by Bill Evans
National Oceanic and Atmospheric Administration/Department of Commerce
When these storms happen over the northern Atlantic Ocean, the Caribbean Sea, the Gulf of Mexico, or the northeastern Pacific Ocean, they are called hurricanes. The word “hurricane” comes from “Hurican,” the Carib god of evil. Hurican was derived from the Mayan god, Hurakan, one of the culture’s creator gods, “who blew his breath across the chaotic water and brought forth dry land.”
In other parts of the world, hurricanes have different names depending on where they occur. In the northwestern Pacific Ocean they are called typhoons. When their wind speeds exceed 150 mph they are called super typhoons. In the southwestern Pacific and southeastern Indian oceans they are called severe tropical cyclones; in the northern Indian Ocean they are called severe cyclonic storms; and in the southwestern Indian Ocean they are called tropical cyclones. Whew! That’s a lot of descriptions from around the world. Sometimes you need a scorecard to keep up in this game!
Hurricanes are also the only natural disasters with their own names. Camille, Wilma, Charlie, Andrew, Hugo, Katrina—each seemed to have a personality and each caused a different sort of disaster.
Hurricanes are very special. A tornado can have winds up to 300 mph, but a hurricane is rarely above 155 mph. A tornado, however, is a very concentrated event compared to a hurricane. A tornado may be more violent, but hurricanes are more deadly! As you will see in this section, hurricanes make tornadoes!
A mile-wide tornado is huge, but a 100-mile-wide hurricane is considered small. Few tornadoes last an hour, whereas a hurricane can last for weeks, producing 12 to 20 inches of rain and flooding hundreds of miles of coastline with 20- to 30-foot-high storm surges.
National Oceanic and Atmospheric Administration/Department of Commerce
Despite improved weather forecasting of hurricanes, loss of life and terrible destruction continue to occur. This was never more evident than in Louisiana and Mississippi in August of 2005, when Hurricane Katrina caused the deaths of 1,836 people and the destruction of hundreds of thousands of homes and other buildings.
Hurricanes generate an incredible amount of energy. In one day, an average hurricane can produce the equivalent of 200 times the total amount of electricity that can be produced worldwide! That’s an incredible amount of energy! Imagine if we could capture or harness that energy—we would never have to rely on fossil fuels again.
Fortunately, only about 3 percent of that energy is converted into wind and waves. I say fortunately because if all of that energy were converted to the surface landmass, it would have the destructive impact of all of the nuclear weapons of the United States and Russia put together!
Ingredients for a Hurricane
The ingredients for a hurricane are simple, yet the formation of a hurricane is a complex process. The key ingredient for a tropical cyclone is a little wind and some warm ocean water—82°F (28°C) and above. Add a humid air mass and weak upper winds flowing in the same direction as those on the surface of the ocean, and you have the perfect environment for a tropical cyclone, especially if the surface winds are rotating.
Frank Picini
Preexisting weather disturbances—like thunderstorms moving across from western Africa—can interact with rotating surface winds, a rising humid air mass, and higher-level winds fto create a tropical cyclone.
This sort of encounter takes place fairly often, but thankfully, only a few of these disturbances become full-blown hurricanes! I can watch the satellite loops all season long and see hundreds of these situations develop, but usually just a handful will have winds of above tropical storm strength at 39 mph!
The warm water must be at least 200 feet deep because as the storm develops, it forces energy into the ocean, which brings cold water up from the depths below.
Winds must be converging at the surface and the wind must also be unstable so that the growing storm will rise. The air must be very humid and go very high into the storm system (as high as 18,000 feet) as the extra water vapor supplies more energy.
To avoid ripping apart the developing storm, all existing winds should be coming from the same direction and at nearly the same speeds. Finally, an upper-atmospheric high-pressure area must exist above the system to pull away the air rising through the storm. This will help the storm to grow stronger.
I think you can understand now why only 10 percent of tropical disturbances grow into tropical storms. The combination of the right ingredients, coming together in the right conditions at the right time, is extremely rare!
Structure of a Tropical Cyclone
The formation of a tropical cyclone is also a process. First, thunderstorms grow and combine, creating a disturbed area in the tropics. Then warm humid air spirals inward as the thunderstorms organize into a circle or a swirl. Next, bands of thunderstorms spiral around the center. Hurricanes are made up of these bands of thunderstorms, called rain bands or spiral bands. They can be 3 to 30 miles wide, and 50 to 300 miles long. The spiral bands contain all of the destructive winds and flooding rains of a hurricane.
Frank Picini
The most recognizable feature of any hurricane is the eye. This is an area, 10 to 40 miles in diameter, around which the entire storm rotates. In the eye, the skies are clear and the winds are light. It’s the calmest part of the tropical cyclone, the area where the lowest surface pressures are found.
A hurricane’s eye becomes visible when some of the rising air in the eyewall is forced toward the center of the storm from all directions. This convergence causes the air to actually sink in the eye. This creates a warmer environment and the clouds evaporate, leaving a clear area in the center.
Air flowing out from the center of the storm begins curving clockwise as water vapor in the air forms cirrostratus clouds, which cap the storm. As the storm approaches the coast of a landmass, a huge mound of water builds to the right of the eye. This is called the storm surge, which is discussed further later on.
These monster storms capture our attention because of their power and their destructive capabilities, as well as their impact on the economy and on our society.
How Water Powers a Hurricane
As I mentioned, a warm ocean and warm air are the key ingredients in a hurricane. The warmer the air, the better. Some of the largest and most destructive hurricanes of all time, like the 1938 hurricane called the “Long Island Express,” and Hurricane Camille in 1969, occurred during years in which there was record heat.
Warm seawater evaporates and is absorbed by the surrounding air. As humid air rises high into the atmosphere, water vapor condenses into water drops. When that happens, heat is released. This is called latent heat—remember, earlier I described a hurricane as a latent heat pump. That latent heat warms the surrounding air, making it lighter and causing the air mass to rise even more. As the warm air rises, more air flows inward to replace it, causing wind.
Frank Picini
Did You Know Hurricanes Produce Tornadoes? It’s True!
While both hurricanes and tornadoes are atmospheric “vortices” (counterclockwise rotation of air), they have little in common. Most tornadoes have diameters of hundreds of feet and are produced from a single convective storm (i.e., a thunderstorm or cumulonimbus). A hurricane, however, has a diameter of hundreds of miles and is comprised of several, sometimes dozens, of convective storms.
Tornadoes are produced in regions of large temperature gradient, meaning areas that are very hot at the surface but very cold high in the atmosphere. Hurricanes are generated in regions of near zero horizontal temperature gradient, meaning the temperature stays close to the same no matter how high you go into the atmosphere. Tornadoes primarily develop over land because the sun’s heating of the land surface usually contributes to the development of the thunderstorm that spawns the vortex. In contrast, hurricanes are purely oceanic phenomena—they die out over land due to a loss of their moisture source. Last, hurricanes have a lifetime that is measured in days, while tornadoes t
ypically last for a number of heart-pounding minutes.
However, hurricanes can spawn tornadoes when certain instability and vertical shear (change of winds with height) conditions are met. In hurricanes, most of the thermal instability is found near or below an altitude of 10,000 feet. Because the instability in hurricanes occurs at relatively low altitudes, the storm cells tend to be smaller and shallower than those usually found over land. But because the vertical shear in hurricanes is also very strong at low altitudes, small super cell storms can be produced inside the hurricane. These super cells are more likely to spawn tornadoes than an ordinary thunderstorm.
Which Hurricane Is the Biggest Tornado Producer?
Hurricane Ivan in 2004 caused a multiday outbreak of 127 tornadoes. Most of the tornadoes occurred on September 17 in the mid-Atlantic region of the United States, two days after Ivan’s landfall in Alabama. State-by-state tornado counts from Ivan include 40 in Virginia, 25 in Georgia, 22 in Florida, 9 in both Maryland and Pennsylvania, 8 in Alabama, 7 in South Carolina, 4 in North Carolina, and 3 in West Virginia. At least seven people were killed and seventeen injured by these tornadoes.
Who’s the Most Damaging Tornado Producer?
One of the tornadoes produced by Hurricane Allen in 1980 did about $50 million worth of damage (in 1980 dollars; about $127 million in 2005 dollars) in the Austin, Texas, area.
More recently, Hurricane Cindy in July 2005 spawned a strong tornado that damaged the Atlanta Motor Speedway and other nearby areas to the tune of some $71.5 million (in 2005 dollars).
Storm Surge—The Biggest Killer
The most dangerous part of the hurricane is the rapid rise in the water’s sea level; this is called the storm surge. The storm surge is caused by the swirling ocean winds driving the water onto the shore. A hurricane’s winds force energy into the ocean, causing the water to pile up. The storm’s winds push the water inward, toward the right of the eye. When the hurricane is out at sea, that water spreads out harmlessly, but as the hurricane approaches land, this water becomes trapped between the shore and the hurricane itself. The sea level rises quickly: in just 2 to 3 hours, 20 feet or more of water can push inland. If the storm surge arrives at high tide, the water could rise even higher: up to 5 to 10 feet higher given the area where this occurs.
A storm surge during a hurricane
National Oceanic and Atmospheric Administration/Department of Commerce
NASA
Let’s say the normal high tide on the shoreline is 5 feet above mean sea level. Add 20 feet of water from a hurricane and you have a surge of 25 feet. If the storm surge were to come during an astronomical high tide (a greater-than-normal high tide caused by the gravitational effects of the moon), the effects would be even greater. The Galveston Hurricane of 1900 came ashore with a devastating storm surge; between 6,000 and 12,000 lives were lost, making it the deadliest disaster in the history of the United States.
Some of the factors that determine the height of the surge are the speed, intensity, and size of the wind field of the hurricane; the angle at which the hurricane approaches the coast; and the physical characteristics of the coastline. The worst effects of storm surge occur where the ocean floor slopes gradually to the shore.
Frank Picini
Because water is so much heavier than air, the storm surge causes even more damage than the hurricane’s fierce winds. Since coastlines today are so densely populated, with too many homes built too close to the ocean, even the smallest storm surges can cause very serious property loss and loss of life.
Stages of Hurricane Development
Tropical Disturbance
Often called the birth of a hurricane, tropical disturbances are areas of showers and thunderstorms with a little wind circulation around an area of low pressure. They may have some towering cumulonimbus clouds and might last for more than 24 hours. In this picture, you can see a tropical disturbance that looks like a blob of puffy clouds. Beneath those clouds are a number of thunderstorms. A circular wind pattern is starting to take place. When meteorologists see a satellite picture like this—either a still or an animated loop—they know that it’s likely that the disturbance will grow stronger.
Tropical Depression
A tropical depression is the next stage of a growing tropical system. In this picture, the low-pressure area, or “depression,” is now surrounded by winds that have begun to blow in a circular pattern. At this point, the maximum wind speed will be 38 mph. The system will continue to draw in more warm, moist air, which will build more thunderstorms and feed existing ones. Meteorologists will plot the depression’s path and watch it for signs that it is continuing to grow larger and stronger.
Tropical Storm
When a tropical depression’s winds exceed 38 mph, it has become a tropical storm. The clouds have taken on a more well-defined, circular shape. Thunderstorm bands are flowing out from the center. Beneath this storm, the winds are so strong and the seas so rough, that ships must now avoid the area. A tropical storm’s winds will range from 39 to 74 mph.
The storm is now drawing more heat and water vapor from the sea. It has a column of warm air near its center. As this column becomes warmer, air pressure at the ocean’s surface falls. The falling pressure draws more air into the storm and the storm grows stronger.
This is the point in the storm’s life where it is given a name.
Hurricane
In these pictures of Hurricanes Felix and Dean, you can see a well-defined, concentric pattern of clouds and a well-formed eye. A storm achieves hurricane status when its winds are in excess of 74 mph. In the eyewall, warm air spirals upward, creating the hurricane’s strongest winds. The speed of the winds in the eyewall is related to the diameter of the eye. A hurricane’s winds blow faster if the eyewall is small; if the eye widens, the winds decrease.
Hurricane Dean
Hurricane Felix
The hurricane is now drawing large amounts of heat and moisture from the sea. Heavy rains are falling from the spiral or feeder bands—as much as 2 inches per hour—and the seas are churning dangerously as the storm surge builds!
How Do Hurricanes Get Their Names?
There’s a great novel by George R. Stewart, Storm, which I assign to all my student interns. Storm is very popular among meteorologists. Written in 1941, and still relevant, it’s about a forecaster who used women’s names for naming storms.
The idea must have made quite an impact on weathermen; during World War II, military forecasters began to formally attach women’s names to storms. Naming helps forecasters avoid confusion and keep track of the storms; it also helps researchers keep things straight. Most times during a hurricane season, there can be two or three storms at a time on the Atlantic and as many as two or three in the Pacific. That’s a lot of storms, and giving them names makes it easier to track and keep up with them as well as to go back and research them later.
Each year, the World Meteorological Organization (WMO), an agency of the United Nations, issues four alphabetical lists of names for tropical storms. There’s one list for the northern Atlantic Ocean and the Caribbean Sea, and one list for each of the regions of the Pacific Ocean: eastern, central, and northwestern. The lists include both men’s and women’s names that are popular in countries that might be affected by the storms in each area. As the equality movement prevailed across the United States, the practice of using men’s and women’s names was started in 1978 in the eastern Pacific and the following year in the Atlantic and Gulf of Mexico.
Except in the northwestern and central Pacific, the first storm of the year gets a name beginning with A—such as Tropical Storm Ana. If the storm intensifies into a hurricane, it becomes Hurricane Ana. The second storm gets a name beginning with B, and so on through the alphabet. The lists do not use all the letters of the alphabet since there are few names beginning with such letters as Q or U. Also, no Atlantic or Caribbean storms receive names beginning with Q, U, X, Y, or Z.
In the northwestern and cent
ral Pacific, the lists include names of animals and things as well as people. For instance, a typhoon was named after a cricket or a cicada in a certain country’s language.
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Atlantic/Caribbean Names
2009
2010
2011
Ana
Alex
Arlene
Bill
Bonnie
Bret
Claudette
Colin
Cindy
Danny
Danielle
Don
Erika
Earl
Emily
Fred
Fiona
Franklin
Grace
Gaston
Gert
Henri
Hermine
Harvey
Ida
Igor
Irene
Joaquin
Julia
Jose
Kate
Karl
Katia
Larry
Lisa
Lee
Mindy
Matthew
Maria
Nicholas
Nicole
Nate
Odette
Otto
Ophelia
Peter
Paula
Philippe
Rose
Richard
