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FREQUENTLY ASKED QUESTIONS ABOUT HURRICANES AND STORMS

The Hurricane Risk of the Hawaiian Islands

George Pararas-Carayannis

How Frequent are Hurricanes?

Fortunately, major destructive hurricanes are relatively rare events at any location.

What is a Tropical Depression?

Tropical cyclone is the general term that descrives a low pressure system that originates over the tropical oceans. By international agreement, tropical cyclones are classified according to their intensity. Tropical Depression is an area of developing counterclockwise (in the northern hemisphere, clockwise in the southern hemisphere) wind circulation that may include localized rain and thunderstorms. Maximum sustained winds up to 38 MPH (33 Knots). Tropical Storm: A well defined area of counterclockwise rotating wind of 39-73 MPH (34-63 Knots). Usually includes rain and thunderstorms. It is assigned a name.

How are Tropical Storms created?

The great storms are driven by the heat released by condesing water vapor, and by external mechanical forces. Once cut off from the warm ocean, the storm begins to die, starved for water and heat energy, and dragged apart by friction as it moves over the land.

What are Hurricanes?

Hurricanes are severe tropical cyclones with winds reaching sustained speeds of 74 miles per hour or more. Hurricane winds spiraling toward a a relatively calm center or eye of low pressure at speeds which may reach more than 150 miles per hour (130 knots). Near the center, hurricane winds may gust to more than 200 miles per hour. Although usually erratic and unpredictable, hurricanes generally follow a westerly to northwesterly path. In the Atlantic, they move toward the Gulf of Mexico or the Eastern U.S. coast causing abnormal water level fluctuations known as hurricane surges. In the Pacific, hurricanes or cyclones (as often called) generally follow the same westerly to northwesterly path. Major hurricane hazards inclulde high winds, heavy rainfall, flooding, storm surge and high surf. If a hurricane has developed from a tropical storm, it retains the same name as the storm. Every year, these violent storms bring destruction toc oastlines and islands in their erratic path.

Stated very simply, hurricanes are giant whirlwinds in which air moves in a large tightening spiral around a center of extreme low pressure, reaching maximum velocity in a circular band extending outward 20 or 30 miles from the rim of the eye. This is called the area of maximum winds. This ci rculation is counterclockwise in the Northern Hemisphere, and clockwise in the Southern Hemisphere. The entire storm dominates the ocean surface and lower atmosphere over tens of thousands of square miles.

The storms move forward very slowly in the tropics, and may remain almost stationary for short periods of time.The initial forward speed is usually 15 miles per hour or less. This is called the translational speed of the hurricane. Then, as the hurricane moves farther from the Equator, its forward speed tends to increase; at middle latitudes it may exceed 50 miles per hour in extreme cases.

NOAA GRAPHIC OF FOUR HURRICANES IN 1998

BE PREPARED

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What makes Hurricanes dangerous?

What makes hurricanes the dangerous storms they are is that they combine the triple hazard of violent winds, torrential rains, and abnormally high waves and storm tides. Each of these by itself can pose a serious threat to life and property. Taken together they are capable of causing widespread destruction. For example, the hurricane that hit Galveston in 1900 resulted in 6,000 deaths and the alsmost complete destruction of the city. Hurricane Camille, which struck the Mississippi coast in 1969, killed 262 persons and caused damages of nearly $1 billion. Recent hurricanes have caused similar damages, but fortunately with a reduced toll on lives, as Warning systems have been implemented.

What is the eye of a Hurricane?

The eye, like the spiral structure of the storm, is unique to hurricanes. Here, winds are light and skies are clear or partly cloudy. But this calm is deceptive, bordered as it is by maximum force winds and torrential rains. Many persons have been killed or injured when the calm eye lured them out of shelter, only to be caught in the maximum winds at the far side of the eye, where the winds blow from a direction opposite to that in the leading half of the storm.

Hurricane Hugo

 

What is the Hurricane Surge?

Hurricane or storm surge is an oceanographic phenomenon of water level fluctuations caused by the atmospheric pressure field and wind stress on the water surface, accompanying the moving hurricane or storm systems. Specific factors which can combine to produce extreme water fluctuations at a coast during the passage of a storm or a hurricane include: storm intensity, size, path, and duration over water; atmospheric pressure variation; speed of translation; winds and rainfall; bathymetry of the offshore region; astronomical tides; initial water level rise; surface waves and associated wave setup and runup.

Hurricane Surge constitutes a greater hazard to lives and coastal property than hurricane winds. Hurricane surges have been estimated to account for 75 to 90 percent of all deaths resulting from a hurricane. Surge inundation is also responsible for extensive damage to coastal property. Since 1900, hurricane damages to coastal property have averaged more than $50 million per year. The per year average has been far greater for the last twenty years.

NOAA GRAPHIC

Are Hurricanes assigned magnitudes?

Yes. Hurricanes are categorized from1 through 5, by the Saffir/Simpson Scale. These magnitudes are assigned in accordance to the amount of potential damage and wind speed.

Description of Damage Wind Speeds (MPH) Storm Surge (feet) Examples

Category 1 : Minimal Damage, Wind speeds of 74 - 95 (MPH), Storm Surge 4 - 5 (feet)

Examples: Hurricane Iwa (Hawaii) , Winds 92 MPH, Nov. 1982

Category 2 : Moderate Damage, Wind speeds 96 - 110 (MPH), Storm surge 6 - 8 (feet). None

Category 3 : Extensive Damage, Winds 110 - 130 (MPH). Storm surge 9 - 12 (feet).

Example: Hurricane Uleki, 128 MPH, Sep. 1992

Category 4 : Extreme damage, Wind speeds 131 - 155 (MPH), Storm surge 13 - 18 (feet),

Example: Horricane Iniki, 145 MPH, Sep. 1992

Category 5 : Catastrophic Wind speeds greater than 155 MPH, Storm surge 18 or more feet.

Examples: Hurricanes Emilia & Gilma, 161 MPH, Jul 94, Hurricane John,/173 MPH Aug. 1994

NOAA GRAPHIC

What is the Hurricane and Tropical Storm risks for Hawaii?

In Hawaii, hurricane winds, especially where augmented by local terrain, have been very damaging to trees, vegetation, and crops, as well as to lightly built dwellings and other structures. Heavy and prolonged hurricane rains falling over steep hillsides can cause landslides and severe flash flooding. Large swell moving out ahead of the hurricane may begin to reach island shores while the storm itself is still several hundred miles away. As the hurricane nears the coastline, rapidly rising water levels from above-normal storm tides and high wind-driven waves will inundate coastal areas, erode beaches, and pound and undermine waterfront structures, highways, and other facilities.

During the last 50 years many hurricanes and tropical storms have come close to the Hawaiian Islands, but only three have had direct impact. In all three cases, Kauai was the hardest hit, although Oahu suffered significant damages as well. Hurricane Iniki was by far the most destructive storm to strike Hawaii in recorded history, with widespread wind and water damage exceeding 2.2 billion dollars. Losses in Hurricane Dot, August of 1959 were about 6 milliion dollars. Hurricane Iwa, in Novenber of 1982 caused over 250 million dollars in damages.

Other hurricanes have occasionally come close enough to cause relatively minor damage, mainly in coastal areas vulnerable to high waves. Thus, Hurricane NINA, in late November 1957, brought surf of 35 feet to Kauai's southern coast, while waves from Hurricane FICO in July 1978 , damaged homes and roads on the Big Island's Ka'u coast when the storm itself was more than 400 miles to the southeast.

Tropical cyclones of less than hurricane strength also have been destructive. For example, in August 1958, flooding rains and high winds from a storm that crossed Hawaii Island caused more than $500 thousand in damage. Most Central Pacific hurricanes originate near the coasts of Central America or southern Mexico. Long before reaching the Hawaiian area, however, many of these storms die off when they move northwestward over cooler water or encounter unfavorable atmo spheric conditions. Of those that survive, most remain far enough away to spare Hawaii their effects. Some hurricanes form nearer the Hawaiian Islands, while a few, like NINA and IWA, orginate far to the southwest.

What is the Hurricane Season for the Hawaiian Islands?

Hurricane season begins in June and lasts through November in the Hawaiian Islands. In some hurricane seasons, many Central Pacific tropical cyclones occur; in others, few or none. In 1978, for example, there were 13, three of them full-fledged hurricanes, while the following year had none. There is no way of telling in advance how active a hurricane season is likely to be. Hurricanes begin as relatively small tropical cyclones, generally off the southwest coast of Mexico or west coast of Central America. Some have, however, slowly formed sout of the state of Hawaii. They then drift to the west-northwest, imbedded in the westward-blowing tradewinds of the tropics. Under certain conditions these disturbances increase in size, speed, and intensity until they become full-fledged hurricanes.

BE PREPARED

Diesel Service & Supply offers a large inventory of backup generators to provide standby power to your business during a hurricane.

(Sources of Information)

1. "Verification Study of a Bathystrophic Strom Surge Model" by George Pararas-Carayannis, Technical Memorandum No. 50, U.S. Army, Corps of Emgineers, Coastal Engineering Research Center, May 1975, a study supported by the U.S. Atomic Energy Commission (AEC)(now, U.S. Nuclear Regulatory Commission)

2. Pamphlets published by the Federal Emergency Management Agency, Hawaii State Civil Defense, and National Oceanic and Atmospheric Administration (NOAA).

3. NOAA Images

See also

Hurricane Surge Prediction - Understanding the Destructive Flooding Associated with Hurricanes

Hurricane Iniki in Hawaii- September 11, 1992

The Bathystrophic Hurricane Surge Model

Tracks of major Destructive Historical Hurricanes (under construction)

Hurricane Imagery - Tropical Cyclones

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