Hurricanes are intense tropical cyclones (storms) that form in the central Atlantic Ocean, Caribbean Sea, Gulf of Mexico or in the eastern Pacific Ocean. Cyclones occur in other places as well but these are not called hurricanes1 and are not covered in this article. A hurricane is a large2, rotating storm, where the winds move around a relatively calm centre called the 'eye'. Each storm usually has a life span of several days. With hurricanes, the combination of size and violence produce a phenomenon with great destructive power.
Every year from 1 June - 30 November3, an average of six hurricanes form in the Atlantic Ocean, Gulf of Mexico, and the Caribbean Sea.
Formation of Hurricanes
Hurricanes are born in tropical waters where the warm ocean and the moist warm atmosphere combine. They require a complex combination of atmospheric processes to grow, mature, and then die.
The process by which a disturbance forms and strengthens into a hurricane depends on at least three conditions. First, a disturbance gathers heat and energy through contact with warm ocean waters. Next, added moisture evaporated from the sea surface powers the infant hurricane like a giant heat engine. The air rises rapidly resulting in an area of very low pressure. Third, the hurricane forms a wind pattern near the ocean surface that spirals air inwards (due to the Coriolis effect4 and interaction with the tradewinds). Bands of thunderstorms form, allowing the air to warm further and rise higher into the atmosphere. If the winds at these higher levels are relatively light, this structure can remain intact and further strengthen the storm.
Once storms mature into hurricanes, they can remain constant for days. Sometimes a hurricane may slow down at sea if the internal pressure drops; this usually results in the central winds strengthening5.
The most violent winds and rain take place in the eye wall, the ring of thunderstorms immediately surrounding the centre. The centre, or eye, of a hurricane is relatively calm with sinking air, light winds and few clouds. At the top of the eye wall (about 15,000m), most of the air is propelled outward, increasing the airís upward motion. Some of the air, however, moves inward and sinks into the eye, creating a cloud-free area.
Hurricanes tend to move faster across water than land. Usually, as soon as the storm makes landfall, it dissipates and the air pressure increases6. However, if the hurricane encounters a large body of warm water before it has completely dissipated, it can reform. This has been known to happen in Central America where a hurricane can pass westwards across the land mass, reach the Pacific Ocean, regroup, and head eastwards again.
Hurricanes don't usually form in the South Atlantic because of heavy vertical shear, or winds from different directions and altitudes that destroy the storm's structure. However, in March 2004, a rare South Atlantic hurricane occurred. This was the first such occurrence since satellites began tracking storms in the 1960s (possibly the first ever). Experts are puzzled as to how this hurricane developed.
Hurricane Life Cycle
Hurricanes do not just appear out of the blue. They actually begin as small and quite harmless weather disturbances. If favourable conditions continue, they develop through a number of stages before becoming a hurricane. These stages are as follows:
This is a kink or bend in the normally straight flow of surface air in the tropics, that forms a low-pressure trough, or pressure boundary, with showers and thunderstorms. There is no organized wind circulation. Tropical waves generally develop off the western coast of Africa at latitudes between 5° south and the equator.
There is evidence of closed wind circulation around a centre with sustained winds of less than 63km/h. It can take anywhere from several hours to several days for a tropical depression to become a hurricane. This phase usually takes place beyond the 5° latitude since the Coriolis force is negligible close to the equator.
Distinct circulation pattern with maximum sustained winds are from 63-117 km/h, the storm is usually named once it reaches tropical storm strength.
Maximum sustained winds that exceed 117 km/h. Hurricanes are classified into categories based on the wind speed.
Storm systems that obtain Tropical Storm or Hurricane status are given names. This is done mainly to reduce error in written and oral communication. It is also quicker and less cumbersome than the latitude/longitude identification methods previously used.
The practice of naming hurricanes began in 1953. Until 1979 only women's names were used. Since then, however, they have alternated between male and female names. There are 6 lists of names that are used in rotation. The list used in 2003 will be used again in 2009. However, storms that cause extensive damage or loss of life usually have their names retired (for reasons of sensitivity and to avoid confusion in historical references). This name is replaced in any future lists. The list for each year is arranged in alphabetical order with alternating male and female names. The names are assigned in sequence with the 1st tropical storm of the region being given the first name on the list and so on.
The Eastern North Pacific uses a different set of 6 lists, and other areas which experience typhoons use names as well. More information about these lists is available here.
How Hurricanes Cause Damage
There are three main conditions that contribute to the damage caused during a hurricane. These are the storm surge, wind and flooding.
The unusual rise in sea level that accompanies hurricanes or other intense storms. The height of the surge is the difference between the observed level of the sea surface and the level that would otherwise have occurred. This surge of high water, topped by waves, is devastating. The stronger the hurricane and the shallower the offshore water, the higher the surge will be. Along the immediate coast, storm surge is the greatest threat to life and property. If the storm coincides with high tide-levels then the effect of the surge is increased.
The wind, due to its sheer speed and the accompanying force, can be very destructive - uprooting trees and downing poles. In addition, pieces of debris - such as signs, roofing material, siding, and small items left outside - become flying missiles in hurricanes.
Flooding is caused by the intense rainfall associated with a hurricane. Since rainfall is not related to the wind speed, it means that weaker storms can sometimes cause more flooding (and more damage) than stronger systems. Inland flooding can occur hundreds of kilometres from the coast causing extensive damage. Heavy rain can also trigger landslides and mud slides. Flooding poses the overall greatest threat to life since it can occur in places remote from the hurricane's path.
Hurricane Categories - Saffir-Simpson Scale
The scale is named after Herbert Saffir, a consulting engineer in Coral Gables, Fla., and Robert Simpson, who was director of the National Hurricane Center from 1967 through 1973. It was developed in 1971.
Hurricane categories are based on the hurricane's current intensity and are intended to give an indication on how much damage is likely to be caused by the storm in the event of it coming onshore. Wind speed is the main determining factor.
Category One (weakest)
Wind speeds range from 119 to 153 km/h. Storm surge generally measures 1.5-2m above normal. No significant damage is caused to building structures. Coastal structures such as jetties and seawalls may suffer minor damage. Damage occurs primarily to plants, trees and possibly road signs.
Wind speeds are between 154 and 177 km/h. Storm surge is generally 2-2.5m above normal. Damage occurs to roofs, doors, and windows of buildings likely. Significant damage to shrubbery and trees takes place with some uprooting. Small craft in unprotected anchorages will break moorings.
Winds measure from 178 to 209 km/h. Storm surge is generally 3-4m above normal. Some structural damage to small residences and utility buildings is inflicted. Damage to plant and trees occurs with large trees uprooted and/or defoliated. Coastal flooding destroys smaller structures with larger structures damaged by battering from floating debris. Areas of low elevation (less than 2m above mean sea level) may be flooded - even as far as 13 km or more inland. Evacuation of low-lying residences may be required.
Wind speeds range from 210 to 249 km/h. Storm surge is generally 4-6m above normal. Complete roof structures can fail on small residences. Shrubs, trees, and all signs are razed. Extensive damage is inflicted upon doors and windows. Major damage to lower floors of structures near the shore can be expected. Terrain lower than 3m above sea level may be flooded requiring massive evacuation of residential areas as far inland as 10 km.
Category Five (strongest)
Winds are faster than 250 km/h. Storm surge generally measures greater than 6m above normal. Complete roof failure occurs on many residences and industrial buildings. Some complete building failures can happen, with small utility buildings blown over or away. All shrubs, trees, and signs are likely to be blown down. Severe and extensive window and door damage is expected. Major damage to lower floors of all structures located less than 5m above sea level and within 500m of the shoreline is likely. Massive evacuation of residential areas on low ground within 8-16 km shoreline may be required.
Meteorologists use advanced methods to track and predict the paths of hurricanes. It's mainly due to advances in hurricane forecasting that not as many lives are lost as in previous times. Some of the technologies in use are:
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Images from geostationary satellites provide valuable information on the size, location and intensity of a storm. Satellites relay day and night images of storm systems and the areas surrounding them.
Pilots fly into the core of a hurricane to measure wind, pressure, temperature and humidity as well as to provide an accurate location of the centre of the storm. Data from aeroplanes also helps scientists in better understanding hurricanes and to improve forecast capabilities.
When a hurricane gets close to the coast, it is monitored by land-based weather radar. Radar provides detailed information on hurricane wind fields and changes. From this, meteorologists are able to provide accurate short-term warnings for floods, inland high winds and any other weather hazards associated with a tropical cyclone.
When a tropical wave forms in the Atlantic, warning centres7 issue8 advisories. These are issued for all storm systems from Tropical Waves to Hurricanes. These give a general description of the storm and give details such as:
- location of the centre (the coordinates and relation to nearest land mass9)
- its direction
- its speed
- the maximum sustained winds
- the width10 of the system
- its internal pressure
- the storm surge and rainfall anticipated
When a system develops in to a Tropical Storm or Hurricane, further alerts are normally made. These take the following forms:
Hurricane (or Tropical Storm) Watch - This is an announcement for specific coastal areas that storm conditions are possible within 36 hours.
Hurricane (or Tropical Storm) Warning - A warning that a storm system is expected in a specified coastal area in 24 hours or less. A hurricane warning can remain in place if the storm surge and wave activity are sufficiently above normal, even though the winds may not be of hurricane strength.
In the event of a hurricane, it is normal for a Hurricane watch (or warning) to be issued for areas that are directly in the path of the storm and a Tropical Storm watch (or warning) for areas that are not on the direct path but are close enough experience Tropical Storm conditions.
In the event of a hurricane it is important to be prepared. Preparations should begin prior to the beginning of the hurricane season (June 1st) even before the first signs of storm activity occur. Hurricanes can cause severe damage and cause loss of electricity and water service for extended periods of time. Being suitably prepared for a hurricane can save lives. Here are some tips on what to do to prepare for a hurricane.
Before the Storm
Stay tuned to weather forecasts and possible warnings. Listen to the radio11 and/or TV for official bulletins of the storm's progress. It may be useful to plot the storm locations on a hurricane map.
Evacuate, if you have been told to do so.
Stock up on drinking water, batteries12, non-perishable foods and medications. Turn refrigerator/freezer to coldest setting. Try to have a 2-week stock of supplies.
Fill up your fuel tanks. This includes your automobile and cooking gas (barbecue cylinders).
Bring in unsecured outdoor objects such as patio furniture, toys, garbage cans and garden tools. Remove roof aerials and satellite dishes.
Prepare your house. Tape windows, or cover them with hurricane shutters.
It is a good idea to have a designated 'safe room' where one can ride out the storm. This is preferably an interior room with the least window/doors. The room should not be air tight as the changing pressure associated with the storm could cause windows to break. Here you should store most of your emergency supplies.
If you have a pool, do not drain it. Drop the water level by 1m or so. If you have no storage for outdoor furniture, place it in the pool. Turn off electricity to the pool pump. Add extra chlorine to compensate for heavy rains.
Check on elderly or others who may need assistance.
Be aware of your nearest shelter locations.
Keep extra cash close at hand.
During the Storm
If you have one, stay in your safe room.
Stay away from unprotected windows. Any loose object becomes a lethal missile during the hurricane/storm.
Stay tuned to radio for weather bulletins and storm progress reports.
Use battery-powered torches/flashlights as source of light - candles can become a fire hazard.
You may hear windows break. Do not leave your safe room until you hear the all clear on radio.
Even if it seems calm outside, do not leave your safe room until you hear the all clear - you may be in the calm eye of the hurricane/storm.
After the Storm
Once the all-clear is given, leave your safe room slowly and carefully. Inspect damage inside your house only. Start cleaning up the most dangerous conditions such as broken glass.
After you have secured your family and cleared your property of dangerous objects, help neighbours.
When going outside be careful. Avoid downed or dangling utility wires. Be especially careful when cutting or clearing fallen trees or walking through water puddles as they may have power lines dangling or lying in them.
If you have evacuated, do not return to the affected area until you have been given permission to do so.
Do not drive across flooded roadways.
Stay clear from moving water, especially near rivers, streams, and drainage systems.
Related BBC Links
The BBC Weather website has lots of additional information about the following aspects of Hurricanes: