Courtesy of NOAA

Impacts

The simplest characterization of hurricane intensity is embodied in the Saffir-Simpson scale: from Category 1 - barely a hurricane - to Category 5 - the worst imaginable. "Major Hurricanes" are those in Categories 3, 4, and 5 with winds stronger than 110 miles per hour equivalent to 100 kt or 50 m s-1. Category 5 hurricanes are the most extreme and also the most rare. Only two, the 1935 Labor Day Storm and Camille in 1969 are recorded to have struck the United States. Andrew, at the very top of Category 4 was the third strongest U.S. landfall, and the second strongest on the mainland, given that the 1935 storm hit the Florida Keys.

In the 20th century, U.S. hurricanes destroyed > $73 billion in property, not corrected for inflation. During the 70-year period from 1925 through 1995, the toll was $61 billion. If the damage from historical hurricanes is normalized for inflation, increased population, and greater individual wealth (Pielke and C. Landsea 1998), the estimate of total damage for the shorter period is $340 billion, equivalent to an average annual loss of $5 billion. During these 70 years, 244 landfalls occurred. The average landfall would have resulted in $1.5 billion in damage with today's prices and costal development. But the average doesn't tell the story. Major hurricanes accounted for 80% of the normalized damage, although they represented only 20% of occurrence.

NOAA Some of the most damaging major hurricanes are predicted to occur over the next two decades.

The 1995 through 1999 seasons inclusive have been the five most active in the > 100-year quantitative climatology. Historically, hurricane landfalls on the U.S. east coast were common during the 1940s through the mid 1960s. In the 1970s and 1980s, landfalls were few. Now activity appears to have returned to the high level that characterized the immediate post-World War II period. These fluctuations in activity are most pronounced for major hurricanes. They also correlate with the observed "North Atlantic Mode", a coherent, multidecadal fluctuation of global sea-surface temperatures. During the active portion of the long-term record, Atlantic Sea-Surface Temperature (SST) anomalies in tropics and high latitudes were warm, and conversely. If the hurricane climatology and the Multi-Decadal Mode prove to be reliable guides, we may expect the first decade or two of the 21st Century to produce as many of the most damaging major hurricanes annually as the last 5 years have.

In terms of hurricane-related mortality, the 20th Century started badly. In Galveston Texas, on a single windy Saturday night, 9 September 1900, the "Great Hurricane" washed > 6,000 souls to their deaths. The total mortality for the century was just a bit more than twice this figure, 13,306 U.S. residents. During the first three decades of the century, the average annual loss of life was 329, or discounting the Galveston tragedy, 129. In the forty years from 1930 through 1969, it was 70. Since 1969, the average annual loss of life has been < 20, notwithstanding a 10-fold increase in coastal population from 1930.

The reason for the dramatic reduction has been effective warnings and timely evacuation from coastal areas inundated by storm surge. Invariably, large loss of life in hurricanes before 1970 stemmed from wind-driven flooding. Since 1970 drowning from inland flooding caused by torrential hurricane rains has come to predominate. Experience shows that when storm surge (or wind for that matter) completely flattens buildings, about 10% of the people present die. Evacuation insures that virtually nobody is present. On the other hand, extreme ( > 30 cm in 6 hr) rainfall places a much larger population at individually smaller risk. For example, Hurricane Floyd, the deadliest U.S. hurricane since Agnes of 1973, killed 49 of the more than 5 million people that it affected in the North Carolina Coastal Plain and Piedmont, for an average risk of dying of less than one in a hundred thousand. People in the developing world, who generally do not have the benefit from farsighted land-use policies or effective building standards, are at greater risk. The tragedy of Hurricane Mitch of 1998, which took 10,000 lives in Honduras, demonstrates that among the many bad consequences of poverty is vulnerability to natural disaster. How do you find an extra-solar planet?

Dynamics

NOAA The power of a hurricane has caused a piece of debris to slice through a tree.

Hurricanes are nearly circular, warm-core vortices, characteristically 1000 km in radius. Strong winds and precipitation are concentrated near, but not at, their centers. Coincident with strongest winds, convective updrafts, fed by inward spiraling surface winds, release latent heat drawn from the ocean to power the storm. The clear eye, 15-30 km in radius, contains the axis of vortex rotation and is surrounded by this ring of strong winds. The eye is characterized by the warmest temperatures in the vortex, low humidity, low pressure, in extreme cases > 10% below that in the undisturbed tropical atmosphere, and calm winds at the very center. The hurricane's eye presents one of the truly magnificent vistas offered to human sight, and the hurricane's workings delight the mind as an intricate and subtle problem in hydrodynamics.

In the center of the hurricane is the cloud-free eye. The clouds that enclose the eye form the eyewall. These clouds draw air from the eye at low levels, causing descent, drying, and warming inside the eye. As the air warms, it becomes less dense so surface pressure must fall. The clouds also draw air inward from outside the eye, thus concentrating the counter-clockwise rotation and increasing the swirling winds. In a hurricane, the strongest winds are near the surface and just outside the eyewall. An unanticipated use of the new GPS sondes was direct observation of low-level wind jets at the inner edge of the eyewall.   

A typical hurricane intensifies slowly, remaining in Category 1 or reaching 2 or even 3 before it runs ashore or drifts north out of the tropics. The strongest hurricanes, such as Andrew in 1992, intensify rapidly and go from Category 1 or 2 to Category 4 or 5 in just a day or two. The process that initiates rapid intensification may begin with atmospheric waves that form on the hurricane vortex at altitudes of greater than 12 kilometers (where the new jet will fly). These waves result from interaction with nearby low-pressure systems. The waves, through a complicated chain of cause and effect, intensify the cumulus clouds that are involved in the energy conversion mechanisms of a hurricane.

Modification?

NOAA It is unlikely unlikely that human intervention will ever prevent hurricanes.

Can human intervention diminish the force of a hurricane? From the mid-1960s through the early 1980s NOAA actively pursued Project STORMFURY, a program of experimental hurricane modification. The general strategy was to reduce the intensity of the storm by cloud seeding. The seeding, it was argued, would stimulate the formation of a new eyewall that would surround the existing eyewall. The new eyewall would contract, strangling the old eyewall and reducing the intensity of the hurricane. However, research carried out at AOML showed clearly that these "concentric eyewalls" happened often in unmodified hurricanes, thus casting doubt on the seemingly positive results of seeding in earlier experimentation. Hurricane Luis provides an example of this behavior. Moreover, observations showed that hurricanes contain little of the supercooled water necessary for cloud seeding to work.

The American Meteorological Society policy statement on planned and inadvertent weather modification, dated October 2, 1998, indicates, "There is no sound physical hypothesis for the modification of hurricanes, tornadoes, or damaging winds in general, and no related scientific experimentation has been conducted in the past 20 years." In the absence of a sound hypothesis, no Federal agencies are presently doing, or planning, research on hurricane modification.

During the 40 years between 1930 and 1959, inclusive, 70 people a year died in the U.S. hurricanes. Since 1950, the midpoint of this interval, population in the 109 coastal counties between Texas and Virginia had increased by a factor of 3.4. If the forecaster's art had not improved during the last century, expected annual mortality to this larger population from hurricanes would be 238, with an economic impact of $1785M. By this estimate the stakes of the annual hurricane game are $8.9B, of which $2.8B, or 31%, is prevented through an investment of < $0.22B in forecasting and research, for a greater than twelve to one favorable benefit to cost ratio. The prevented impact is dominated by reduced mortality.