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Hurricane Laura: How hurricanes form and why they are so frequent in the USA, Mexico and the Caribbean

 

They are all tropical cyclones, but the name hurricane is used exclusively for those in the North Atlantic and the Pacific Northwest.

How do they form and why do they usually occur in this part of the world?

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The most common mechanism of hurricane formation in the Atlantic - which causes more than 60% of these phenomena - is a tropical wave.

When it reaches the Atlantic Ocean, the tropical wave may be the beginning of a hurricane, but in order for it to form, it needs energy sources, such as adequate humidity, heat and wind.

In particular, it is necessary that the temperature of the ocean surface is higher than 27º C, as well as that of the water layer that extends for at least 50 meters below the surface.

Specific types of wind are also needed. On the one hand, winds with horizontal rotation, so that the storm is concentrated.

On the other hand, it is necessary that the winds rising from the surface of the ocean keep their strength and speed constant.

If there is a sharp wind, that is, variations in the wind with height, this can interrupt the flow of heat and humidity that causes the hurricane to take shape.

There must also be a concentration of clouds laden with water and high relative humidity in the atmosphere.

All of this needs to occur at the appropriate latitudes, usually between the 10 ° and 30 ° parallels of the northern hemisphere, since in this region the effect of the Earth's rotation causes the winds to converge and ascend around the low pressure area.

When the tropical wave meets all these ingredients, an area of ​​about 50 to 100 meters is created, where they begin to interact.

"The movement of the tropical wave acts as the trigger for this storm," Jorge Zavala Hidalgo, general coordinator of Mexico's National Meteorological Service, explained to BBC News Brazil.

And this storm works as a catalyst: a ballet of heat, air and water begins.

The low pressure area causes the hot, humid air that comes from the ocean to rise and cool, which feeds the clouds.

The condensation of this air releases heat and causes the pressure on the surface of the ocean to drop further, which attracts more moisture from the ocean, strengthening the storm.

The winds converge and ascend within this area of ​​low pressure, rotating in the opposite direction to the needles of the clock - due to the Earth's rotation.

It is this rotation that gives hurricanes their characteristic image.

As the storm gets more powerful, the eye of the hurricane, a central area of ​​up to 10 km remains relatively quiet.

The wall of the eye rises around it, composed of dense clouds, where the most intense winds are. Beyond it, there are the spiral-shaped bands, where there is more rain.

The speed of the winds is what determines when we can call this phenomenon a hurricane. At its birth it is a tropical depression, when its strength increases it becomes a tropical storm and becomes a hurricane when it exceeds 118 km / h.

From there, they can be classified into five categories according to the sustained speed of their winds. To measure the destructive power of Atlantic hurricanes, the Saffir-Simpson scale is used.

The strength of tropical cyclones is so strong that their winds could produce energy equivalent to almost half of the world's electricity generation capacity, according to the United States National Oceanic and Atmospheric Administration.

However, the main responsible for the destruction and loss of life when a hurricane passes are the storm surge in coastal cities and the floods caused by the rains that it brings.

In the United States, for example, storm tides accounted for almost half of deaths related to tropical Atlantic cyclones between 1963 and 2012, according to data from the American Meteorological Society.

In addition to these factors, the destruction caused by a hurricane depends on other circumstances, such as the speed with which it passes, the geography of the territory and the infrastructure of the affected region.

Mexico, United States and the Caribbean: the most vulnerable areas
One of the factors that explains why this region is more prone to receiving hurricanes is that the Atlantic Ocean, in tropical latitudes, has the appropriate temperature for its formation for more months in the year.

Another factor is the circulation of the winds that push hurricanes.

Trade winds, the main winds in tropical low latitudes, run from east to west, taking cyclones to the coasts of the Caribbean, the Gulf of Mexico and the United States.

The course of these winds is also influenced by the rotation of the Earth - the so-called Coriolis effect - that causes them to tend to deviate towards the north.


Typically, as hurricanes advance, they also move slightly north.

When passing the parallel 30 ° N, they usually meet the winds from the west, another of the

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