The skipper he stood beside the helm,
His pipe was in his mouth,
And he watched how the veering flaw did blow
The smoke now West, now South.
Then up and spake an old Sailòr,
Had sailed to the Spanish Main,
“I pray thee, put into yonder port,
For I fear a hurricane.”
—From “The Wreck of the Hesperus,” by Henry Wadsworth Longfellow
It seems inconceivable that something so large and deadly could possibly have a beneficial side. But hurricanes are really just giant heat engines that pick up warmth from the oceans in the warmer latitudes and transport it to the colder climates, helping to balance the earth’s warm and cool zones.
Tropical disturbances often form when stormy weather over Africa moves offshore near the Cape Verde islands and begins to make its way east across the Atlantic. During the peak of the Atlantic hurricane season in September and October, some of the largest and most dangerous hurricanes form in this region.
When ocean waters are warm, humidity is high, and winds are light, the conditions are right for a hurricane to form. The tropical Atlantic, the Caribbean, and the Gulf of Mexico provide this atmospheric mix every year from June through November. Hurricanes also need something to set them spinning, a job handled by the Coriolis force that is created by the earth’s rotation. Because the Coriolis force is zero at the equator, storm systems must migrate at least 5 degrees north of it before they can pick up enough spin to become a hurricane.
Most hurricanes—about two-thirds—form between 10 and 20 degrees north latitude, along a constant feature called the Intertropical Convergence Zone, or ITCZ. This zone marks the area where northeasterly breezes meet southeasterly trade winds, causing the warm, humid tropical air to rise. As the air rises, it naturally condenses into clouds and rain, which often gather into groups of thunderstorms and move along the ITCZ from east to west.
Like tornadoes, hurricanes can form only when a specific set of conditions is in place. Because they feed on heat, hurricanes need water that is at least 80°F from the surface down to a depth of about 150 feet. The atmosphere above a developing storm must be substantially cooler than the surface, allowing the system to develop towering thunderclouds. The midlevel layers of the atmosphere must be saturated with water vapor, adding to the fuel that powers the storm. As noted earlier, hurricanes can’t form at the equator, so a developing storm has to be at least 500 miles away from zero latitude to get going, and there must be at least some low-level spin in the atmosphere to get it started.
Unlike tornadoes, which depend on winds blowing in different directions at varying levels of the atmosphere to get them spinning, wind shear is a hurricane’s enemy. Many potential hurricanes have been ripped apart by areas of strong wind shear aloft. But once conditions are right and a low-pressure system has developed at the surface, it can become a self-sustaining atmospheric juggernaut.
Although hurricanes usually begin in the tropics, they can wander far from their breeding grounds, bringing their own brand of mischief to northern climates. Many major hurricanes have battered New York City, and one model predicts that a Category 3 storm could cause a storm surge of 20 feet at the Statue of Liberty.
As converging winds spin toward the center of the low, they are drawn into its core and rise upward, producing an area of high pressure above the storm. Although the upper air that helped the thunderstorms build initially is much colder than the surface air, it isn’t long before the rising, warmer air heats up the air aloft and it begins to flow away from the top of the storm. This provides an exhaust port for the developing system, lowering the pressure at the surface even more and causing large quantities of humid air to be drawn into the rotating cyclone.
As the storm spins up, a chain reaction begins: more outflow at the top lets more hot air in at the bottom, resulting in faster surface winds pouring in and spiraling up through the storm’s center. Until the winds reach 39 miles per hour, the storm is known as a tropical depression, and its main threat is rain and the potential for flooding if it moves over mountainous terrain or stalls over any landmass for an extended period.
When wind speeds exceed 39 miles per hour, the system is called a tropical storm and the National Hurricane Center assigns it a name from a list established by the WMO (World Meteorological Organization). The Atlantic Basin is assigned six lists of names, with one list being used each year. After six years, the first list is used again. If an especially destructive hurricane develops, its name is retired and no future storm will ever bear that name.
A tropical cyclone is known as a tropical storm until its winds reach 73 miles per hour, when it officially becomes a hurricane. At that point, it can be measured on the Saffir-Simpson Hurricane Wind Scale, a method of rating hurricanes developed by Herbert Saffir, an engineer who became interested in how wind causes damage to buildings, and Robert Simpson, who was director of the National Hurricane Center in the 1970s. The scale rates hurricanes by their wind speed, barometric pressure, storm surge height, and damage potential in categories from 1 to 5. After Hurricane Floyd in 1999, which caused extensive flooding in eastern North Carolina, a need for an additional scale that would measure the risk of flooding was identified; its creation is still a work in progress.
Although no recent hurricane has exceeded Category 5, MIT researchers theorize prehistoric hurricanes may have reached 750 miles per hour or more. These “hypercanes” could have formed after asteroid impacts, and might have wiped out whole species.
In the meantime, the categories and their characteristics break down as follows:
Wind speeds of 73 to 95 miles per hour. Category 1 storms are the least destructive hurricanes where winds are concerned. Most damage is usually confined to trees, shrubs, and unanchored mobile homes, although flooding can cause much more harm than wind. As the hurricane grows, it forms an eye in the center, a circle of relative calm where the sky may be blue and the winds light. Surrounding this area is an eye wall, a ring of intense thunderstorms that spin around the center of the hurricane.
Winds of 96 to 110 miles per hour. The storm surge, a giant dome of water that moves along with the hurricane, can be from 6 to 8 feet above normal, causing coastal areas to flood in advance of the storm. Doors, windows, and roofing materials are all at risk in a Category 2 hurricane.
Winds range from 111 to 130 miles per hour. This storm kicks it up another notch, flooding low-lying areas near the coast with a storm surge 9 to 12 feet above sea level. The surge can move inland up to 8 miles, requiring some evacuations. At these wind speeds, mobile homes are completely destroyed and even concrete block homes sustain some damage. The foliage of shrubs and trees is blown off, and many trees are uprooted by the wind. Large structures near the beach are battered by floating debris carried by high waves.
Winds of 131 to 155 miles per hour. Once a hurricane has this strength, you know it’s extremely dangerous. Low-lying areas may be flooded hours before the storm moves ashore, and mass evacuations are necessary. The storm surge can reach a towering 13 to 18 feet above normal, causing major damage to any structures near the shore. The roofs are stripped off some homes and other buildings, and even external walls may fail. Signs, trees, and shrubs are torn from the ground and become flying missiles.
Winds are sustained at over 155 miles per hour. Fortunately, Category 5 storms are rare, but they are among the most treacherous winds on Earth. With a storm surge of more than 18 feet, these hurricanes leave major devastation in their wake. There is extensive damage to even strong buildings, and the complete destruction of others, especially those near the shoreline. Category 4 and 5 hurricanes making landfall near inhabited areas have caused some of the worst damage and loss of life in our nation’s history. One such storm was Hugo, a giant storm that brought the highest storm surge ever recorded on the east coast of the United States—nearly 20 feet above sea level.
NOAA estimates that nine out of every ten victims of hurricanes are killed by storm surge. Because hurricanes are huge low-pressure systems, some think the surge is caused by the storm’s lower atmospheric pressure raising the sea level. While that effect is real and measurable, it is so small as to be insignificant when a hurricane makes landfall.
The storm surge is caused by the hurricane’s winds piling water up ahead of the eye as it moves toward shore. As long as the hurricane is in the open ocean, there is no storm surge to speak of, because water that gets pushed up ahead of the storm has room to flow away. But as the storm approaches the coast, there is no room for the water to escape and it rapidly rises, sometimes in a matter of minutes, into a mountain of water. How high the storm surge will be above average sea level and how much damage it will do depends on several factors. One is wind speed: a Category 4 storm will have a higher storm surge and cause much more destruction than a Category 2, for instance.
The storm surge isn’t just another wave pushed ahead of a storm; it acts like a gigantic bulldozer that can destroy anything in its path. Think of the storm surge as a moving wall of water weighing millions of tons.
But just as important as the hurricane’s wind speed is where you are in respect to the hurricane’s eye. If you’re behind and above a hurricane looking toward its direction of motion, the front part on the right side will have the highest wind speeds. That’s because the hurricane’s forward speed is added to the wind speed in that area, called the right front quadrant (RFQ). So, if a hurricane approaching land has sustained winds of 100 miles per hour, and it’s moving forward at a speed of 20 miles per hour (a fairly fast-moving hurricane, by the way), winds in the RFQ will be measured by a stationary ground observer at 120 miles per hour. As you might guess, winds on the left side of a hurricane are rotating in the opposite direction, and won’t cause as much damage.
The storm’s angle of attack is a key factor in its impact. The highest level of destruction is caused by a hurricane hitting the coastline head-on, just as in an automobile accident. If a storm travels up the coast, with its left side brushing the seashore, the most dangerous part of the storm stays offshore and the net effect will be much less damage.
The shapes of the shoreline and the ocean bottom have a great deal to do with the magnitude of a storm surge. The worst damage occurs when a developing surge meets a shallow seabed sloping gently to the beach. This is why areas like New Orleans are especially at risk.
The worst-case scenario would be a hurricane arriving onshore at high tide. With the ocean level already at its highest point of the day, the storm surge from a Category 4 or 5 hurricane can add another 15 or 20 feet of water, with abnormally large waves breaking on top of that. Water weighs around 1,700 pounds per cubic yard, and there are few structures that can stand up to the abuse a high storm surge can produce.