Isabel as a Category 5 Hurricane in the mid-Atlantic on 9 September 2003.

Summer and fall bring hurricanes to the Atlantic Ocean, Caribbean, and Gulf of Mexico. Like angry apparitions, they ride the prevailing winds out of the tropical Atlantic toward North America. Many miss the continent and die in the cold over the North Atlantic, and a few arrive in Europe as soggy rainstorms. Still, during an average hurricane season one or two reach US shores. Over the last 30 years accurate warnings have reduced the number of people killed as hurricanes rearrange coastal neighborhoods. Nonetheless, each untimely death was a tragedy and the cost of ruined property has been astonishing.

Hurricanes are geographically smaller, but more intense, than the frontal cyclones that make midlatitude weather, and larger, but less intense, than the tornadoes that provide springtime excitement in the heartland. Hurricanes draw energy from the warm surface of the tropical ocean. Their motion is dominated by the large-scale "steering flow" that surrounds them, and an inherent tendency to drift slowly poleward and westward due to rotation of our spherical planet. Typically, the steering carries them westward in low latitudes, then northward, and finally northeastward in middle latitudes. Individual tracks are variable. The change in track from westward to northeastward is called "recurvature." It reflects the hurricane's migration from the subtropical, trade-wind easterlies (winds blowing from the east) into the mid-latitude westerlies. After hurricanes recurve, they usually pass over water too cold to sustain their intensity. If they run onshore, they die quickly, but often spread deluges of rain and flooding far inland before they finally expire.

Throughout spring and early summer, the sun shines on the tropical sea, raising temperatures over broad stretches to 27-29° C by early August. When the ocean is that hot, the process of bringing the atmosphere from the surface to the tropopause into thermodynamic equilibrium with the water can lower surface pressure by about 10%---equivalent to the pressure at the center of the strongest hurricane. The air above the tropical ocean is drier and cooler than it would be at thermodynamic equilibrium . Evaporation of seawater takes heat from the ocean and stores it in the air as "latent" heat. When the vapor condenses to form rain, the air warms as its latent heat becomes sensible heat-an increase in temperature.

In a hurricane, the strong winds blend warm, moist air in contact with the ocean upward shifting air within a few hundred meters of the surface toward equilibrium. Thus, hurricanes always form from preexisting disturbances, which start the heat transfer. Once the hurricane gets going, the energetic, near-surface air is carried upward by convective clouds (thunderstorms) that reach from the surface to the tropopause. The most intense convection lies in the eyewall, a ring of cumulonimbus clouds that encircles the cloud-free eye. It is primarily in the eyewall that condensation causes the heaviest rain and fuels the storm, driving the surface pressure lower.

To a first approximation, hurricanes are, round; the wind blows cyclonically (counterclockwise north of the equator) in circular orbits around the lowest pressure at the center of the eye. The low pressure tends to draw the air inward, providing the "centripetal" acceleration required for the air to orbit around the vortex center--much as gravity provides the centripetal acceleration that keeps satellites in their orbits. This relation between pressure and wind is called "cyclostrophic" balance---or more generally, when the rightward apparent force due to the Earth's rotation becomes an additional factor, "gradient" balance. Within a few hundred meters of the surface, friction between the wind and the waves slows the wind incrementally below the balanced speed. Thus the inward acceleration due to the pressure difference predominates, and the air spirals inward, gaining heat energy from the ocean to feed the convective clouds around the eye.

The strongest surface winds lie just at the outer edge of the cloud-free eye, where the inflow stops and turns upward to feed the convection. At the very center of the eye, the winds are calm, and at some distance outward, between 300 and 1000 km (200-600 miles), they merge into the large-scale surrounding flow.

On the right side of the storm, facing in the direction of vortex motion, the translation and circulating wind add. Mariners know this region as the "hazardous semicircle." On the left side, the translation subtracts from the circulation. Mariners know this region as the "navigable semicircle." When a hurricane strikes a north-south coast, the hazardous semicircle is north of the impact site, and the navigable semicircle is to the south. Off the US mid-Atlantic coast, many hurricanes move northward or northeastward parallel with the coast. Consequently, the navigable semicircle is over land until the storm encounters east-west tending peninsulas, like Cape Hatteras or Cape Cod, and the seaward end of these features experiences the fury of the hazardous semicircle.

By definition, winds in a hurricane must be greater than 65 nautical miles per hour (65 kt is 75 statute miles per hour, or 33.5 meters per second). Weaker cyclones with winds stronger than 35 kt (gale force, 40 mph, or 18 m/s) are called Tropical Storms; whereas still weaker cyclones in which the wind still blows around a well-defined center are called Tropical Depressions. Finally the weakest members of the family are Tropical Disturbances which lack a closed wind circulation altogether.

Damage from Hurricane Andrew, August 1992 in southern Miami-Dade County, Florida

In 1974, Herbert Saffir, a distinguished structural engineer, and Robert Simpson, who was then Director of the National Hurricane Center, proposed the Saffir-Simpson Scale for ranking hurricanes by maximum wind. On this scale, Category 1 is barely a hurricane and Category 5 is the worst imaginable. Only three Category 5 hurricanes, are recorded to have struck the United States(the Florida Keys Labor Day Hurricane of 1935, Camille of 1969, and Andrew of 1992). The total damage caused by Category 5 hurricanes is only a third of the total damage due to Category 4 hurricanes, because a total of nine Category 4 hurricanes have struck the US-many where expensive real estate happened to be in the way.

An average hurricane season (June through November) produces nine tropical storms and six hurricanes, three of which become "major" or "intense" hurricanes in Saffir-Simpson Categories 3, 4, and 5, with winds stronger than 96 kt (111 mph or 50 m/s). Variability of hurricane occurrence can be extreme. The greatest recorded number of hurricanes (tropical storms plus hurricanes) was 12 in 1969 (19 in 1995) and the least number of hurricanes was 2 in 1982 (4 in 1983). Records from 1933 indicate 21 tropical storms and hurricanes, but data from before 1944 (when aircraft reconnaissance began) are considered unreliable. The largest recorded number of major hurricanes was 7 in 1950. Atlantic hurricanes have been recorded in every calendar month but February.

For a hurricane to form the ocean must be warmer than 26° C (79° F) and the horizontal wind must not change too much with height (typically 12 m/s from the surface to 14 km altitude). Worldwide tropical cyclones are not observed to form within 2.5° of the Equator. Regardless of where tropical cyclones form, they require a preexisting disturbance. In most of the world, the monsoon trough, a band of low pressure between the northern and southern hemisphere trade winds, provides the spawning ground. In the Atlanic however "easterly waves" that form along the boundary between the searingly hot air of the the deserts of North Africa and the humid, but not quite so hot, air of the savannahs to the south provide most of the hurricane seedlings. One of these waves crosses from Africa to the Atlantic every four days on average from late spring though fall. Most dissipate of pass over Central America into the Pacific Ocean, but about one in ten finds conditions just right to become a tropical storm and perhaps a hurricane.

Most hurricanes intensify gradually before they recurve or make landfall. The average hurricane intensity is Category 1. It is the occasional hurricane that outperforms the average that makes the history books. These overachieving storms both become spectacularly intense and reach the potential very quickly. A process that meteorologists call "rapid deepening," which can transform a hurricane from Category 1 or 2 to Category 4 or 5 overnight, appears to be the only way that nature makes a major hurricane in the Atlantic. Rapid deepening, which is generally not well forecast, is a problem both because it causes the most destructive hurricanes and because its pace can outrun the 6 to 12 h forecast cycle.

The last sweep of the Miami National Weather Service Radar before the winds of Hurricane Andrew blew it from the roof of the National Hurricane Center building in Coral Gables Florida, 24 August 1992.

In contrast with the difficulties posed by intensity, forecasts of the hurricanes path, "track," have improved steadily by one or two percent a year since meteorologists began keeping score in the early 1950s. This pace sounds modest, but it adds up. At the crucial 24-h time horizon that triggers evacuations, the average forecast error decreased from 120 nautical miles to 85 between 1970 and the turn of the 21st Century. Since the error increases more-or-less linearly with lead-time, errors are typically about 170 nautical miles at 48 h lead time and 250 nautical miles at 72 h.

Track forecast uncertainties are not an academic exercise; they translate into people displaced from their homes and businesses closed unnecessarily. The figure "a million dollars a mile" is often cited as the cost of raising warnings on the US Atlantic or Gulf coast. The provenance of this number is somewhere between questionable and disreputable, and the actual cost depends upon what mile is being warned. Nonetheless, it is clear that hurricane-warning costs steal a multiple of $100M from the economy every year, even before hurricane winds blow the first shingle from the first roof. Weighed against the financial cost of warnings is demonstrable saving of an average of 200 lives annually through evacuation. In 1950, hurricanes claimed an average of 67 lives a year. In 2000, they claimed an average of 20. The coastal population has increased by 314%. If the forecasting enterprise functioned now as it did a half century ago hurricanes would kill 210 people in the US. While assigning economic value to saved lives is problematic, the conventional valuation gives savings of about a billion dollars, a figure somewhere between two and three times the cost of posting warnings and responding to them.

Low hurricane mortality is no reason for complacency. The record is only as good as the next landfall forecast made in the face of dynamically chaotic steering flow and the possibility of rapid deepening in the last few hours before landfall some summer night. Still, the evident challenge is to redesign human institutions and their built environments to survive-even prosper-where land and civilization on meet the sea and the tempest.

Inside the eye of Hurricane Georges, seen from a NOAA WP-3D research aircraft on 19 September 1998.