HURRICANES
What is a hurricane ?
A hurricane is a very large, disc-shaped mass of clouds that rotates on itself. It is between 200 and 1000 km in diameter and rotates counter-clockwise in the northern hemisphere. A hurricane is formed when warm, moist air is released from the ocean. The warm waters (temperature above 26°C) of the intertropical zone are the fuel of the hurricane and feed the powerful clouds that form.
At the center is the eye, surrounded by the eyewall, a dense wall of clouds reaching heights of 10 to 17 km. With a highly variable diameter, averaging around 40 km, the eye is a relatively calm area where winds are weak and the sky is only slightly cloudy.
Elements of the hurricane
Each year, from June to November, the Caribbean experiences a period of heightened activity known as "hurricane season." The region is exposed due to its location in the tropical Atlantic, an area where the ideal conditions for hurricane formation are present.
● Warm waters: sea surface temperatures often exceed 26°C, the energy needed to power cyclones.
● High atmospheric humidity promotes the development of dense clouds, which are known to be powerful.
● Winds similar in altitude, allowing the weather systems strengthening.
● Distance of at least 550 km from the equator: the Coriolis force due to the Earth's rotary motion causes the vortex effect of hurricanes. Hurricanes cannot therefore form when the Coriolis force is zero. However, this force is zero at the equator and increases towards the poles.
The stages of the formation of a hurricane
In the Caribbean, the majority of hurricanes are formed by tropical waves originating from the African coast and moving westward at speeds ranging from 20 to 40 kilometers per hour, driven by the prevailing trade winds.
Representation of the path of hurricanes
The manifestations of a hurricane
A hurricane is manifested by extreme and destructive :
- Winds : gusts can exceed 250 km/h. changes of direction often brutal, including the passage of the eye, can be the cause of considerable damage, and transform objects in real missiles.
- Heavy rainfall can lead to various forms of natural disasters, including floods and landslides (such as rockslide and mudslide)
- Wave / Flood (or Swell Cyclonic) : the waves generated by the wind, high and several meters, can be seen up to 1,000 km of the hurricane. Spreading faster than him, the swell can be the first sign of his arrival. This phenomenon adds a raising of the level of the sea called " Storm Tide ". By the sea, it can be seen in the floods.
- Lightning activity : a localized around the center of the hurricane, it is due to the instability of the air and the temperature differences between the high and low altitudes.
Classification : depression, tropical storm or hurricane ?
Depression, tropical storm, or hurricane ?
There are three categories of tropical systems, classified according to wind intensity:
Tropical depression: wind speeds below 39 mph.
Tropical storm: wind speeds between 39 and 73 mph (at this stage, the system is given a name)
storm system is given a name) ;
Hurricane: wind speeds exceeding 73 mph
In the Northern Hemisphere, hurricanes are classified using the Saffir–Simpson Scale, which includes five categories based on wind strength.
What is the link with climate change ?
With climate change, the water of the oceans becomes increasingly hot. The cyclones will not necessarily be more numerous, but they may be more intense. In addition, the climate change raise the level of the sea, which makes it all the more dangerous storm surges.
Source : Booklet teacher, cycle 3 – Project "Pare pa Pare" – French Red Cross – 2022
Scale Saffir-Simpson scale
To find out more
Georisques.gouv.fr – Hurricanes Ressource in order to understand what is a hurricane, and the useful gestures to adopt. ( French)
National Geographic hurricanes Article on how hurricanes starts and how to prevent it (English).
Frequently asked questions : hurricanes - Laboratoire océanographique et météorologique de l’Atlantique. Toutes les informations pour comprendre ce que sont les ouragans sous forme de questions – réponses. NOAA (Anglais)
CDEMA Ouragans Pratical ressources on understanding the hurricanes.
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