Cyclone Categories in the North Indian Ocean: The IMD Classification

A recurring source of confusion during cyclone events is that the same system may be described in quite different terms depending upon which agency is quoted. This is not disagreement about the storm; it reflects genuinely different classification systems and different measurement conventions.

The India Meteorological Department serves as the Regional Specialised Meteorological Centre for the North Indian Ocean, which means it holds official responsibility for tracking, naming and issuing warnings for tropical cyclones across the Bay of Bengal and the Arabian Sea.

The classification scale

The IMD classifies systems according to maximum sustained surface wind speed, averaged over three minutes.

A system is assigned a name upon reaching Cyclonic Storm intensity, at 34 knots. Below that threshold it is identified by number and basin only.

Naming

Names for North Indian Ocean cyclones are drawn from a list contributed by the thirteen member countries of the WMO/ESCAP Panel: Bangladesh, India, Iran, Maldives, Myanmar, Oman, Pakistan, Qatar, Saudi Arabia, Sri Lanka, Thailand, the United Arab Emirates and Yemen.

Names are used sequentially rather than alphabetically, and unlike Atlantic practice they are not reused. Each name is retired after use, and the list is extended as required.

Seasonality

The North Indian Ocean exhibits a distinctive bimodal cyclone season, unlike most basins which have a single peak.

The pre-monsoon peak occurs during April and May, before monsoon onset. Sea surface temperatures are high and vertical wind shear is comparatively low, favouring development. Storms forming in this period can be intense.

The monsoon period from June to September is comparatively unfavourable for intense cyclone development, as strong monsoon flow produces high vertical wind shear which disrupts the vertical alignment a cyclone requires. Depressions and deep depressions form frequently, but few intensify into major storms.

The post-monsoon peak from October to December is the more active of the two. Sea surface temperatures remain high following the monsoon, while shear decreases as the monsoon circulation withdraws. The most damaging cyclones in Indian history have predominantly occurred during this period.

The Bay of Bengal experiences considerably more cyclone activity than the Arabian Sea, historically by a factor of roughly four, although Arabian Sea activity has attracted increased attention in recent years.

Why the Bay of Bengal is particularly dangerous

The hazard from a North Indian Ocean cyclone arises disproportionately from storm surge rather than from wind.

The Bay of Bengal presents an unusually adverse combination. It is shallow, and shallow water permits a surge to build to greater height. It narrows towards the north, funnelling water towards the head of the bay. The coastal terrain of West Bengal, Bangladesh and coastal Odisha is low-lying and densely populated. Major river deltas allow surge to penetrate far inland.

Historically, the great majority of cyclone fatalities in this region have resulted from surge and consequent flooding rather than from wind damage.

Warning stages

The IMD issues a graduated four-stage warning sequence for coastal areas:

• Pre-Cyclone Watch — issued 72 hours in advance, indicating the possibility of a developing threat
• Cyclone Alert — issued 48 hours in advance for areas likely to be affected
• Cyclone Warning — issued 24 hours in advance, with expected landfall area and intensity
• Post-Landfall Outlook — issued 12 hours in advance, addressing conditions expected after landfall

Warning accuracy for the region has improved markedly over recent decades. Track forecast errors have reduced substantially, and this improvement, combined with organised evacuation, has produced a dramatic reduction in mortality even as economic losses have risen with coastal development.