Hurricane Mechanics: Nuts and Bolts of Heat Engines

Tropical thunderstorms that organize around the center of a hurricane act like carburetors, injecting fuel into the hurricane's eye, the nearly circular cylinder of relative calm at the center of the storm. The fuel injected into the eye is latent heat, a high-premium grade released when rising, invisible water vapor condenses into cloud droplets within thunderstorms. Interrupt or lessen the flow of latent heat into the eye and the engine sputters or stalls. Before we tinker with the parts of this engine, we need to read its operating manual.

Hurricane Engine: Basic Operating Manual

Just as the operating manual for your car's engine offers tips for peak performance under various weather conditions, there are optimum conditions that promote and support the ignition (and subsequent smooth running) of the heat engine of a hurricane. Here is the tropical manual for successfully starting up a hurricane's heat engine:

  1. Start heat engine over warm tropical seas, with water temperatures above 78oF (25.5oC) to a depth of at least 60 meters (about 200 ft). This insures high rates of evaporation and high octane ratings of water vapor. The atmosphere's water vapor fuel tank should be topped off to the middle troposphere to promote ignition.

  2. Start heat engine far enough from the equator so that a sufficiently strong torsion from the Coriolis effect can engage a circulation (counterclockwise in the Northern Hemisphere, clockwise in the Southern).

  3. Before starting, make sure that there are weak winds blowing in a uniform direction throughout the troposphere. Strong winds or winds changing direction with increasing altitude (that is, vertical wind shear) will cause leaks in the fuel line.

Of the three guidelines, warm ocean water is the most important for ignition. Water evaporating from warm tropical oceans tops off the lower and middle troposphere's gas tank of water vapor. A full tank ensures a great release of latent heat once thunderstorms organize around the center of a developing hurricane, helping to push the heat engine to full throttle. In coming sections, we will learn how the release of latent heat drives the development of a hurricane.

For maximum efficiency, water beneath the surface of tropical oceans needs to be warm as well. As a hurricane develops, its revving winds stir the ocean, mixing warm surface water with water below. If the water below the surface is also warm, the mixing will keep surface water temperatures high, thereby maintaining high rates of evaporation and guaranteeing a rich supply of water vapor.

The heat engine of a hurricane has a rotary motor in which fuel-injected thunderstorms spiral around a cylindrical-shaped eye. For engine parts to turn, the Coriolis effect must be sufficiently strong. Climatological studies of the tropics reveal that ignition of hurricane heat engines does not occur within 5 latitude of the equator, where the Coriolis effect is simply too weak to induce a circulation (note that the basins of hurricane genesis do not extend to the equator in Figure 11.2). Ignition becomes increasingly more likely as latitudes increase from 5o to 15o. Without a contribution from the Coriolis effect, winds would blow almost directly from high to low pressure, precluding the development of the circulation needed to organize thunderstorms around an eye.

It is also crucial to the ignition and smooth running of a hurricane for the engine's cylinder to receive an uninterrupted supply of high-octane latent heat. If upper-level winds are too strong, latent heat released from the tops of thunderstorms will be blown away from the eye. Essentially, strong upper-level winds blowing latent heat away from the center of a hurricane can be likened to a leak in the fuel line of a hurricane.

In addition, winds blowing from a uniform direction throughout the troposphere are important for ignition and smooth operation. Later in the chapter, we will learn that many hurricanes are steered from east to west on the equatorward flanks of subtropical high pressure systems. Sometimes, however, high-level winds at or around the 250-mb level may blow against the grain from west to east. These westerlies shear the tops off hurricanes, decapitating thunderstorms and interrupting the flow of fuel. Warning: tropical heat engines may sputter and stall if steered into regions with strong high-level westerlies.

Hurricane Development: Spark-Plug Disturbances

If you were to take a sharpened pencil to the task of isoplething temperature and pressure in the tropics, the point would probably not get very dull (assuming there aren't any hurricanes). You wouldn't draw many isotherms or isobars because the tropics, in general, have uniform distributions of surface temperature and pressure. Therefore, recalling earlier mid- latitude analyses, you might conclude that the tropics should be devoid of low pressure systems. After all, low pressure systems in the mid-latitudes derive their strength from horizontal gradients of temperature and moisture in the presence of strong upper-level divergence and well-defined fronts. How can a pencil-dulling, isobar-packed low pressure system like a hurricane emerge from such uniformity?

As discussed in Chapter 7, the Intertropical Convergence Zone (ITCZ) meanders around the globe in tropical regions. The ITCZ is a necklace of showers and thunderstorms that girdles equatorial regions, evidence of the powerful convergence and lifting that occurs there. Occasionally, a cluster of thunderstorms breaks away from the ITCZ and provides the spark to ignite the heat engine of a hurricane.

Another spark that can ignite the hurricane heat engine is an easterly wave, a cluster of thunderstorms that travels from east to west (hence, its name) in the tropical troposphere. In the equable tropics, locating the pressure signature of an easterly wave on surface weather maps can be rather difficult. Instead, meteorologists look at maps of low-level streamlines to locate areas of wind convergence associated with easterly waves (see Figure 11.4). These areas of convergence promote clusters of thunderstorms that can, under the operating conditions outlined in the heat engine manual, fuel a hurricane.

Sometimes meteorologists lump disorganized clusters of tropical thunderstorms into the generic classification of tropical disturbances. Assuming that the optimum conditions outlined in the operating manual apply, we will now discuss the nuts and bolts of how tropical disturbances evolve into the highly structured tropical system we call a hurricane.

Starting the Engine: Ignition to Full Throttle

As latent heat is released from clusters of thunderstorms that form a tropical disturbance, ill-defined areas within the cluster begin to warm. In response, air density lowers, prompting surface pressures to fall. Embryonic tropical disturbances often toss the point of lowest pressure around like a hot potato. But eventually, an area of weak low pressure will emerge if thunderstorms within an especially intense cluster congregate and combine their latent heat to establish a central warm core of air.

In response, tropical surface winds begin to increase in speed and converge around the incipient low, importing richer supplies of moisture toward the center. In turn, thunderstorms increase in intensity and begin to multiply in number. The release of latent heat now escalates, and surface pressure, responding to the warming, falls even more, causing converging, moist winds to accelerate further. If the system is far enough from the equator (generally at least 8 to 9o of latitude), the Coriolis effect will induce these fledgling winds to circulate counterclockwise inward towards the area of lowest pressure. When sustained winds reach 37 km/hr (23 mph), the tropical disturbance graduates into a tropical depression. The heat engine of the budding hurricane begins to chug to life.

Meanwhile, air pressures near the tropopause, in response to the warming from latent heat release, begin to increase (recall Chapter 4). In response to higher pressure aloft, air begins to flow outward (that is, diverge) around the top of the center of the tropical depression. Like a chimney, this upper-level area of high pressure vents the tropical depression, preventing air converging at lower levels from piling up around the center (which would raise surface air pressures and squelch the storm). Assuming optimum conditions in the heat engine manual still apply, this feedback process between the release of latent heat, the subsequent drop in surface pressure, and the corresponding increase in surface winds, will continue. When winds become sustained at 62 km/hr (39 mph) or greater, the tachometer mounted on the heat engine now reads tropical storm. Figure 11.5 is a visible photograph of Tropical Storm Iniki (1992), taken from the space shuttle. The circulation around a center is clearly visible in the cloud pattern, but there is no eye (a lack of an eye is characteristic of a tropical storm).

Once the system reaches tropical storm status, it is given a name, a tradition started in 1950 (for storms in the Atlantic Basin) with the use of World War II vintage code names such as Able, Baker, Charlie, Dog, and Easy. Female names were first used in 1953, and the alternation of male and female names for Atlantic Basin storms began in 1979. There are separate lists of names for storms forming in the various basins. The National Hurricane Center (NHC) near Miami has responsibility for keeping tabs on storms in the north Atlantic and eastern Pacific Oceans. Between longitude 140oW and the International Date Line, the Hurricane Center in Honolulu assumes responsibility for monitoring tropical systems. Once a storm moves west of the Date Line, the Hurricane Center on Guam takes over. These three hurricane centers all shared responsibility in tracking Hurricane John in August and early September of 1994. John, whose track is shown in Figure 11.6, was the longest-lived named storm on record and the most powerful hurricane ever observed in the central Pacific, at one time packing winds of 276 km/hr (170 mph).

Like great baseball players, names used for hurricanes can be retired if the storm is exceptionally noteworthy. No future Atlantic storm will ever bear the infamous names of Camille (1969), Agnes (1972), Hugo (1989), Bob (1991), Andrew (1992) or Opal (1995), to name a few. Names not retired are rotated into use every six years. Table 11.2 shows the names selected for tropical storms through the year 2000 in both the Atlantic and eastern Pacific Basins.

Table 11.2:  Tropical storm and hurricane names for the Atlantic and eastern Pacific Basins, 1997-2000.

Atlantic Basin

1997      1998      1999      2000

Ana       Alex      Arlene    Alberto
Bill      Bonnie    Bret      Beryl
Claudette Charley   Cindy     Chris
Danny     Danielle  Dennis    Debby
Erika     Earl      Emily     Ernesto
Fabian    Frances   Floyd     Florence
Grace     Georges   Gert      Gordon
Henri     Hermine   Harvey    Helene
Isabel    Ivan      Irene     Isaac
Juan      Jeanne    Jose      Joyce
Kate      Karl      Katrina   Keith
Larry     Lisa      Lenny     Leslie
Mindy     Mitch     Maria     Michael
Nicholas  Nicole    Nate      Nadine
Odette    Otto      Ophelia   Oscar
Peter     Paula     Philippe  Patty
Rose      Richard   Rita      Rafael
Sam       Shary     Stan      Sandy
Teresa    Tomas     Tammy     Tony
Victor    Virginie  Vince     Valerie
Wanda     Walter    Wilma     William

Eastern Pacific Basin

Andres    Agatha    Adrian    Aletta
Blanca    Blas      Beatriz   Bud
Carlos    Celia     Calvin    Carlotta
Dolores   Darby     Dora      Daniel
Enrique   Estelle   Eugene    Emilia
Felicia   Frank     Fernanda  Fabio
Guillermo Georgette Greg      Gilma
Hilda     Howard    Hilary    Hector
Ignacio   Isis      Irwin     Ileana
Jimena    Javier    Jova      John
Kevin     Kay       Kenneth   Kristy
Linda     Lester    Lidia     Lane
Marty     Madeline  Max       Miriam
Nora      Newton    Norma     Norman
Olaf      Orlene    Otis      Olivia
Pauline   Paine     Pilar     Paul
Rick      Roslyn    Ramon     Rosa
Sandra    Seymour   Selma     Sergio
Terry     Tina      Todd      Tara
Vivian    Virgil    Veronica  Vincente
Waldo     Winifred  Wiley     Willa
Xina      Xavier    Xina      Xavier
York      Yolanda   York      Yolanda
Zelda     Zeke      Zelda     Zeke

We are now ready to bring our test engine to full throttle. Once again, assuming optimum conditions still prevail as outlined in the hurricane operator's manual, the feedback process between the release of latent heat, decreasing air pressure at the surface, and increasing surface winds will ultimately upgrade sustained wind speeds to 119 km/hr (74 mph). The heat engine has gone from the high gear of a tropical storm to the turbo-charged overdrive of a hurricane.

Conditions across the entire Atlantic basin were certainly near optimum in late August 1996 when the visible satellite image of Figure 11.7 was taken. This view of the Atlantic basin shows formidable Hurricane Edouard (packing maximum sustained winds of 190 km/hr (120 mph)), strong Tropical Storm Fran (with winds of nearly 110 km/hr (70 mph)), weak Tropical Storm Gustav (with winds of barely 65 km/hr (40 mph), on the verge of weakening to a tropical depression), and a tropical disturbance off the coast of Africa (which, nearly two weeks later, would skirt the eastern Bahamas as Hurricane Hortense with winds of 215 km/hr (135 mph)). Situations when four tropical systems are engaged in the Atlantic Basin are rather unusual, although the record-setting hurricane season of 1995 also produced several windows to catch four storms (or budding storms) simultaneously.

Hurricane Structure: A Look Under the Hood

A look under the hood at the heat engine of a full-fledged hurricane reveals its working parts (see Figure 11.8). The eye, undoubtedly the most distinctive feature of a hurricane, is an island of tranquility in the midst of a sea of storminess. The eye is a nearly circular cylinder of calm or light winds and partly cloudy skies, usually with a diameter ranging from 8 to 80 km (5 to 50 mi). The sunny breaks that often develop in the eye are caused by compensating subsidence from the powerful thunderstorms that surround the eye. Sinking air warms by compressional heating. Working in tandem with latent heat, compressional warming evaporates cloud droplets within the eye. The notion that heating is greatest in the cylinder of the eye (forming the warm core) is supported in Figure 11.8 by the relatively high altitude of the 32oF (0oC) isotherm above the eye.

To explain the relative calm in the eye, consider a parcel of air spiraling in toward the center of a hurricane. As the air parcel gets closer to the center, its velocity increases because it will tend to conserve its angular momentum (defined in Chapter 7). Like ice skaters whose bodies spin more rapidly as their arms are drawn inward, parcels near the surface attempt to speed up as they spiral in towards the center of the hurricane. Suppose, for sake of argument, we allowed the parcels to spiral inward to the exact center of the eye. By the law of conservation of angular momentum, the velocity of these parcels would become infinite. But a hurricane (or any weather system) doesn't have an infinite amount of energy to support such speeds, because the hurricane's maximum output of energy is fixed by the temperatures of the tropical oceans over which it moves. For water temperatures in the 80 to 90oF (27 to 32oC) range, maximum sustained winds seldom exceed 325 km/hr (200 mph). So in order not to violate the conservation of energy, the parcel must stop short of reaching the center, creating a cylinder of relative calm.

Typically, in an intense hurricane that has abundant energy, parcels can spiral closer to the center without violating the law of conservation of energy, thereby narrowing the diameter of the eye. Usually, a small, well-defined eye is the signature of a powerful hurricane. This general rule certainly was true in the case of Hurricane Gilbert (1988), which struck Jamaica, then crossed the Yucatan Peninsula and finally came ashore in northern Mexico. The satellite image of Gilbert in Figure 11.9a shows the hurricane at 13Z on September 12, 1988, when its central pressure was 960 mb and its maximum sustained winds were about 200 km/hr (125 mph). The eye was approximately 55 km (35 mi) in diameter. Figure 11.9b shows Gilbert almost 36 hours later, at 2330Z on September 13, when its central pressure had fallen to 888 mb, with maximum sustained winds of 296 km/hr (184 mph). This pressure of 888 mb set the record for the lowest sea-level pressure ever observed in the Western Hemisphere. Notice how at this time, the eye had shrunk to only 15 km (9 mi) in diameter.

To further probe the characteristics of the eye of a hurricane, consider Figure 11.10, an aerial view of Beaver Stadium on the University Park campus of Penn State University. No, the authors' zeal for Nittany Lion football has not distracted us. Rather, there is an inkling of tropical meteorology in the way successive rows of seats slope upward and away from field level. Indeed, this inclined profile of Beaver Stadium bears a striking resemblance to the eye structure of some hurricanes. For proof, look at Figure 11.11, a close-up view of the eye of a Pacific typhoon in 1988. Note how the clouds surrounding the eye slant upward and away from the storm's center as altitude increases, forming a caricature of a stadium (please use a little imagination here).

To explain this so-called "stadium effect," we rely on the observation that, above approximately the 700-mb level, the strength of a hurricane typically decreases with increasing altitude (that is, the strongest winds surrounding the eye are always found in the lower troposphere, with wind speeds falling off at high altitudes). As evidence, consider Color Plate 38, which shows a series of horizontal radar slices at various altitudes through Hurricane Erin, taken when the hurricane swirled near Florida on August 2, 1995. Note how the circular, rain-free zones associated with the eye expand with increasing altitude. The cheerleader for this megaphone pattern of rain-free rings is the conservation of angular momentum. Because winds circulating around the eye taper off above 700 mb, parcels of cloudy air, which are sworn to conserve their angular momentum, stop their inward spirals farther and farther from the center of the storm as altitude increases. The end result is that the cloudy walls of the eye slope upward away from the storm's center, forming a stadium built of clouds.

Even within the relative calm of the eye, conditions can, at times, be turbulent. Figure 11.12 is a close-up of the eye of Hurricane Emilia, taken from the space shuttle in July 1994. At the time, Emilia was fast becoming the strongest hurricane ever observed in the central Pacific (Gilma and John eclipsed Emilia later in August 1994, closing the books on one of the most active periods ever recorded in the central Pacific). The turbulent look to the clouds in Emilia's eye suggests that low-level vortices are disrupting the relative calm of the eye. Emilia's eye is also not clear, but there are breaks in the overcast. Sometimes, clouds can even completely fill the eye of a hurricane (even a powerful one), as was the case with Hurricane Opal. As Opal was an hour from landfall in Figure 11.13 at 21Z on October 4, 1995, it packed maximum sustained winds of around 200 km/hr (125 mph) and a central pressure of 940 mb, yet there were few (if any) breaks in the clouds in its eye.

Figure 11.14 shows the three-dimensional air flow in a computer model of a hurricane, demonstrating how parcels of air near the surface spiral inward toward the eye of the storm, stop short of the center, and rise, eventually flowing outward from the eye at the top of the hurricane (also see the cross section in Figure 11.8). These rising parcels of air support powerful thunderstorms that surround the eye. This fierce doughnut of thunderstorms, called the eye wall, contains the hurricane's strongest winds and heaviest rains. Figure 11.15 is a visible image of Hurricane Elena (1985) over the Gulf of Mexico. The eye wall appears as the elevated area of intense convection and overshooting tops surrounding the eye. As a hurricane develops, birds sometimes get trapped in the eye by the towering, fierce storms in the eye wall. In effect, the eye wall becomes a tropical bird cage until the hurricane begins to fizzle. In September 1985, thousands of birds, presumably trapped by the eye wall, were observed in the eye of Hurricane Gloria as the storm came ashore in southern New England.

Another distinctive feature of a hurricane is its spiral bands, tentacles of thunderstorms that pinwheel cyclonically around and into the center of the hurricane (these bands often bear a striking resemblance to the spiral arms of some galaxies). Hurricane Felix, seen in Figure 11.16 spinning in the Sargasso Sea on August 14, 1995, provides a textbook example of spiral bands. At the time, Felix was a weak hurricane, packing winds of about 135 km/hr (85 mph). Often, waves of narrow spiral bands will precede the arrival of a hurricane, producing fitful rains and gusty winds as the bands come and go. Hurricanes often spawn tornadoes when they make landfall (as will be discussed in a coming section), and about 80% of these twisters form from thunderstorms in spiral bands.

Hurricane Movement: Who's Driving?

Look once again at the storm track of Hurricane John shown in Figure 11.6. John's path closely follows the clockwise circulation around the robust subtropical high pressure system that resides over the North Pacific Ocean during the summer. Among their many atmospheric duties, subtropical highs provide the steering currents for many tropical systems. The subtropical high's clockwise (in the Northern Hemisphere) flow of air near the 500-mb lvel is crucial to a meteorologist's assessment of the direction of movement of a tropical storm or hurricane. Often, hurricanes moving westward on the equatorial side of a subtropical high curve poleward (like Hurricane John did). When and where a hurricane will pivot poleward and recurve is often difficult to forecast, marking a period of restlessness and uncertainty for meteorologists, especially if the storm is nearing land.

In the North Atlantic Ocean, the Bermuda high provides the steering currents that escort many tropical systems in a predictable, recurving path. A good example is that of Hurricane Gilbert, whose rampage through the Caribbean and Gulf of Mexico is shown in Figure 11.17. Other infamous storms, however, have deviated from the original course set by the Bermuda high. Such deviations are often caused by upper-level low pressure systems that sometimes wield more influence than the Bermuda high. This was the situation in the case of Hurricane Hugo.

Hurricane Hugo: September 1989

Until Andrew struck southern Florida in 1992, Hugo was the costliest hurricane in United States history, causing $7 billion in damages. Hugo was especially devastating to areas near Charleston, SC which bore the brunt of the powerful storm. Figure 11.18a shows Hugo's storm track, while Figure 11.18b is a visible satellite picture of Hugo during the morning of September 21, 1989. Figure 11.19 shows the 500-mb charts during the three- day period surrounding Hugo's entry into the United States. Arrows show the subsequent movement of the storm. The upper- level steering currents for Hugo were constant for the duration of the storm's approach to land: the combination of the Bermuda high and an upper-level low pressure area in the north-central Gulf of Mexico coaxed Hugo into the South Carolina coast. Hugo was then absorbed by an approaching trough of low pressure after coming ashore, helping to zip the remnants of the storm quickly to the northeast.

Hurricane Elena: Late August and early September 1985

The strength and reach of subtropical highs varies over time, creating situations where hurricanes are left to wander somewhat aimlessly. The result can be a hurricane path that resembles the crayon scribble of a four-year old. In a coloring book, that's not a problem. But with a major hurricane, as was the case with Hurricane Elena, the result is unsettling - residents from Tampa to New Orleans fled the coast in the largest peacetime evacuation in United States history. Elena's track is shown in Figure 11.20 (and recall the satellite image of Elena in Figure 11.15).

Hurricane Elena carved out an erratic path over the Gulf of Mexico during Labor Day weekend, 1985. Figure 11.21 shows the 500-mb chart over four of the five days that Elena threatened the Gulf Coast. The arrows indicate the subsequent movement of the storm. Elena developed near central Cuba and moved northwestward through the Gulf of Mexico, steered by the westward-shifting Bermuda high near the Georgia coast. On August 30, 1985, however, Elena was drawn toward a trough of low pressure that had quickly dipped into the eastern United States. As this trough moved eastward, Elena made a 90o turn and headed east, threatening the west coast of Florida. But the course change was temporary, given that the upper-level trough zipped to the northeast. Elena could not keep up. Abandoned by steering winds, Elena remained stationary for more than 24 hours. In response, the storm's battalion of hurricane-force winds were camped out just off the coast of the Tampa Bay area. Finally, late on September 1, Elena began to be influenced by a 500-mb high pressure system building to its north, which nudged the storm into Mississippi and Louisiana. This last maneuver forced many residents in those states to evacuate a second time.

Subtropical highs lead many hurricanes to their demise, eventually luring them poleward toward land, colder water and strong westerlies. All three are lethal. As a hurricane succumbs to the alien environment over middle latitudes, it releases its energy and moisture to the surroundings. In this way, a hurricane transfers energy and moisture out of the tropics and into the middle latitudes. In effect, hurricanes aid the earth's general circulation in mitigating the large temperature contrasts between the poles and equatorial regions. Although hurricanes and tropical storms, taken as a whole, are responsible for only about 2% of energy transport out of the tropics, they account for as much as 30% of the energy transport during the peak of hurricane season.

Hour of Danger: Landfall and Storm Surge

Land is where hurricanes sometimes go to die. But as the eye of a hurricane comes onshore (that is, as the hurricane makes landfall), it does not give up without a ferocious fight. Heavy rain, powerful winds, and tornadoes are weapons that a hurricane brings to bear on coastal communities. But its most destructive weapon is the storm surge, a rise in ocean levels of up to 9 meters (about 30 ft) that accompanies the landfall of a hurricane.

Hurricane Andrew's ferocious winds caused considerable damage in southern Florida, but the brute force of the storm surge, as evidenced in Color Plate 39, was truly remarkable. When people think of a storm surge, they often envision dramatic "Hawaii Five-O" style tidal waves ripping into the coastline. Some Hollywood movie producers perpetuate a similar notion that is riddled with misconceptions about storm surges. The 1979 film Hurricane featured defiant beach condominium owners ignoring orders to evacuate as they partied against the wind. Then, in dramatic fashion, a distant rumble signalled the approach of a mountain of water that slammed into the condo, instantaneously wiping out the rebellious revelers.

The storm surge does indeed have destructive impact upon the coast, but it makes its presence felt in a more gradual manner. Over the open ocean, a hurricane's violent winds push and churn up surface waters, creating waves of many sizes. These waves propagate away from the hurricane, eventually organizing into swells that break on distant shores, foretelling the approach of the storm (for example, during hurricane season, surfers flock to the southern shores of Hawaii to ride the big waves generated by storms passing south of the Islands). Likewise, in the vicinity of a seafaring hurricane, there is little rise in ocean levels. There is virtually no storm surge.

It's a different matter once the storm approaches shore. Building onshore winds start to push water toward land. As water approaches the coast, it "feels" the bottom and starts to slow down and pile up near shore. Slowly but surely, ocean waters rise and, like a swelling monstrous tide, swamp everything in their path. This "surge" of water, with battering waves rolling on top of it, is very powerful, capable of leveling houses and small-story buildings. When Hurricane Camille struck the central Gulf Coast in 1969, large ships were carried as much as 1.6 km (1 mi) inland by a storm surge that reached 7 meters (23 ft) near Biloxi, MS.

Destruction is greatest when the storm surge arrives around the time of high tide. As Hurricane Gloria bore down on southern New England in 1985, destruction was expected to be catastrophic. Though damage was extensive, Gloria's landfall did not coincide with high tide, lessening the impact of the storm surge. It could have been a lot worse.

The storm surge is always highest on the side of the eye corresponding to onshore winds, which is on the right side of the point of landfall in the Northern Hemisphere (point R is in this region in Figure 11.22a). On this side of the hurricane, the forward motion of the storm also contributes to a larger storm surge. To understand why the storm's forward motion makes a difference, consider a train robber running at 5 km/hr on top of a train moving at 80 km/hr. If the robber is running in the same direction as the train is moving, his speed relative to the ground is actually 85 km/hr. Similarly, a hurricane moving at 40 km/hr with peak winds measured at 160 km/hr in the right front quadrant of the storm (Figure 11.22a) will effectively have a maximum wind speed of 200 km/hr. The combined rise in ocean waters inundates coastal locations as shown in Figure 11.22(b-c).

On the opposite side of the point of landfall of the storm (point L is in this region in Figure 11.22a), the water level sometimes actually decreases as the storm makes landfall. Here, winds are offshore, opposing the direction of the storm's movement (if the train robber had been running opposite the direction of the train's movement, his speed relative to the ground would have been only 75 km/hr). The shallow sounds of eastern North Carolina (with average depths around 2 meters (6 ft)) have actually been observed to nearly empty as a hurricane makes landfall north of North Carolina. When Hurricane Hugo slammed into South Carolina in September 1989, a low water warning was issued for the coastal waters off Jacksonville, FL, about 300 km (186 mi) south of the point of landfall.

In the early 1970s, a system was designed by Herbert Saffir, a consulting engineer, and Robert Simpson, then the director of the National Hurricane Center, to quantify (for disaster agencies) what level of damage to expect from a hurricane. Using a mix of structural engineering and meteorology, they constructed the Saffir-Simpson Scale, which consists of five categories which correspond to a hurricane's central pressure, maximum sustained winds, and storm surge, as given in Table 11.3. Categories 3, 4 and 5 are intense hurricanes, with the potential to inflict great damage and loss of life. Since 1988, several intense hurricanes have made landfall into the United States, including Hugo (1989), Andrew (1992), and Iniki (1992), all category 4 storms. Camille (1969) and the Florida Keys' Labor Day Hurricane of 1935 are the only two category 5 hurricanes to strike the United States in this century.

Table 11.3:  Saffir-Simpson Hurricane Damage Potential Scale

   Category      Pressure (mb; inches mercury)   Wind (knots; mph)   Storm Surge (m; ft)

1: Minimal         980        28.94             64-82     74-95         1.0-1.7   4-5
2: Moderate        965-979    28.50 - 28.91     83-95     96-110        1.8-2.6   6-8
3: Extensive       945-964    27.91 - 28.47     96-113    111-130       2.7-3.8   9-12
4: Extreme         920-944    27.17 - 27.88     114-135   131-155       3.9-5.6   13-18
5: Catastrophic    < 920       < 27.17           >135      >155          >5.6     >18

Another danger that a hurricane poses when it reaches land is the potential for tornadoes. A tornado watch is usually issued when a hurricane comes ashore, especially for areas in the right front quadrant of the storm (see Figure 11.22a). Here, air moving from water to land experiences an increase in friction (because the land is rougher), causing surface winds to slow and thus cross the isobars at a larger angle (recall that as the wind slows, the magnitudes of both friction and the Coriolis effect are reduced, so the pressure gradient force has more of an upper hand, pushing parcels more directly towards lower pressure). Wind speeds and directions above the surface are less affected by friction, creating a zone of vertical wind shear that can spin up some F0 or F1 tornadoes.

About 20% of tornadoes spawned by a landfalling hurricane occur near the outer edge of the eye wall. The rest form in bands of thunderstorms that lie farther from the eye. Some of the spiral bands of Hurricane Allison, which made landfall near Apalachee Bay, FL in June 1995, spun up tornadoes across northern Florida and southern Georgia. Allison's penchant for spin is clearly evident in Figure 11.23. One of Allison's twisters seriously damaged an elementary school in Jacksonville, FL. Unlike "traditional" twisters that develop primarily in the heat of the afternoon and evening when severe thunderstorms are most common, there is no preferred time for the formation of tornadoes associated with a tropical storm or hurricane. These tornadoes typically develop when the storm makes landfall, whether it's day or night.

Hurricane Breakdown: The Engine Stops Running

The guidelines in the manual for the smooth ignition and operation of the heat engine of a hurricane are akin to the legs that support a table - if one is kicked out, the table almost always topples. When hurricanes die over land, their demise is often attributed to the greater roughness of the land which causes the winds to slow. Indeed, friction eventually helps to sap hurricane winds. But the primary reason that hurricanes dissipate over land or over higher latitude waters is that they are removed from their one true source of energy warm, tropical waters. The weakening effect that landfall had on Hurricane Bertha in July 1996 can be seen by comparing the two Doppler radar reflectivity images in Color Plate 40 and Color Plate 41, taken seven hours apart surrounding Bertha's landfall just east of Wilmington, NC. Although heavy rain persists in the later image, primarily north of the storm center, Bertha's circulation is clearly more ragged just a few hours after coming ashore.

Colder water takes an obvious toll by lowering evaporation rates and thus reducing the amount of available water vapor. A hurricane moving north up the East Coast of the United States over progressively cooler coastal waters must move quickly if it is to hit New England without being sapped of its full fury. A slow-moving storm inevitably weakens following such a path (provided it doesn't move over the relatively warm waters of the nearby Gulf Stream - then all bets are off!).

In the Pacific, relatively cool water south and east of Hawaii usually protects the Islands from the full fury of hurricanes. This cool pool of water owes its existence to the high rates of evaporation and low annual rainfalls in the latitudes of the subtropical highs. This combination makes surface water rather salty and relatively dense. Owing to its density, the saline surface water periodically sinks and cooler water from below rises to take its place. This cooler water then acts as an outer defense for the Islands. When storms approach the Islands from the east (guided by the Pacific subtropical high), they cannot avoid these cool seas and are gradually sapped of strength.

For a hurricane to deliver a devastating blow to Hawaii, it must be sneaky and fast. On occasion, while a hurricane is seemingly passing safely south of the Islands over warmer water, a trough of low pressure from higher latitudes can dip southward and latch onto the storm. In response to the relatively strong southerly and southwesterly steering winds ahead of the trough, the hurricane can turn and accelerate northward, its faster pace limiting the time spent over cool water. With most of its power intact, the storm can then deal Hawaii a big blow. Hurricane Iniki (1992) was such a storm, breaking through these outer defenses and striking the western part of the island of Kauai (see Color Plate 42 taken during landfall) with sustained winds of 210 km/hr (130 mph). Iniki was the worst hurricane to strike Hawaii this century. At the time, Steven Spielberg's Jurassic Park was being filmed on Kauai, and the production was halted by the dramatic actions of the hurricane.

Finally, wind shear can cause a hurricane's heat engine to sputter and stall, given that latent heat released in the upper part of the storm can become separated from the low-level circulation, disassembling the hurricane's engine. In late autumn, strengthening westerlies, in combination with cooling tropical seas, help to close the door on the Atlantic hurricane season. In November 1994, however, Hurricane Gordon challenged late-autumn odds and formed off the coast of Nicaragua (see Figure 11.24). Over the Caribbean, searching westerlies eventually found the storm and sheared off its top. Defiantly, the tropical storm regrouped near Key West, FL, cut across the Sunshine State, and then headed north off the Atlantic Coast. Gordon intensified into a hurricane as it passed over the warm waters of the Gulf Stream and approached Cape Hatteras on November 18 (see Figure 11.25), marking the closest approach by a late-season hurricane to the Outer Banks since December 2, 1925. Inevitably, the bullying winds of the mid- latitude jet stream forced Gordon to retreat and die, shearing off the tops of its thunderstorms. November is a hostile month for tropical storms and hurricanes.

Hurricanes certainly do not die gracefully. Though eventually downgraded to tropical depressions after striking land, hurricanes and tropical storms often remain dangerous. As tropical systems enter the alien world of the mid-latitudes, they are like cornered bulls, tormented by their extratropical surroundings. Depending on the nature of these surroundings, a hurricane sometimes charges, accelerating through the mid- latitudes and spreading strong winds forward. For example, Hurricane Hazel (1954) stampeded up the East Coast, with its horns lowered in a forward charge until mid-latitude conditions corralled its center of circulation near Toronto, Ontario. Other times, a hurricane can stop dead in its tracks, menacingly pawing its tropical rain over the same ground and goring a region with massive flooding. The moisture-laden remnants of Hurricane Camille (1969), which stalled over the central Appalachians, dumped as much as 63.5 cm (25 in) of rain, causing devastating floods that killed 150 people. Sometimes a tropical storm will interact with fronts in the mid-latitudes, as the remnants of Hurricane Agnes did in June 1972. The joining was catastrophic, leading to torrential rains that produced all-time record flooding in eastern Pennsylvania, Maryland and northeastern West Virginia (see Chapter 15 for more on Agnes).

Go to previous section: Hurricane Climatology: Engine Specs
Go to next section: Hurricane Andrew: A Case Study