Cyclones: tropical and temperate

In 1999, a super cyclone struck Odisha and killed around ten thousand people. In 2013, Cyclone Phailin — of comparable strength — struck the same coast, and the death toll was a tiny fraction of that, largely because around a million people were evacuated in time. Understanding cyclones, and how India learned to prepare for them, is part of both GS I (geography) and GS III (disaster management).

What is a cyclone?

A cyclone is a system of winds rotating around a centre of low pressure — anticlockwise in the northern hemisphere and clockwise in the southern hemisphere, because of the Coriolis force. The two main types, tropical and temperate, form in very different ways.

Tropical cyclones

Conditions for formation

1. Warm sea surface — generally above about 26–27°C, with warm water extending to a significant depth. Warm water provides the moisture and latent heat that power the storm.
2. Coriolis force strong enough to start rotation — which is why tropical cyclones rarely form within about 5° of the equator.
3. Low vertical wind shear — little change in wind speed and direction with height, so the storm's structure is not torn apart.
4. A pre-existing low-pressure disturbance to start with.
5. Upper-level divergence — air flowing out at the top, allowing more air to rise.

Structure

• The eye — a calm, clear centre with the lowest pressure.
• The eyewall — a ring of towering clouds around the eye, with the strongest winds and heaviest rain.
• Rainbands — spiral bands of cloud and rain extending outward.

Tropical cyclones weaken rapidly over land or cold water, because they lose their supply of warm, moist air.

Names in different regions

The same phenomenon has different names: hurricanes in the Atlantic and eastern Pacific, typhoons in the western Pacific, and cyclones in the Indian Ocean.

Why they are dangerous

• Storm surge — seawater pushed onto the coast by winds and low pressure; historically the biggest killer.
• Extreme winds, which destroy buildings and infrastructure.
• Heavy rainfall and flooding, often far inland.

Cyclones in the north Indian Ocean

• The Bay of Bengal sees considerably more cyclones than the Arabian Sea — commonly cited as around four times as many — because of its warmer waters, the remnants of Pacific typhoons moving in from the east, and the funnel-like shape of the bay. The low-lying, densely populated coasts of Odisha, West Bengal, Andhra Pradesh and Bangladesh are especially vulnerable.
• There are two main seasons: pre-monsoon (April–May) and post-monsoon (October–December). During the monsoon itself, strong vertical wind shear tends to suppress cyclone formation.
• Recent years have seen more intense cyclones in the Arabian Sea, which scientists have linked to warming of its waters.

IMD classification

The India Meteorological Department classifies systems by maximum sustained wind speed:

Naming

Cyclones in the north Indian Ocean are named from a list contributed by the member countries of the WMO/ESCAP Panel on Tropical Cyclones. A new list of names was adopted in 2020. The Regional Specialised Meteorological Centre in New Delhi, run by IMD, issues advisories and names the storms for the region.

Temperate (extra-tropical) cyclones

Temperate cyclones form in the middle and high latitudes (roughly 35° to 65°), where warm and cold air masses meet along a front.

Western disturbances

For India, the most important temperate systems are western disturbances — low-pressure systems originating over the Mediterranean region that travel east with the westerlies. They bring winter rain to north-west India, which is valuable for the rabi wheat crop, and snowfall to the western Himalaya.

How India cut cyclone deaths

Odisha's transformation after 1999 is a widely cited model:

• Better forecasting — IMD's track and intensity forecasts have improved markedly.
• Early warning reaching villages through multiple channels.
• Cyclone shelters along the coast, often doubling as schools.
• Large-scale, planned evacuation — as in Phailin (2013) and Fani (2019).
• Institutions — the Odisha State Disaster Management Authority (set up after 1999) and, nationally, the Disaster Management Act, 2005, the NDMA and the National Disaster Response Force.
• Community preparedness — volunteers and drills.

The international community, including the UN, has recognised Odisha's approach as a model of disaster risk reduction.

Practice questions

1. Prelims-type: Why do tropical cyclones rarely form near the equator? What are western disturbances?
2. Mains-type (150 words): Why is the Bay of Bengal more prone to cyclones than the Arabian Sea?
3. Mains-type (250 words): "Odisha's cyclone preparedness shows that disasters are not purely natural." Discuss.