• Tropical cyclone: An intense low-pressure system over warm tropical seas with organised convection and a closed, rotating circulation. It spins anticlockwise in the Northern Hemisphere because of the Coriolis force.

Structure:

• The eye is a calm, clear centre with sinking air.
• The eyewall is a ring of tall clouds around the eye, with the strongest winds and heaviest rain.
• Spiral rainbands extend outward from the eyewall.

Conditions for formation:

• Sea surface temperature of about 26.5 °C or more, to a depth of around 50 m.
• Sufficient Coriolis force (beyond roughly 5° latitude).
• Low vertical wind shear and high humidity in the lower and middle troposphere.
• A pre-existing low-level disturbance. Latent heat released as moist air condenses powers the system.

• IMD classification (by sustained wind speed): Depression (31–49 km/h), deep depression (50–61), cyclonic storm (62–88), severe cyclonic storm (89–117), very severe cyclonic storm (118–166), extremely severe cyclonic storm (167–221) and super cyclonic storm (222 km/h or more).
• Major hazards: Storm surge (the biggest killer on a low-lying coast), heavy rainfall and flooding, and destructive winds, especially at landfall.

Why the Bay of Bengal forms cyclones:

• Its waters are warm and a large river discharge (Ganga, Brahmaputra, Mahanadi, Godavari) makes the surface layer less saline. This stratification helps retain heat near the surface.
• It is shallow, humid and sits in the monsoon trough region, with plenty of moisture.
• Systems from the West Pacific often cross Southeast Asia and re-intensify in the Bay.
• Its funnel-shaped northern end and low-lying coastline amplify storm surge.
• As a result, it produces several times more cyclones than the Arabian Sea.

Why the post-monsoon months are more prone:

• Sea temperatures are at their annual peak after monsoon heating.
• As the monsoon withdraws, the monsoon trough and ITCZ shift south over the Bay, bringing moisture and low-level convergence.
• Vertical wind shear weakens. It is high during the peak monsoon (June–August), which suppresses cyclones and favours monsoon depressions instead.

• Note: The Bay’s two cyclone seasons are April–May and October–November, and the post-monsoon season is the more dangerous one for Odisha. September mostly brings monsoon depressions and low-pressure areas, with October the first real cyclone month.

• Cyclone naming: Cyclones in the North Indian Ocean are named from lists prepared by the WMO/ESCAP Panel on Tropical Cyclones, and the IMD issues the names.
• Odisha’s disaster record: The 1999 Super Cyclone led to the Odisha State Disaster Management Authority (OSDMA), now regarded as a model for early warning and evacuation.
• Legal framework: The Disaster Management Act, 2005 provides for the NDMA, SDMA and DDMA, and for the NDRF and SDRF as response funds.
• East Coast Railway: Headquartered at Bhubaneswar, it regulates train services during cyclones.

Cyclone formation and structure, the IMD classification, the Bay of Bengal versus the Arabian Sea, the 1999 Super Cyclone and OSDMA, and the Disaster Management Act, 2005 are recurring themes for OPSC Prelims (Odisha Geography, Disaster Management, Current Events) and OPSC Mains GS Paper I (Geography) and GS Paper III (Disaster Management).

For structured notes on Odisha-specific current affairs, aspirants preparing in Bhubaneswar can join the OPSC Foundation Batch at Strive Edge IAS — recognized as one of the best OPSC OAS coaching institutes in Bhubaneswar.