• Forecast Accuracy:
  • Track prediction improved to 3–5 days in advance with high confidence.
  • Intensity prediction remains less accurate (error ~15–25%).

Broader Analysis

• Improved Preparedness: Post-1999 Odisha super cyclone, India established IMD’s Regional Specialized Meteorological Centre (RSMC) and INSAT-based alert systems.
• Disaster Risk Reduction: Cyclone shelters, early warning dissemination, and community resilience have reduced mortality rates drastically.
• Climate Link: Warming oceans → increase in frequency of Very Severe Cyclones (VSCS), though total cyclone count remains stable.
• Data Gap: Absence of reconnaissance flights in Indian Ocean affects real-time accuracy; dependence on satellite estimates continues.

Conclusion

• Cyclones are heat engines of the tropics, driven by oceanic and atmospheric interactions.
• While track prediction has achieved notable precision, intensity estimation still faces uncertainty due to high wind shear, sea temperature variability, and data resolution.
• Sustained investment in ocean monitoring, AI-based modeling, and regional cooperation (like BIMSTEC & ESCAP) is essential for enhanced cyclone resilience.

---