What is a cyclone ?

A cyclone is a very strong atmospheric disturbance that originates above the warm waters of the oceans of the intertropical zone, and is accompanied by large movements of winds, sometimes very violent, and torrential rains. It should be noted that these depressions change names depending on their origin. They are called hurricanes or hurricanes in the Caribbean Sea and the Gulf of Mexico and typhoons in the western Pacific and cyclones in the Indian Ocean and the southwest Pacific.

A cyclone is characterized by the conjunction of 3 main phenomena which can cause damage when they pass :

• Very strong winds blowing in a circular pattern around a huge mass of clouds that spiral around the central part called the eye.
• A strong atmospheric depression at the surface of the ocean which is maximum at its center (lowest depression value at the eye of the cyclone).
• Heavy rains that can occur in spiral cloud bands sometimes extending up to 1,000 km from the eye of the cyclone.

Cyclones move at varying speeds, usually along a path that resembles the curve of a parabola. At low latitudes, their speed ranges from 10 to 30 km/h, and at higher latitudes, it can reach 80 km/h.

Evolution of a cyclone

A cyclone can undergo changes depending on its speed :

1. As soon as a disturbance takes on a swirling form with winds below 63 km/h (34 knots), it is called a tropical depression. At this level, the meteorological services assign it a number so that it can be identified.
2. When this tropical depression reaches wind speeds between 63 and 118 km/h (34 to 63 knots), it is classified as a tropical storm and is assigned the first available name in the predefined list. At this level, the phenomenon can already cause significant damage (strong swell, violent winds, heavy rains, floods, landslides).
3. This storm reaches hurricane stage when winds exceed 118 km/h (63 knots). Classified into five categories according to their power, hurricanes are very violent storms generating winds whose average speeds can reach at least 118 km/h. The eye of the cyclone generally appears from category 2. Category five is very rare. Hurricanes are accompanied by a very strong drop in atmospheric pressure which generates extremely violent winds, devastating seas as well as torrential rains which can cause major damage.

Formation and consequences of a cyclone

Cyclones form in the middle of the North Atlantic Ocean, cross the Caribbean arc, and then generally die along the southwest and west coasts of the United States. Their characteristics vary depending on where they form. For example, “Caribbean” cyclones are cyclones that originate in the Caribbean Sea, sometimes near the northern continental areas of South or Central America.

A cyclone manifests its devastating power in three fundamental forms: wind, rain, and sea. Each hurricane is unique and has different effects depending on the combined importance of these three elements. That is, cyclones that are accompanied by heavy rainfall can cause damage in mountainous and flood-prone areas. This damage can be landslides, mudslides, floods, etc. Whereas when they are accompanied by very strong winds, damage can be recorded at the level of homes and various infrastructures throughout the territory. For example: Roofs blown away, electricity and telephone networks destroyed, etc.

The colors of vigilance

The keystone of meteorological information to security actors and the population is the vigilance system whose role is to symbolize the different levels of danger by colors. The colors change according to the proximity of the intensity of the danger and the probability of the impact allowing to ensure the management of the cyclone in real time, this system is composed of the following colors:

1) Green vigilance :

Green is the color of the normal situation, that is, without any significant risk.

2) Yellow vigilance : « Be attentive ! »

Vigilance in the literal sense begins with the color yellow. In yellow alert, the danger is imprecise and this symbol is announced two or three days in advance and/or with limited impact and consequences. In yellow alert, it is advisable to remain vigilant and limit risky travel, etc.

3) Orange vigilance : « Get ready! Stay mobilized! »

Orange alert announces the likelihood of a hazard with moderate effects and therefore a more significant and serious threat in twenty-four to thirty-six hours. At this level of alert, one must be aware of the danger of the sea at the coast or of heavy rain. Therefore, one must remain very attentive to weather and information reports.

With orange alert now, it’s time to stock up on supplies to ensure you’re self-sufficient for a few days. Orange alert is, in a way, the preparation phase for the cyclone’s passage, with a view to limiting the likely damage.

4) Red vigilance : « Get ready! »

This alert indicates a significant likelihood of a severe storm or hurricane occurring within six to eighteen hours. On red alert, everyone is urged to return home and take the necessary steps to protect themselves, their pets, important documents, etc., from the threat of a hurricane.

5) Purple vigilance : « Comply !, take shelter ! »

The purple level is the emergency level. At this level, the danger is evident and the impacts are beginning to be felt. The most threatening and difficult phase is expected in three to six hours and its consequences are potentially catastrophic: take shelter, do not go outside under any circumstances because objects will start flying everywhere, trees, antennas, poles, etc. could fall at any moment, mudslides or landslides could occur at any time. At purple alert, caution is advised, information is inevitable.

6) Gray vigilance : « Stay careful »

Gray alert means the hurricane is moving away, but the danger is not over. This is the phase of damage assessment or recognition and the activation of emergency response. Gray alert signals a gradual return to normal life. However, it is not recommended to travel as usual; caution is still required.

Some supporting links :

• http://la.climatologie.free.fr/cyclone/cyclone.htm
• https://www.youtube.com/watch?v=9F5TCB-3u1I
• https://www.youtube.com/watch?v=TyqbwiDPYDs