Every year, between the months of June and November, they hit the Caribbean area, the Gulf of Mexico and the east coast of the United States, sometimes destroying buildings and towns.
Their counterparts are typhoons, which affect the Northwest Pacific Ocean, and cyclones, which affect the South Pacific and the Indian Ocean.
They are all tropical cyclones, but the name “hurricane” is used exclusively for those in the North Atlantic and Northeast Pacific.
But how are they formed and why do they usually affect this area of the world?
Hurricanes, energy bombs
The most common mechanism for the formation of hurricanes in the Atlantic – which causes more than 60% of these phenomena – is a tropical wave.
The wave begins as an atmospheric disturbance that creates an area of relatively low pressure.
It is usually generated in east africa from mid-July.
If it finds the right conditions to stay or develop, this low-pressure area begins to move from east to west, with the help of the trade winds.
When it reaches the Atlantic Ocean, the tropical wave can be the germ of a hurricane, but for it to form it needs Energy sources, like the heat and the wind suitable.
In particular, it is necessary that the surface of the water is above 27ºC and that there is a thick layer of warm water in the ocean.
There also has to be, on the one hand, winds with a horizontal turn for the storm to concentrate. On the other hand, winds that maintain their strength and speed constant as they rise from the surface of the ocean.
If there is wind shear, or variations in the wind with height, this can disrupt the flow of heat and moisture that causes the hurricane to form.
In addition, there must be a concentration of water-laden clouds and a high relative humidity present in the atmosphere.
All this has to happen in the suitable latitudes, in general between the parallels 10 ° and 30 ° of the northern hemisphere, since here the effect of the Earth’s rotation causes the winds to converge and rise around the area of low pressure.
When the tropical wave finds all these ingredients, an area of about 50-100 km is created, where they begin to interact.
“The movement of the tropical wave functions as the trigger for that storm,” explains Jorge Zavala Hidalgo, general coordinator of the National Meteorological Service of Mexico, to BBC Mundo.
And it is this storm that acts as a catalyst: the dance of heat, air and water begins.
The area of low pressure It makes the hot, humid air that comes from the ocean rise and cool, which feeds the clouds.
The condensation of this air releases heat and causes the pressure on the ocean’s surface to drop even further, which draws more moisture from the ocean, thickening the storm.
The winds converge and rise within this low pressure area, turning in the direction counter clockwise – by the influence of the Earth’s rotation – and giving hurricanes that characteristic image.
As the storm grows more powerful, the eye of the storm – the central area of up to 10 km – remains relatively quiet.
Around it rises the wall of the eye, composed of dense clouds where the most intense winds are located.
Beyond are the spiral-shaped cloud bands, where there is more rainfall.
The speed of the winds It is what determines when we can call this phenomenon a “hurricane”: at its birth it is a tropical depression, when it increases in strength it becomes a tropical storm and it becomes a hurricane when it passes 118 km per hour.
From there, they are usually classified into five categories based on sustained wind speed. In the Atlantic, the Saffir-Simpson Wind Scale is used to measure their destructive power.
Such is their strength that the winds from a hurricane could produce the same energy as almost half of the world’s electricity generating capacity, according to the US National Oceanic and Atmospheric Administration (NOOA). ).
However, it is not the wind but the storm surge and flooding that causes the rain that the hurricane discharges which generally causes the most destruction and loss of life.
In the United States, for example, storm surge caused by tropical cyclones in the Atlantic was responsible for nearly half of the deaths between 1963 and 2012, according to data from the American Meteorological Society (AMS).
In addition to these factors, the destruction caused by a hurricane will depend on other circumstances, such as the speed at which it passes, the geography of the territory and the infrastructure of the affected area.
“The damage or danger associated with a tropical cyclone does not necessarily correspond to its category. For example, the highest category cyclone does not have to be associated with more precipitation ”, Jorge Zavala Hidalgo tells BBC Mundo.
Mexico, the United States and the Caribbean: the most vulnerable areas
One of the factors that explains why this part of the world is prone to hurricanes is that the Atlantic Ocean, in tropical latitudes, has the right temperature for their formation for more months of the year.
Another is the movement of great wind currentss that push hurricanes.
Trade winds – the global wind currents in the tropics – run from east to west, carrying them toward the coasts of the Caribbean, the Gulf of Mexico and the southern United States.
The path of these winds is also influenced by the rotation of the Earth – the so-called coriolis effect – which makes them tend to drift north.
In the Atlantic, as the hurricanes advance, they deviate slightly to the north; and when they exceed approximately 30 ° N, they tend to meet the westerly winds, another of the great global currents, which cause them to bend towards the east.
On their way they will run into him Bermuda-Azores anticyclone which will determine if they are heading towards the Gulf of Mexico or towards the United States.
Anticyclones are regions of high atmospheric pressure with drier air, fewer clouds, and winds that rotate in a clockwise direction in the Northern Hemisphere.
The Bermuda anticyclone acts as a obstacle and if the hurricanes want to advance they have to skirt around it. For this reason, the size and position of the anticyclone can determine where a tropical cyclone is going.
If it is weak and more positioned towards the It is, hurricanes surround it and they continue north, moving away from the Caribbean.
On the contrary, if it is stronger and is located to the southwest, a tropical cyclone can head towards the Gulf of Mexico or towards Florida.
The position of the anticyclone changes according to the year, the seasons and can change in a matter of days.
“Because of these variations, a hurricane can follow a very different trajectory today than one that passes three or five days later,” explains Jorge Zavala Hidalgo, of the National Meteorological Service of Mexico.
Following the same logic, anticyclones and other air masses are responsible for a hurricane rolling westward., as happened in 2012 with Hurricane Sandy, for example.
After making landfall in Cuba, Sandy began to move northeast, but an anticyclone in Greenland and a cold front blocked her path. That caused Sandy to recede toward the east coast of the United States, causing destruction in New York and New Jersey.
At Pacífico EastAlthough it is a more active area than the North Atlantic, fewer hurricanes make landfall.
“What happens is that these storms tend to go towards the west or northwest. Some can recede towards the coasts of Mexico if the winds are adequate, but most go to higher latitudes, find colder waters and disappear ”, says Gary M. Barnes, retired professor at the University of Hawaii, to BBC Mundo. USA.
Why we hardly see in South America
Although the northern part of the Atlantic may offer ideal conditions for the formation of hurricanes, the same does not happen below the equator.
“The South Atlantic is calmer because there is no tropical wave – it is a more common phenomenon in the northern hemisphere – and there are more variations in the speed and direction of the wind, something that inhibits the formation of hurricanes,” explains Barnes.
Also, tropical cyclones do not normally form if they are not at least 500 kilometers from the Equator, as the Coriolis effect is too weak to make the winds turn and form a hurricane.
Although it is a phenomenon that happens with very little frequency in South America, hurricanes have been registered in the coasts of southern Brazil.
In 2004, tropical cyclone Catarina left 11 dead and more than 30,000 people displaced.
And how can climate change impact?
“Climate change causes the temperature of the ocean surface and the thick layer to be warmer and that is a problem. We have theories that if the ocean is warmer it can translate into stronger and more intense storms. ”Says meteorologist Gary M. Barnes.
There are indications that the areas in which a cyclone finds conditions to stay and survive are expanding over time, according to Jorge Hidalgo, coordinator of the National Meteorological Service of Mexico.
“Perhaps the number of cyclones will not increase but the distribution of categories may change. In other words, there are more hurricanes of a major category and fewer of a minor category ”, adds Zavala.
Scientists agree, however, that it is too early to measure the impact of climate change on the formation and progression of hurricanes.
“The storms are likely to intensify very little by little, but we are going to need a lot of data to prove that global warming is going to cause stronger hurricanes. In 25 years we may have evidence, “concludes Barnes.
Thanks to José Manuel Gálvez, meteorologist for the National Oceanic and Atmospheric Administration (NOAA).