6.8 Tropical Cyclones

Tropical cyclones^? are intense low-pressure systems confined to the area lying between 30° N and 30° S latitudes, around which high-velocity winds blow in a circular pattern. Horizontally, a mature tropical cyclone extends 500–1,000 km; vertically, it stretches from the surface up to 12–14 km. The system is, in effect, a giant heat engine — energised by the latent heat released as moisture condenses inside the rising air column over warm seas.

6.8.1 Conditions for Cyclone Formation

Scientists do not yet fully agree on the exact mechanism of a tropical cyclone, but four initial conditions must be present:

6.8.2 Anatomy of a Tropical Cyclone

A mature tropical cyclone has three diagnostic features — the calm eye, the violent eye-wall, and the broad spiral rain-bands. Inside the eye, descending air keeps the sky clear and the wind almost still. Surrounding it, the eye-wall holds the strongest winds and the heaviest rain on Earth. Beyond, the spiral bands of cloud feed moisture into the system from the warm sea below.

6.8.3 Spatio-Temporal Distribution in India

India's peninsular shape is flanked by the Bay of Bengal in the east and the Arabian Sea in the west — both warm tropical seas in which cyclones can form. Most Indian cyclones originate between 10°–15° N latitudes during the monsoon season. In the Bay of Bengal, cyclones develop most often during October and November; they originate between 16°–21° N and west of 92° E. By July, the place of origin shifts to around 18° N and west of 90° E, near the Sundarban delta.

Figure 6.F: Approximate frequency of cyclones recorded over the Bay of Bengal versus the Arabian Sea over recent decades — the eastern basin clearly dominates.

6.8.4 Consequences of Tropical Cyclones

The energy of a tropical cyclone comes entirely from the latent heat released by warm, moist air. As distance from the sea increases, the cyclone loses fuel and weakens. In India, therefore, the force of the cyclone falls rapidly with distance from the Bay of Bengal and the Arabian Sea. The coastal areas, however, are often struck by severe cyclonic storms with average velocities of 180 km/h. Often this leads to an abnormal rise in sea level called a storm surge^?.

A storm surge is generated by the interaction of air, sea and land. The cyclone provides the driving force in the form of a very high horizontal pressure gradient and very strong surface winds. Sea water flows across the coast along with strong winds and heavy downpours, inundating human settlements and agricultural fields, damaging crops, and destroying built structures. Loss of life is amplified because India's coast is densely populated and central to the national economy.

6.9 Landslides

Have you ever read of the road to Srinagar being blocked by falling boulders, or of trains stopped on the Konkan Railway by debris on the tracks? These are landslides^? — the rapid sliding of large masses of bedrock down a slope. Although less dramatic than earthquakes or cyclones, their cumulative impact on the natural environment and the national economy is no smaller.

Unlike most other disasters, landslides are largely controlled by highly localised factors — geology, geomorphic agents, slope, land use, vegetation cover, and human activities. This makes monitoring difficult and cost-intensive: each potential landslide site must be assessed individually rather than predicted from regional patterns.

6.9.1 Landslide Vulnerability Zones of India

6.9.2 Consequences of Landslides

Landslides have a relatively small and localised area of direct impact, but their consequences are far-reaching. Roads are blocked, railway lines destroyed, and river channels dammed by rock-falls. Diversion of river courses by landslide dams can cause downstream floods and loss of life and property. Spatial interaction becomes risky and costly — tourism, defence supplies and ordinary economic activity all suffer.

6.9.3 Mitigation

The right approach to landslides is always area-specific. NCERT recommends a mix of restrictive and constructive measures:

• Restrict construction and developmental activities — especially roads and dams — in the high-vulnerability zones.
• Limit agriculture to valleys and areas with moderate slopes.
• Control the development of large settlements in vulnerable zones.
• Promote large-scale afforestation programmes; build bunds to reduce the flow of water down slopes.
• Encourage terrace farming in the north-eastern hill states where jhumming (slash-and-burn / shifting cultivation) is still prevalent.

6.10 Disaster Management

Some disasters are more predictable than others. Cyclones, unlike earthquakes, tsunamis and volcanic eruptions, are largely predictable in time and place — modern satellite-based monitoring of the cyclone's intensity, direction and magnitude has made cyclonic-hazard management possible to a fair extent. Cyclone shelters, embankments, dykes, reservoirs and afforestation to break the wind speed are some of the steps that minimise damage. Yet losses keep rising because of the high vulnerability of dense coastal populations in countries like India, Bangladesh and Myanmar.

6.10.1 The Disaster Management Bill, 2005

The Act laid the institutional foundation for India's modern disaster-management framework. Two anchors stand out:

• NDMA — the National Disaster Management Authority, chaired by the Prime Minister, which lays down policies and guidelines for disaster management at the national level.
• NDRF — the National Disaster Response Force, the multi-disciplinary specialist force trained for search, rescue and relief at the time of a disaster. State and District Disaster Management Authorities (SDMA, DDMA) extend the same architecture downward.

6.10.2 Three Stages of Disaster Mitigation and Management

NCERT identifies three stages in the disaster-management cycle:

India needs this framework with special urgency: about two-thirds of its geographical area, and an equal proportion of its population, are vulnerable to one disaster or another. The introduction of the Disaster Management Act, 2005 and the establishment of the National Institute of Disaster Management are concrete steps in this direction.

6.10.3 Conclusion — Hazards Need Not Become Disasters

Disasters can be either natural or the result of human activities, and not every hazard turns into a disaster. Since natural disasters are difficult to eliminate, the next best option is mitigation and preparedness. The three pillars — vulnerability mapping, public awareness, and a trained response force — transform hazards from helpless tragedies into manageable risks. The story of India's disaster management is, in the end, a story of humans learning to share the planet with its tantrums.