Hurricanes
What is a hurricane ?
A hurricane is a set of clouds to be very extended in the form of a disc that moves by turning on itself. Its diameter is between 200 and 1 000 km and it rotates in the opposite direction, clockwise in the Northern hemisphere. A hurricane is formed through the hot and humid air that emerges from the ocean. The warm waters (temperature greater than 26°C) of the inter-tropical zone are the fuel of the hurricane and feed the mighty clouds that are forming.
The eye, in the centre, is bounded by the wall, a wall of cloud, and from 10 to 17 km in height. Diameter very variable, of the order of 40 km on average, the eye is in fact a quiet area where the winds are low and the slightly cloudy sky.
Elements of the hurricane
Each year, between June and November, the Caribbean is full of “hurricane season”. The region is exposed, because it is located in the tropical Atlantic, an area where gather all the ideal conditions to the formation of hurricanes:
● Hot water : the temperature of the surface of the sea often exceeds 26 °C, the energy necessary to power the cyclones.
● High humidity in the atmosphere, promoting the development of dense clouds, and 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 rotational movement of the earth causes the effect swirling hurricane. The latter, therefore, can not 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, with the majority of hurricanes from waves tropical formed off the african coast and move westwards by the prevailing winds (trade winds) at a speed of the order of 20 to 40 km/h.
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 rain : they can result in floods or landslides (landslide, landslide or cast muddy).
- 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 : cyclone, storm or depression ?
Depression, tropical storm, or hurricane ?
There are 3 categories for events such as cyclone, classified according to the intensity of the winds :
● if the wind is less than 64 km/h, it is a tropical depression ;
● if the wind is between 64 km/h and 119 km/h, it is a tropical storm (at this point, the
storm system is given a name) ;
● if the wind exceeds 119 km/h, it is a hurricane
When they are considered as the hurricane, the systems cyclone in the northern hemisphere the North-West are classified according to the scale Saffir-Simpson scale into 5 categories according to the intensity of the winds.
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 (French) Resource to understand what is a cyclone, its risks, the actions necessary to protect and grants available French to deal with.
National Geographic hurricanes (English) Article explaining how hurricanes form, how to cope before, during and after.
Frequently asked questions : hurricanes – (NOAA Laboratory, naval meteorology and oceanography of the Atlantic). All the information to understand what are the hurricanes in the form of questions and answers. (English)
CDMEA Ouragans (English) Resources practices in order to understand the hurricanes and prepare before, during, and after their passage.
Our publications on the hurricanes
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