Cyclones: reading the four warning classes

A traveller who hears “Class II” on Mauritian radio is hearing a precise piece of information, though not the one they think. The warning classes describe neither the strength of the system, nor its distance, nor its category. They describe the time remaining before a single threshold: gusts of 120 kilometres per hour. The four classes apply to tropical depressions, tropical storms and tropical cyclones likely to affect Mauritius or Rodrigues.

Four classes, one threshold

The system reads like a countdown. The higher the class number, the less time remains before the gusts, and the last class means the threshold has been crossed.

A Class I warning is issued not less than 36 hours and not more than 48 hours before the onset of gusts of 120 kilometres per hour. It is the warning to prepare: it leaves two nights and a day, or a little less.

A Class II warning is issued so as to allow, as far as practicable, twelve hours of daylight before those same gusts. The nuance matters: not twelve hours, but twelve hours of daylight. A warning is not designed to send people out to prepare in the dark.

A Class III warning follows the same logic, with six hours of daylight. A Class IV warning, for its part, is issued when gusts of 120 kilometres per hour are recorded in some places and are expected to continue. A Class IV warning therefore forecasts nothing: it states a fact.

How a warning ends

There are two ways out, and they are not to be confused.

After a Class III or Class IV warning, a safety bulletin lifts the warning once observations show that the risk of cyclonic gusts has passed and that the cyclone is moving away. This bulletin also informs the public of the weather and environmental risks that remain: the end of the gusts is not the end of everything.

After a Class I or Class II warning, or after a safety bulletin, it is a termination bulletin that is issued. There is nothing automatic about it: it follows consultation with the National Crisis Committee and its advice that outdoor risks have considerably decreased. The end of a warning is therefore not a purely meteorological decision.

System categories and the season

The warning classes should not be confused with the categories of tropical systems, which form a separate scale. That scale has six grades: tropical depression, moderate tropical storm, severe tropical storm, tropical cyclone, intense tropical cyclone and very intense tropical cyclone. The category describes the system itself. The class speaks only of the time left to those in its path.

The cyclone season runs from 1 November to 15 May of the following year. For summer 2026-2027, between five and seven named storms are expected in the South-West Indian Ocean basin. A named storm in the basin does not necessarily affect the country: a system passing well offshore may bring nothing but swell, and a storm surge only if it passes very close.

What a visitor should take away

Three practical consequences follow from the definitions themselves.

The first is that the time available is known in advance. A Class I warning is not background noise: it marks out a window of 36 to 48 hours. This is the moment for shopping, charging devices and putting questions to the hotel, not for setting off on a boat trip.

The second is that the classes do not follow one another mechanically. A warning can be lifted at Class I or Class II by a termination bulletin, without ever moving on to the next. This is not a foretold escalation but an assessment revised as the system advances.

The third is that the threshold is the same everywhere. The 120 kilometres per hour does not depend on where one is staying. What changes from one part of the island to another is the likelihood of the threshold being reached there, and that is for the bulletin to say, not the class.

As for the safety instructions specific to each class, they are broadcast by the national authorities at the time of the warning. That is when to listen to them, and to follow them, without trying to guess for oneself what a still-calm sky may portend. For a visitor, the hotel reception is the first place to ask what the current class means for the day and for getting about.