|Weather Reference - Hurricanes
Overview | Quick Facts | Safety Information | Other Resources | Disclaimer
Tropical storms which have sustained wind speeds of 74mph or more are known as Hurricanes over the North Atlantic, Cyclones over the Indian Ocean and Typhoons over the north-west Pacific. The name 'hurricane' is derived from the aboriginal Caribes (Indigenous peoples of the West Indies at the time of the arrival of Christopher Columbus) expression for 'evil spirit.'.
Hurricanes can often be seen on satellite photos. The image to the left shows a satellite photo taken on the 8th September 2003 with EU Met Satellite (MET-8) and shows Hurricane "Isabel" over the Atlantic. You can clearly see the eye of the hurricane in the centre and the anti-clockwise rotation of the hurricane.
For a hurricane to form, the sea surface temperatures must exceed 26°C. Hurricanes are one of the most powerful weather phenomenon powered by heat energy released from condensation of water vapour. Wind speeds can occasionally reach up to 200mph spiraling around a clear, calm low pressure 'eye' of the storm. A ring of thunderstorms where the winds are the strongest and the rain is torrential surround the eye. There can be hundreds of thunderstorms in a mature hurricane. To give an indication of the strength of the winds produced by a hurricane, they can give a coconut the force of a cannonball.
The cluster of storms that begin the initial production of a hurricane only occur where the sea temperature is at least 26°C , therefore they will normally only develop in the tropics.
Additional hazards of hurricanes include ocean surges where the the intense low pressure system at the eye combining with the effect of the severe winds can raise the ocean surface by up to 4 metres. The sea level rises approximately 1cm for every millibar. (e.g. if the pressure is 950mb, the sea surge will be about 50cm). Costal areas can also suffer additional damage from huge waves of up to 15m high on to of the sea surge. Sea levels can also fall as wind blows back around the hurricane pushing water away. Flooding can also occur inland from the torrential rainfall that falls from the storm's cumulonimbus clouds. A hurricane can pick up around 2 billion tons of moisture per day which is released as rain (and other precipitation such as hail).
To develop the distinctive rotation, the system must be at least 5 degrees form the equator. This is where the curiolis effect begins to have an influence. Once spinning, a storm system tends to move further away from the equator although it is unlikely to continue beyond 30 degrees north or south.
Hurricanes can last for weeks and can cover many thousand miles. Hurricanes travel at up to 30mph. An average hurricane can travel around 300-400 miles per day. (About 3,000 miles before it dies out).
In the Atlantic and the Pacific north of the equator, hurricanes generally occur between July and October. South of the equator, off Australia and the Indian Ocean, they generally occur between November and March.
The picture on the left was taken near the edge of Hurricane Isabelle in 2003.
You can see the severity and intensity of this hurricane from the pictures.
The strength of hurricanes is measured on the 'Saffir-Simpson Hurricane Scale' . The scale was formulated in 1969 by Herbert Saffir, a consulting engineer, and Dr. Bob Simpson, director of the National Hurricane Center. The World Meteorological Organization was preparing a report on structural damage to dwellings due to windstorms, and Dr. Simpson added information about storm surge heights that accompany hurricanes in each category.
All Hurricanes are dangerous, but some are more dangerous than others. The way storm surge, wind and other factors combine determines the hurricanes destructive power. To make comparisons easier and to make the predicted hazards of approaching hurricanes clearer to emergency managers, National Oceanic and Atmospheric Administration's hurricane forecasters use a disaster-potential scale which assigns storms to five categories. This can be used to give an estimate of the potential property damage and flooding expected along the coast with a hurricane.
|Saffir-Simpson Hurricane Scale
| No real damage to building structures. Damage primarily to unanchored mobile homes, shrubbery, and trees. Also, some coastal road flooding and minor pier damage.
|Some roofing material, door, and window damage to buildings. Considerable damage to vegetation, mobile homes, and piers. Coastal and low-lying escape routes flood 2-4 hours before arrival of center. Small craft in unprotected anchorages break moorings.
|Some structural damage to small residences and utility buildings with a minor amount of curtainwall failures. Mobile homes are destroyed. Flooding near the coast destroys smaller structures with larger structures damaged by floating debris. Terrain continuously lower than 2m above sea level may be flooded inland 8 miles or more.
|More extensive curtainwall failures with some complete roof structure failure on small residences. Major erosion of beach. Major damage to lower floors of structures near the shore. Terrain continuously lower than 3m above sea level may be flooded requiring massive evacuation of residential areas inland as far as 6 miles.
|Complete roof failure on many residences and industrial buildings. Some complete building failures with small utility buildings blown over or away. Major damage to lower floors of all structures located less than 15 feet above sea level and within 500m of the shoreline. Massive evacuation of residential areas on low ground within 5 to 10 miles of the shoreline may be required.
Major destruction is caused when hurricanes travel over land. When a hurricane moves over land, its energy source is depleted and the friction as it moves across the land distorts the air flow which results in the eye being filled with cloud which marks the destruction of the hurricane.
Over the British Isles, hurricanes do not normally occur. Deep depressions can form but in order for a storm to be classed as a hurricane, it must have sustained wind speeds of at least 74mph for ten minutes or more. Severe gales in Britain can produce gusts in excess of this, but it is very unusual for sustained wind speeds of this magnitude. One other most famous storms was the widely publicised depression 'The Great Storm' which occurred on 16th October 1987. By this definition, Hurricane Force winds occurred locally, such as at Lee on Solent and Gorleston, but were not widespread. Michael Fish was made famous because of this storm.
The British Isles can be effected from the remnants of hurricanes. These are the remains of prior hurricanes which are no longer of hurricane strength or building in intensity.
Hurricanes are given names, the naming convention currently used are in alphabetical sequence with alternate male and female names in the order of occurrence. In the Atlantic basin and in the Eastern Pacific the first storm of each year is is given an 'A' name. The letters Q, U, X, Y, and Z aren't used because too few names begin with these letters.
The hurricane path can only be forecast once it has begun to develop as other than knowledge of hurricane occurrence there are no atmospheric conditions that can be measured and combined to predict where a hurricane will develop. Satellites can detect early stages of hurricane formation and can help to provide early warning of imminent hurricanes. Specially designed aircrafts fitted with instruments fly through and above hurricanes, and weather radar can locate storms within a few hundred miles of a radar station.
Hurricane Quick Facts
- Hurricanes are one of the most dangerous natural hazards to effect people and the environment.
- To be classified as a hurricane, a storm must have sustained wind speed for 10 minutes of at least 74mph.
- The strength of a hurricane is measured using the Saffir-Simpson Hurricane Scale.
- Hurricanes as a rule tend not to occur over the British Isles.
- Storm surges pose an additional hazard.
- In land flooding occurs as a result of the billions of tones of moisture picked up.
- Hurricanes form between 5 and 30 latitude over seas.
- Hurricanes require the sea surface temperature to be over 26°C in order to begin formation.
- Hurricanes can travel about 3,000 miles before they die out.
- Wind speeds can be up to 200mph.
- Hurricanes transfer heat and energy between the equator and the poles.
The following information is some advice on safety precautions to take if you are likely to be or are affected by a hurricane. Most of the information is basic common sense. We strongly advise you to research all information if you are likely to be affected by a hurricane. This information is purely ideas and not a substitute for any other official information. Use this information at your own risk.
If visiting an area affected by hurricanes during the hurricane season, it is advisable to:
- Plan an evacuation route.
- Learn safe routes inland.
- Be ready to drive 20 to 50 miles inland to locate a safe place.
- Have disaster supplies on hand;
- Carry Flashlight and extra batteries
- Carry a Portable, battery-operated radio and extra batteries
- First aid kit and manual
- Emergency food and water
- Non-electric can opener
- Essential medicines
- Cash and credit cards
- Sturdy shoes
- Make sure that all family members know how to respond after a hurricane.
- Teach family members how and when to turn off gas, electricity, and water.
- Teach children how and when to call 9-1-1, police, or fire department and which radio station to tune to for emergency information.
- Protect your windows.
- Trim back dead or weak branches from trees.
- Check into flood insurance.
- Homeowners polices do not normally cover damage from the flooding that accompanies a hurricane.
- Develop an emergency communication plan.
- In case family members are separated from one another during a disaster, have a plan for getting back together.
During a hurricane watch (A Hurricane Watch is issued when there is a threat of hurricane conditions within 24-36 hours.) it is advisable;
- Listen to a battery-operated radio or television for hurricane progress reports.
- Check emergency supplies.
- Fuel car.
- Bring in outdoor objects such as lawn furniture, toys, and garden tools and anchor objects that cannot be brought inside.
- Secure buildings by closing and boarding up windows.
- Remove outside antennas.
- Turn refrigerator and freezer to coldest settings. Open only when absolutely necessary and close quickly.
- Store drinking water in clean bathtubs, jugs, bottles, and cooking utensils.
- Store valuables and personal papers in a waterproof container on the highest level of your home/accommodation.
- Review evacuation plan.
- Moor boat securely or move it to a designated safe place. Use rope or chain to secure boat to trailer. Use tiedowns to anchor trailer to the ground or house.
During a hurricane warning (A Hurricane Warning is issued when hurricane conditions (winds of 74 miles per hour or greater, or dangerously high water and rough seas) are expected in 24 hours or less.) it is advisable to;
- Listen constantly to a battery-operated radio or television for official instructions.
- If in a mobile home, check tiedowns and evacuate immediately.
- Avoid elevators.
- If at home:
- Stay inside, away from windows, skylights, and glass doors.
- Keep a supply of flashlights and extra batteries handy. Avoid open flames, such as candles and kerosene lamps, as a source of light.
- If power is lost, turn off major appliances to reduce power "surge" when electricity is restored.
- If officials indicate evacuation is necessary:
- Leave as soon as possible. Avoid flooded roads and watch for washed-out bridges.
- Secure your home by unplugging appliances and turning off electricity and the main water valve.
- Tell someone outside of the storm area where you are going.
- If time permits, and you live in an identified surge zone, elevate furniture to protect it from flooding or better yet, move it to a higher floor.
- Take pre-assembled emergency supplies, warm protective clothing, blankets and sleeping bags to shelter.
- Lock up home and leave.
After the storm it is advisable to;
- Stay tuned to local radio for information.
- Help injured or trapped persons.
- Give first aid where appropriate.
- Do not move seriously injured persons unless they are in immediate danger of further injury. Call for help.
- Return home only after authorities advise that it is safe to do so.
- Avoid loose or dangling power lines and report them immediately to the power company, police, or fire department.
- Enter your home with caution. Beware of snakes, insects, and animals driven to higher ground by flood water.
- Open windows and doors to ventilate and dry your home.
- Check refrigerated foods for spoilage.
- Take pictures of the damage, both to the house and its contents for insurance claims.
- Drive only if absolutely necessary and avoid flooded roads and washed-out bridges.
- Use telephone only for emergency calls.
Inspecting utilities in a damaged home - it is advisable to;
- Check for gas leaks--If you smell gas or hear blowing or hissing noise, open a window and quickly leave the building. Turn off the gas at the outside main valve if you can and call the gas company from a neighbour's home. If you turn off the gas for any reason, it must be turned back on by a professional.
- Look for electrical system damage--If you see sparks or broken or frayed wires, or if you smell hot insulation, turn off the electricity at the main fuse box or circuit breaker. If you have to step in water to get to the fuse box or circuit breaker, call an electrician first for advice.
- Check for sewage and water lines damage--If you suspect sewage lines are damaged avoid using the toilets and call a plumber. If water pipes are damaged, contact the water company and avoid the water from the tap. You can obtain safe water by melting ice cubes.
The following lists useful resources for information on hurricanes from other websites. Click the links to visit the website.
All information is presented in good faith. We cannot accept any liability for any incorrect or incomplete information. You are strongly advised to seek clarification on any information presented. If you find any errors or omissions, please contact us to let us know so that we can put it right.