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What are hurricanes?

By: Juliana Rocha

Category-5 Hurricane Isabel*

Category-5 Hurricane Isabel*

Hurricanes, typhoons and cyclones are regional names for strong tropical storms. Meteorologists use the term “tropical cyclones” to refer to the large amounts of low pressure air that move in an organized fashion over the seas around the Equator. Not all tropical cyclones turn into hurricanes; a few disappear hours after having been formed.

In order for a tropical cyclone to be called a hurricane, its winds need to be blowing at more than 75 mph. When that happens, the cyclone takes the shape of a donut and is baptized by meteorologists with names such as Catarina, Andrews, Ophelia, etc.

Did you know that there is a difference between the hurricanes formed in the Northern Hemisphere and those of the Southern Hemisphere? The winds of hurricanes that occur in the Northern Hemisphere flow counterclockwise, while, in the Southern Hemisphere, they flow clockwise. This happens because of the Earth’s rotation and because of the so-called Coriolis effect, which causes the wind to blow in opposite directions in each of the hemispheres.

But don’t go thinking that the Earth’s rotation is capable, for instance, of affecting the drainage of toilets and sinks. This motion is two slow to be perceived that way! The direction in which the water twirls in sinks and toilets is determined by the shape of these objects and by the initial direction of the water. You are able to make the water drain both clockwise and counterclockwise, no matter which hemisphere you’re in. Don’t believe it? Then try it for yourself!

How is a hurricane formed?

Have you ever noticed that seawater becomes warmer by the end of a sunny day? This happens because the sea absorbs and retains the heat it receives during the day. Especially in the summer, tropical seas receive a great amount of energy and, therefore, become warm. When the surface of the sea reaches 26o Celsius, the natural process of evaporation speeds up. Therefore, the air right above the surface absorbs the water vapor caused by the evaporation, thus becoming warmer and more humid. When it gets warm, the air begins to rise and forms a low pressure column above which wind starts to blow. As this column of hot and humid air rises, the water vapor condenses forming droplets. After a few hours, the droplets are joined together and form clouds. After a few days of cloud formation, rain and thunder occur.

 Typhoon Amber

Typhoon Amber's eye**

When winds flowing around the hot air column reach 75 mph, the pressure in a small area in the column declines steeply: that’s when the so-called “eye of the hurricane” forms. The eye is a calm place, where winds blow slowly, no faster than 20 mph. If you could cross a hurricane, first you would go through very strong winds blowing against you, then you would find a warmer area with a breeze and, finally, would reach a new area with violent winds again. Hurricane winds can reach up to 150 mph.

On average, hurricanes last for six days and travel at a speed between 12 and 20 mph. Fully developed storms move quicker than young storms. Hurricanes also cause waves of up to 40 feet in height to form, as well as up to 215 more inches of rain than usual for the region.

Classification of hurricanes

Marilyn, Isabel, Floyd... Do you know any of these names? Well, besides being the name of a famous American actress from the 1950s, a princess who freed the slaves in imperial Brazil and a character in the trilogy Back to the Future, these are names of a few of the greatest hurricanes in history.

The need to tell one storm apart from the other led meteorologists to use the alphabet as a naming system. This way, the first storm of the season will receive a name with the letter “a”, such as Audrey, the name of the second will start with the letter “b”, as in Barbara, the third storm’s name will begin with the letter “c”, as in Charles, and so on. Every season, different names are used so new hurricanes are not confused with those in different seasons.

Although this is the most common way of naming a hurricane, there are other systems as well. In Saudi Arabia, for instance, hurricanes are named with the letters ARB, followed by the last two digits of the year and a number indicating the sequence in which they took place, that is, whether it was the first, the second, the third and so on to occur that year. When a hurricane hits a region or a country with great devastation, the country might request to the international meteorological authorities that its national meteorological center baptize it.

The World Meteorological Organization (WMO) names hurricanes that are formed over the North Atlantic Ocean and the Philippines. Hurricanes formed in other regions (the seas of China or the Indian Ocean, for instance) are named by a Regional Center of Tropical Cyclones. There are five regional centers that cover the areas where hurricanes are most commonly formed: RSMC La Réunion-Tropical Cyclone Centre, RSMC Miami-Hurricane Centre, RSMC Nadi-Tropical Cyclone Centre, RSMC Tropical Cyclones New Delhi, and RSMC Tokyo-Typhoon Centre.

Damages from Category-3 Hurricane Katrina**

Damages from Category-3 Hurricane Katrina**

Hurricanes are also classified differently, with a scale called Saffir-Simpson, which considers the pressure within the eye, wind speed and the volume of storms. This scale ranges from 1 to 5 and measures the destructive power of hurricanes.

Category-1 hurricanes have wind speeds that range from 74 to 95 mph, with a storm surge of about 4 to 5 feet, and cause little structural damage. Category-2 hurricanes have winds that range between 96 and 110 mph, with 6 to 8 feet more rain, and cause damage to trees and roofs. Category-3 hurricanes have winds between 111 and 130 mph, with storm surges between 8 to 12 feet, and cause floods and damages to homes.

Category-4 hurricanes have winds that range between 131 and 155 mph, with an increase in rain between 13 and 18 feet, causing severe damage to roofs and great structural damage to houses. The most devastating are category-5 hurricanes, with wind speeds of above 155 mph, a storm surge of above 19 feet, serious floods and severe structural damage to houses and buildings.


* September 6-20, 2003. Photo: NASA.

** August 27, 1997. Satellite photo/University of Wisconsin Cooperative Institute for Meteorological Satellite Studies.

**Photo: Petty Officer Kyle Niemi. US Coast Guard/Wikipedia.

Back to the main article:

Cyclones, hurricanes, tornadoes...

See also:

What are tornadoes?

What are cyclones and tropical storms?


Tropical Cyclone Programme - World Meteorological Organization (TCP/WMO)

National Oceanic and Atmospheric Administration (NOAA)

Hurricane Research Division - Atlantic Oceanographic and Meteorological Laboratory (HRD/AOML/NOAA)

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