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The Anatomy of a Storm

The Life Cycle of a Hurricane


The Saffir-Simpson Scale

Hurricane Effects

Factors in Hurricane Damage


When Is Hurricane Season?

2005: A Season to Remember

The Naming of Storms


International Cooperation

Storm Tracking Tools



Hurricane Watch

Hurricane Warning

Other Advisories

Information Sources

Storm Summaries


Identify the Risks

Family Emergency Plan

Other Emergency Plans

Prepare Your Home

Prepare Yourself

Prepare Your Pets and Livestock

Emergency Kits

Practice Your Plan

Your Community


Strengthening Your Home

Hurricane Watch





Downed Power Lines

Power Outages

Danger: Carbon Monoxide

Safe Evacuation


What to Expect

After the Storm Has Passed

Returning Home

Keep Food Safe

Keep Water Safe


First Steps

Cleanup Safety Tips

Keep or Discard

Dry Out Your Home

Before Moving Back In

The Road to Recovery


First Aid Kit

Emergency Kit

Additional Householding Emergency Items

Vehicle Emergency Kit

Workplace Emergency Kit

Important Documents



Index to Hurricanes





The Anatomy of a Storm

A tropical cyclone is an almost circular storm of extremely low pressure and high winds that spiral inward, accompanied by heavy rainfall. There are three types of tropical cyclones: tropical depressions, tropical storms, and hurricanes, which are the most intense and dangerous.

Hurricanes have three distinctive parts: the eye, the eye wall, and spiral rain bands.

The Eye

One of the most familiar parts of a tropical cyclone is the eye, produced by the intense spiraling of the storm. It is the region where the surface pressure is lowest and the temperature aloft is warmest, and the air in it is slowly sinking. The eye is the innermost zone of the tropical cyclone, but it is not always in the center of the storm. Sometimes it turns or moves in various directions with the storm itself, which continues to move forward on its own course.

The eye is surprisingly calm, with little or no wind. Within it, the skies are often clear, despite the fact that winds and clouds continue to rage around the edge of the eye. As the eye passes over a site, the sky clears and calm prevails. Then, the storm strikes again with high winds from the opposite direction.


The eye of a hurricane. U.S. Air Force

The Eye Wall

The eye of a tropical cyclone is surrounded by a ring of thunderstorms called the eye wall. The heaviest rain, strongest winds, and worst turbulence are normally in the eye wall. The mechanisms by which the eye and the eye wall are formed are not well understood, but it is generally thought that the feature is a fundamental component of all rotating fluids.

Spiral Rain Bands

Hurricanes are surrounded by bands of heavy convective showers that spiral inward toward the storm’s center. Cumulus and cumulonimbus (thunderstorm) clouds rise, and lightning develops in the spiral rain bands at the hurricane’s edge.


Do not relax your precautions if a hurricane suddenly dies down. The eye may be passing directly over you.


The upper eye wall of Hurricane Katrina, captured from a NOAA Hurricane Hunter aircraft over the Gulf of Mexico. The day before making landfall on August 28, 2005, Katrina was a strong Category 4 storm, and the eye was approximately 48 km (30 miles) in diameter. Note the spiral banding in the eye wall clouds and the clear sky above the eye. NOAA

The Life Cycle of a Hurricane


Many hurricanes that strike North America form in the tropical waters of the Atlantic Ocean or Caribbean Sea, move on a westerly to northerly track, steered by the prevailing wind direction, and strike the mainland on either the Gulf or Atlantic coast.

Hurricanes develop only in certain areas of the earth. Climatologists have defined seven regions of tropical storm formation: the western north Pacific, the eastern north Pacific, the Atlantic, the southwest Indian Ocean, the north Indian Ocean, and two areas off Australia: the southeast Indian Ocean and the southwest Pacific Ocean. The shape of the south Atlantic Ocean basin does not allow storm clouds from Africa enough time over warm water to develop into cyclones, and the waters of the eastern south Pacific are simply too cold for hurricane formation.

In order for tropical cyclones to form, several environmental conditions must be present:

• The cyclone must originate over ocean water that is least 26.5°C (80°F). Hurricanes draw their energy from the warm water of the tropics and the latent heat of condensation.

• The atmosphere must quickly become cooler as the altitude increases. This condition creates instability. If the air is unstable, the warm surface air will continue rising. If winds at all levels of the atmosphere are blowing at the same speed and from the same direction, the disturbance will grow.

• Cyclones form no closer to the equator than approximately 500 kilometres (300 miles). This is because of the Coriolis Force, an effect caused by the turning of the earth, which starts the cyclone spiral and maintains the low pressure of the disturbance. Close to the equator, the Coriolis Force is too weak.

• An upper atmosphere high-pressure area above the growing storm should be present. The air in such high-pressure areas flows outward, pushing away the air that is rising in the storm and drawing even more air up from the low levels.

The Growth of a Hurricane

A hurricane progresses through a series of stages from birth to dissipation. First, it begins as a tropical disturbance: a large area of organized thunderstorms that maintain their identity for more than 24 hours.

If the area of thunderstorms organizes so that a definite rotation develops and winds become strong, the system is upgraded to a tropical depression. At this point, a low-pressure center exists and the storm is given a number.

If winds continue to increase to 63 kilometres per hour (39 miles per hour, or 34 knots), the system becomes a tropical storm and is given a name. The system becomes more organized and the circulation around the center of the storm intensifies.

As surface pressures continue to drop, the storm becomes a hurricane when wind speed reaches 118 kilometres per hour (74 miles per hour, or 64 knots). An eye develops near the center of the storm, with spiral rain bands rotating around it.


Typhoon Tip (1979), in the northwest Pacific, was the largest tropical cyclone on record. The circulation around it measured 2174 kilometres (about 1350 miles) across.

Storm Track

Tropical cyclones tend to travel common paths, following the predominant wind in the region. For example, late season hurricanes over the Atlantic often begin near Africa, drift west on the trade winds, and then veer northeast as they meet the prevailing winds coming eastward across North America.

The storm track is also affected by the Coriolis Force and by troughs and subtropical ridges in the atmosphere. Predicting the tracks of tropical cyclones would be much easier if the wind currents that drive these storms were fixed in time. The reality is that these currents change constantly, so the tracks of tropical cyclones are difficult to predict.

Power Loss

A hurricane will begin to dissipate when the conditions that enabled it to form and grow no longer exist. When it passes over cooler waters or makes landfall, it will begin to dissipate because its main energy source, the warm ocean water, is no longer there. Since land is a rough surface, friction will slow down the movement of the hurricane once it makes landfall, weakening it by disrupting the low-level inflow of warm air.

Hurricane Size

Hurricanes are typically about 480 kilometres (300 miles) wide, although they vary considerably. Size does not necessarily indicate a hurricane’s intensity. At nearly the same location, Floyd (1999, left) was at least twice the size of Andrew (1992, right), although both were Category 4 hurricanes.



Tropical Cyclones in Canada

Since Atlantic Canadian waters are much cooler than the tropical waters where hurricanes are formed, hurricanes usually lose their energy source, and most are in the decaying stage of their life cycle by the time they reach these latitudes. Storms that have moved inland also weaken rapidly because they are beyond their source of energy, and the frictional drag of the land wears them out. The energy of a dissipated storm can continue to move through the atmosphere even when the storm is no longer visible on a weather map. Heavy rain from the very moist tropical air may continue to fall over the region.

Some tropical cyclones undergo post-tropical transition while near Atlantic Canada. As they move into the stronger air streams and cooler air temperatures of the middle latitudes, the storms accelerate, and their rain and wind patterns change. The heaviest rains shift to the left side of the accelerating storm, while the strongest winds occur on the right side. In addition, very high waves can mark the right-hand side of the post-tropical storm’s track. Because of these pattern changes, land areas in Canada are frequently affected by the heavy rains but seldom experience the strongest winds.

Saffir-Simpson Scale





The Saffir-Simpson Scale

Once a tropical cyclone reaches hurricane strength, it is given a rating from 1 to 5 on the Saffir-Simpson Hurricane Intensity Scale. The Saffir-Simpson Scale gives public safety officials an assessment of the potential wind and storm surge damage from a hurricane. Scale numbers are publicized when a hurricane is within 72 hours of landfall, and they are revised regularly as new observations result in new estimates of the hurricane’s disaster potential.

A Category 1 storm has the lowest wind speeds, while a Category 5 hurricane has the highest. These terms are relative, though. Lower category storms can sometimes inflict greater damage than higher category storms, depending on where they strike and the particular hazards they bring. In fact, tropical storms that haven’t developed into hurricanes can also produce significant damage and loss of life, mainly due to flooding.


The destruction caused by Hurricane Camille (1969) provoked a major change in the perception of hurricanes and led to the development of the Saffir-Simpson Hurricane Intensity Scale.

What the Numbers Mean

Category 1 (minimal): Damage primarily to shrubbery, trees, foliage, and unanchored homes. No real damage to other structures. Some damage to poorly constructed signs. Lowlying coastal roads inundated, minor pier damage, some small craft in exposed anchorage torn from moorings.

Category 2 (moderate): 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 the arrival of the hurricane’s center. Considerable damage to piers. Marinas flooded. Small craft in unprotected anchorage torn from moorings. Evacuation of some shoreline residences and low-lying areas required.

Category 3 (extensive): Foliage torn from trees; large trees blown down. Practically all poorly constructed signs blown down. Some damage to roofing materials of buildings; some window and door damage. Some structural damage to small buildings. Mobile homes destroyed. Serious flooding at coast, with 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 of 1.5 metres (5 feet) or less above sea level flooded inland 13 kilometres (8 miles) or more. Evacuation of low-lying residences within several blocks of shoreline possibly required.

Category 4 (extreme): Shrubs and trees blown down; all signs down. Extensive damage to roofing materials, windows and doors. Complete failure of roofs on many small residences. Complete destruction of mobile homes. Flat terrain of 3 metres


A 1x4 board driven through the trunk of a royal palm during Hurricane Andrew (1992). NOAA

(10 feet) or less above sea level flooded inland as far as 9.5 kilometres (6 miles). 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 metres (500 yards) of shore and of single-story residences within 3 kilometres (2 miles) of shore possibly required.

Category 5 (catastrophic) — Shrubs and trees blown down; considerable damage to roofs of buildings; all signs down. Very severe and extensive damage to windows and doors.

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