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What is a Category 5 Hurricane?

Can hurricanes be weakened using oil slicks or other techniques?

Can hurricanes be weakened using oil slicks or other techniques?
Rhett A. Butler,
September 21, 2005

Hurricane Katrina was the most expensive natural disaster in the history of the United States. Hurricane Rita threatens to add to the 2005 hurricane season’s toll. Is there anything that can be done about these deadly and destructive storms? The answer is someday there may be ways to reduce the intensity of these tropical storms but in the meantime, the best option is to avoid new construction in hurricane-prone regions.

In the 1960s and 1970s, the United States government attempted to weaken hurricanes by seeding storms with silver iodide. The experiment, called Project Stormfury, sought to test the theory that the seeding would augment precipitation outside the storm’s eyewall, causing it to collapse and thus reduce the winds. The experiment proved inconclusive but it seems unlikely that such a mechanism could significantly impact the strength of a large hurricane.

Today researchers are experimenting with other approaches to moderate hurricanes and tropical storms. Some meteorologists believe that small changes in the temperature in and around a hurricane can shift its path or disrupt its intensity.

Hurricane Formation
from Wikipedia

In meteorology, a tropical cyclone (or tropical disturbance, tropical depression, tropical storm, typhoon, or hurricane, depending on strength and geographical context) is a type of low pressure system which generally forms in the tropics.

Structurally, a tropical cyclone is a large, rotating system of clouds, wind and thunderstorm activity. The primary energy source of a tropical cyclone is the release of the heat of condensation from water vapor condensing at high altitudes. Because of this, a tropical cyclone can be thought of as a giant vertical heat engine.

The ingredients for a tropical cyclone include a pre-existing weather disturbance, warm tropical oceans, moisture, and relatively light winds aloft. If the right conditions persist long enough, they can combine to produce the violent winds, incredible waves, torrential rains, and floods associated with this phenomenon.

Condensation as a driving force is the primary difference which distinguishes tropical cyclones from other meteorological phenomena. Mid-latitude cyclones, for example, draw their energy mostly from pre-existing temperature gradients in the atmosphere. In order to continue to drive its heat engine, a tropical cyclone must remain over warm water, which provides the atmospheric moisture needed. The evaporation of this moisture is driven by the high winds and reduced atmospheric pressure present in the storm, resulting in a sustaining cycle. As a result, when a tropical cyclone passes over land, its strength will diminish rapidly.

Five factors are necessary to make tropical cyclone formation possible:
  1. Sea surface temperatures above 26.5 degrees Celsius to at least a depth of 50 meters. Warm waters are the energy source for tropical cyclones. When these storms move over land or cooler areas of water they weaken rapidly.
  2. Upper level conditions must be conducive to thunderstorm formation. Temperatures in the atmosphere must decrease quickly with height, and the mid-troposphere must be relatively moist.
  3. A pre-existing weather disturbance. This is most frequently provided by tropical waves—non-rotating areas of thunderstorms that move through the world’s tropical oceans.
  4. A distance of approximately 10 degrees or more from the equator, so that the Coriolis effect is strong enough to initiate the cyclone’s rotation.
  5. Lack of vertical wind shear (change in wind velocity over height). High levels of wind shear can break apart the vertical structure of a tropical cyclone.

Tropical storm and hurricane season peaks from August through October. Nearly all tropical cyclones form within 30 degrees of the equator and 87% form within 20 degrees of it.

Scientists at the Atmospheric and Environmental Research (AER), an R&D consulting firm, are modeling hurricanes to see what effect atmospheric heating might have on their path and strength. In an article in the September 27th, 2004 issue of Scientific American, Ross N. Hoffman of AER suggests that an array of earth-orbiting solar power stations could eventually be used to supply sufficient energy for the disruption of hurricanes. By heating an area of ocean, scientists in the future may be theoretically able to “steer” a hurricane off its projected path if it threatens population centers. Hoffman concedes though that the amount of energy needed to achieve such an objective would be substantial.

Since hurricanes draw their power from evaporating sea water, cutting off the supply of warm water available to them could reduce their strength. Operating on this premise, Hoffman suggests another tactic involving the application of “a thin film of a biodegradable oil” to slow the evaporation that serves as the fuel for a hurricane. A great deal more research is needed to determine where such a strategy is feasible especially with a large hurricane that may cover hundreds of square miles of ocean surface.

Other approaches involving the towing of icebergs or detonation of nuclear weapons pose more risk that potential meteorological protection.

Meteorological control raises issues

While technical solutions to such weather phenomena may someday be possible, there are a host of issues that must be resolved. For example, who would determine what tropical storms and hurricanes would be targeted? Each year there are several dozen weather disturbances, of which only five or so actually become hurricanes. Further, “what if intervention causes a hurricane to damage another country’s territory?” asks Hoffman. Or, what if countries start to use weather as a weapon of mass destruction?

At the moment these questions are somewhat academic. For now the focus should be on the wisdom of continuing to build on lands that are highly susceptible to hurricane damage. As the oceans warm, hurricanes are only going to get stronger and pose an ever greater threat to low-lying cities and coastal areas.

Can hurricanes be stopped? Related articles:

Could a hurricane hit California? – 20-August-2007
San Diego has been hit by hurricanes in the past and could be affected by such storms in the future according to data from the National Oceanic and Atmospheric Administration (NOAA). While a hurricane in San Diego would likely produce significantly less damage than Hurricane Katrina in New Orleans, it could still exact a high cost to Southern California especially if the region was caught off guard.

Nuclear war could cause global cooling (i.e. block global warming) 11-December-2006
Nuclear war would disrupt global climate for at least a decade according to new research presented Dec. 11 at the annual meeting of American Geophysical Union in San Francisco. The research, based on findings from historic volcano eruptions, found that a small-scale, regional nuclear war could produce millions of tons of “soot” particles that could block solar radiation, in effect, cooling the planet.

What is a Category 5 Hurricane? – 21-September-2005
Hurricane Rita just strengthened to a Category 5 hurricane. A Category 5 hurricane is the strongest and most severe class of hurricane. The scale, known as the Saffir-Simpson Hurricane Scale, classifies hurricanes by the intensity of their sustained winds, storm surge and flooding, developed in 1969 by civil engineer Herbert Saffir and National Hurricane Center director Bob Simpson.

Tampa Bay could be hit by 25-foot storm surge in Category 4 hurricane – 16-September-2005
A Category 4 hurricane could cause a storm surge of as much as 25 feet in Tampa Bay, according to a University of Central Florida researcher who is looking at the risks Florida cities face from tidal surges and flooding.

Number of Category 4 and 5 hurricanes has nearly doubled over past 35 years – 16-September-2005
The number of Category 4 and 5 hurricanes worldwide has nearly doubled over the past 35 years, even though the total number of hurricanes has dropped since the 1990s, according to a study by researchers at the Georgia Institute of Technology and the National Center for Atmospheric Research (NCAR). The shift occurred as global sea surface temperatures have increased over the same period. The research appears in the September 16 issue of Science.

Environmental problems worsened Hurricane Katrina’s impact – 31-August-2005
The loss of coastal marshlands that buffer New Orleans from flooding and storm surges may have worsened the impact of Hurricane Katrina.

Hurricanes getting stronger due to global warming says study – 29-August-2005
Late last month an atmospheric scientist at Massachusetts Institute of Technology released a study in Nature that found hurricanes have grown significantly more powerful and destructive over the past three decades. Kerry Emanuel, the author of the study, warns that since hurricanes depend on warm water to form and build, global climate change might increase the effect of hurricanes still further in coming years.

Further reading

This article used information from Wikipedia, the National Hurricane Center, and Scientific American.

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