Geography · Ch 6 — Natural Hazards and Disasters
Spatio-temporal Distribution of Tropical Cyclone in India
Spatio-temporal Distribution of Tropical Cyclone in India
The diagram is a map of India, colour-coded to show how vulnerable different parts of the country are to damage from strong winds and cyclones. The risk is graded by the maximum wind speed a region is expected to face, and the map divides the country into six zones. From most to least dangerous, these are: Very High-A (55 m/s), Very High-B (50 m/s), High (47 m/s), Moderate-A (44 m/s), Moderate-B (39 m/s), and Low (34 m/s). Each zone is a band of wind velocity, so the map is essentially telling you the design wind speed — the speed buildings and infrastructure in that area should be built to withstand.
The most striking feature is that the very-high-risk zones are not where you might first expect them. They appear over Jammu & Kashmir and parts of the Northeast, not along the cyclone-prone coasts. This is because the map measures wind hazard from all sources, not just tropical cyclones. The strong winds over the Himalayas and the Northeast come from western disturbances, thunderstorms, and the fierce local winds that funnel through mountain valleys. The moderate-risk zones, by contrast, hug the coastlines — much of the east and west coasts fall into Moderate-A or Moderate-B. This is where tropical cyclones actually strike, and the map captures that threat well, even though the wind speeds there are lower than in the mountainous interior. …
India’s geography dictates where its tropical cyclones are born. The peninsula juts into the sea with the Bay of Bengal on one side and the Arabian Sea on the other, so cyclones that strike the country originate in one of these two basins. The book’s focus here is on where and when these systems form — the spatio-temporal pattern.
Most cyclones affecting India originate between 10° and 15° north latitudes during the monsoon season. That is the general rule. But the Bay of Bengal behaves differently from the Arabian Sea, and even within the Bay, the preferred birthplace shifts with the calendar.
For the Bay of Bengal, the peak months are October and November. During this post-monsoon window, cyclones typically develop between 16° and 20° north latitudes, and to the west of 92° east longitude. This places their origin in the central-western Bay, close enough to the eastern coast to pose a threat.
By July, the picture changes. The place of origin shifts northward to around 18° north latitude and west of 90° east longitude, near the Sunderban Delta. So the same basin produces storms at different locations depending on the season — the monsoon-season storms cluster further north and closer to the delta, while the October–November storms form a bit further south and east.
| Aspect | Bay of Bengal (October–November) | Bay of Bengal (July) |
|---|---|---|
| Latitude of origin | 16°–20° N | around 18° N |
| Longitude of origin | west of 92° E | west of 90° E |
India has a long coastline and is exposed to nearly 10 per cent of the world's tropical cyclones. Due to its peninsular shape, India is exposed to nearly 10 per cent of the world's tropical cyclones, with the Bay of Bengal on the east and the Arabian Sea on the west; the majority of the cyclones (nearly 71%) originate over the Bay of Bengal, and the rest over the Arabian Sea. Most of these cyclones originate between 10° and 15°N latitude during the monsoon season. Cyclones originating in the Bay of Bengal mostly develop during the months of October and November, mainly between 16°N and 21°N latitudes and to the west of 92°E lo …