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How Hurricanes Work
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Hurricane Dangers

Degrees of possible disaster
The Saffir-Simpson Hurricane Scale rates each storm on a scale of 1-to-5. The scale is based primarily on the strength of the winds. This scale gives an idea of the damage and flooding islanders can expect when a hurricane hits. See further below for a description of each category.

Category Winds* (km per hour) Severity Storm Surge
Tropical Storm

Less than 118 kph


very weak
0.61-1.21 m
119-153 kph
1.22-1.81 m
154-177 kph
1.82-2.72 m
178-209 kph
2.74-3.99 m
210-249 kph
very strong
4-5.50 m
above 249 kph
more than 5.50 m
*Note: Most weather stations and meteorologists measure wind speed in "knots". Popular media converts this to "kilometers per hour", or "miles per hour."

"Hurricane Ismael's winds have been clocked at 200 kph, making it a Category 3 hurricane. It is going to cross your island."

This announcement might make a great start for an exciting computer game or a white-knuckle, action movie. However, it is a grim reality every year to millions of people living in the tropics. The devastation is caused in a variety of ways.

Winds. A photograph taken after one hurricane showed a wood plank that was seized by the wind and driven through a palm tree as if the plank were a well-sharpened knife. Other photographs showed large sugar mills reduced to rubble; hamlets of wood and thatch homes erased from the face of the planet; telephone and electrical cables balled up in masses of tangled wires; plantations of bananas and lime trees destroyed. The winds move through with such force that pieces of wood, sections of zinc roofs, flowerpots, lawn chairs, and tricycles become deadly projectiles.

Rains. More rain can fall on one mountainside during a seven-hour hurricane — 635 mm. is not a record-breaking amount — than falls on Southern California in an average year! The waters in streams and on coastal flood plains can grow up to ten times their normal size. As they tumble to the sea, they'll sweep people, cars, and buildings with them as if they were mere toys. The rains also trigger massive landslides of super-saturated soil down the steep slopes, which are common on many of the volcanically-formed Caribbean islands.

Waves. Mariners once recorded hurricane waves that dragged a ship with a buoy, chain, and anchor weighing more than 13,600 kg, for 16km! Imagine what these waves can do when they hit a low-lying coastal community. Tidal surges temporarily raise the local sea level and submerge entire coastal areas. Tidal surges cause the greatest number of hurricane deaths.

While all this is happening, rescuers can do nothing but sit and wait for the hurricane to pass, for it is far too dangerous to venture out. When the hurricane does finally pass, roads are often impassable, lines of communication destroyed. Rescue efforts are extremely difficult.

Hurricane Watch

A hurricane watch is issued when there is a threat of hurricane conditions within 24 or 36 hours.

What should I do?

Everyone in the area covered by the "watch" should listen to their radios and television for weather updates and be prepared to act promptly if a hurricane warning is issued. Prepare to either bring in or place in a secure location any lawn furniture, outdoor decorations, trash cans, hanging plants, and anything else that can be picked up by the wind. You should also cover all windows of your home. Also, prepare a Disaster Safety & Supply Kit.

Hurricane Warning

A hurricane warning is issued when hurricane winds reach 74 miles an hour or higher, or a combination of dangerously high water and rough sea conditions are expected in 24 hours or less.

What should I do?

Everyone in the area covered by the "warning" should take precautionary actions to prepare for the full force of the hurricane. You should listen to local officials and leave if they tell you to do so. If you are not advised to evacuate, stay indoors and away from windows. You should also be alert of tornadoes and remain indoors, in the center of your home, in a closet or bathroom without windows

Tropical Systems Terms
Tropical disturbance, tropical wave: Unorganized mass of thunderstorms, very little, if any, organized wind circulation.

Tropical depression: Evidence of closed wind circulation around a center with sustained winds from 37-63 kph.

Tropical storm: Maximum sustained winds are from 64-118 kph. The storm is named once it reaches tropical storm strength.

Hurricane: Maximum sustained winds exceed 119 kph.

Saffir Simpson Hurricane Intensity Scale

Category One - A Minimal Hurricane

Winds: 119-153 kph, 64-83 kts.
Minimum surface pressure: higher than 980 mbar.
Storm surge: 1.22 - 1.81m.
Damage primarily to shrubbery, trees, foliage, and unanchored homes. No real damage to other structures. Some damage to poorly constructed signs. Low-lying coastal roads inundated, minor pier damage, some small craft in exposed anchorage torn from moorings. Example: Hurricane Jerry (1989)

Category Two - A Moderate Hurricane

Winds: 154-177 kph, 84-96 kts.
Minimum surface pressure: 979-965 mbar.
Storm surge: 1.82 - 2.72 m.
Considerable damage to shrubbery and tree foliage; some trees blown down. Major damage to exposed mobile homes. Extensive damage to poorly constructed signs. Some damage to roofing materials of buildings; some window and door damage. No major damage to buildings. Coast roads and low-lying escape routes inland cut by rising water 2 to 4 hours before arrival of hurricane center. Considerable damage to piers. Marinas flooded. Small craft in unprotected anchorages torn from moorings. Evacuation of some shoreline residences and low-lying areas required. Example: Hurricane Juan (2003)

Category Three - An Extensive Hurricane

Winds: 178-209 kph, 97-113 kts.
Minimum surface pressure: 964-945 mbar.
Storm surge: 2.74 - 3.99 m.
Foliage torn from trees; large trees blown down. Practically all poorly constructed signs blown down. Some damage to roofing materials of buildings; some wind and door damage. Some structural damage to small buildings. Mobile homes destroyed. Serious flooding at coast and many smaller structures near coast destroyed; larger structures near coast damaged by battering waves and floating debris. Low-lying escape routes inland cut by rising water 3 to 5 hours before hurricane center arrives. Flat terrain 1.5 meter or less above sea level flooded inland 13 km. or more. Evacuation of lowlying residences within several blocks of shoreline possibly required. Example: Hurricane Gloria (1985)

Category Four - An Extreme Hurricane

Winds 210-249 kph, 114-135 kts.
Minimum surface pressure: 944-920 mbar
Storm surge: 4 - 5.50 m.
Shrubs and trees blown down; all signs down. Extensive damage to roofing materials, windows and doors. Complete failures of roofs on many small residences. Complete destruction of mobile homes. Flat terrain 3 meter or less above sea level flooded inland as far as 10 km. Major damage to lower floors of structures near shore due to flooding and battering by waves and floating debris. Low-lying escape routes inland cut by rising water 3 to 5 hours before hurricane center arrives. Major erosion of beaches. Massive evacuation of all residences within 500 meters of shore possibly required, and of single story residences within 3.2 km. of shore. Example: Hurricane Andrew (1992)

Category Five - A Catastrophic Hurricane

Winds: greater than 250 kph, 135 kts.
Minimum surface pressure: lower than 920 mbar
Storm surge: higher than 5.5 m
Shrubs and trees blown down; considerable damage to roofs of buildings; all signs down. Very severe and extensive damage to windows and doors. Complete failure of roofs on many residences and industrial buildings. Extensive shattering of glass in windows and doors. Some complete building failures. Small buildings overturned or blown away. Complete destruction of mobile homes. Major damage to lower floors of all structures less than 4.6 meters above sea level within 500 meters of shore. Low-lying escape routes inland cut by rising water 3 to 5 hours before hurricane center arrives. Massive evacuation of residential areas on low ground within 8 to 16 km. of shore possibly required. Example: Hurricane Camille (1969)

Review Questions

  1. What type of damage would you expect to find in a Category 5 hurricane?
  2. Identify three causes of devastation during a hurricane.
  3. A hurricane hits a nearby island. Damage is found to shrubbery and trees, but most homes and buildings are not affected. Minor damage was done to piers and small boats. None of the roads on the island flooded. The above description is most likely of a Category _________ hurricane.