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Canadian Hurricane Centre Glossary

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Anticyclone
A high pressure system with an anticyclonic circulation. It is also called a "high," and is generally associated with fair weather.
Anticyclonic
Rotation that is counter-clockwise in the Southern Hemisphere and clockwise in the Northern Hemisphere. It is the opposite of cyclonic.
Apogee (lunar)
The point in the moon’s orbit that is farthest from the Earth. It is the opposite of perigee.

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Building Codes
Standards and guidelines for construction of buildings to ensure a minimum level of safety for the occupants.

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Cape Verde-type Hurricanes
Cape Verde-type hurricanes are Atlantic basin tropical cyclones that develop into tropical storms fairly close (less then 1000 kilometres) to the Cape Verde Islands, off the west coast of Africa, and then become hurricanes before reaching the Caribbean. Typically, this occurs in August and September.
Coriolis Force
Apparent effect of the earth’s rotation to deflect the direction of any object - including the wind - to the right in the northern hemisphere and to the left in the southern hemisphere. The Coriolis Force is responsible for giving a cyclone its spin, and without it, tropical cyclones would not form.
Cyclone
A low pressure system with a cyclonic circulation. It is also called a "low," or a "depression," and is generally associated with poor or stormy weather. The point of lowest atmospheric pressure marks the centre of the cyclone.
Cyclonic
Rotation that is counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. It is the opposite of anticyclonic.

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Easterly Wave (also called Tropical Wave)
Also known as easterly waves or African easterly waves, they are a type of atmospheric trough, oriented north to south, which move from east to west across the tropics causing areas of cloudiness and thunderstorms. A tropical wave can develop into a tropical cyclone.
Extratropical Cyclone
The generic term for the class of frontal low pressure systems. They are unlike tropical cyclones in that they are not symmetric in their temperature, precipitation or wind patterns. The driving mechanism for these systems is the difference in temperature and humidity across the cyclone.
Extratropical Transition
The transformation of a tropical cyclone into an extratropical cyclone. More than 40% of Atlantic tropical cyclones undergo such a transformation at the end of their tropical existence. The metamorphosis typically starts as the system begins to be driven by atmospheric temperature differences rather than latent heat release. The cyclone begins losing its tropical characteristics and often develops front-like qualities. During the transformation the rain area usually shifts to the left of the storm track while the strongest winds diminish but become elongated to the right of the storm track. Some extratropical transition events can result in a more powerful storm than the originating tropical cyclone.
Eye of the storm
The roughly circular area of comparatively light winds found at the centre of a hurricane and surrounded by the eyewall. Within the eye, the skies are often clear, despite the fact that winds and clouds continue to rage around the edge of the eye in the eyewall. Over the ocean the sea can be treacherous under the eye because high waves are converging. The eye of a hurricane is often obscured by overriding high cloud by the time it reaches Canadian latitudes. The diameter of a hurricane eye can range from 10 to more than 100 kilometres.
Eyewall
The ring of thunderstorms that surrounds a storm's eye. The heaviest rain, strongest winds and worst turbulence are found in the eyewall.

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Frontal Cyclone
Any cyclone associated with a front. It is often used synonymously with extratropical cyclone (as opposed to tropical cyclone, which is nonfrontal).

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Hurricane
Hurricanes are tropical cyclones with maximum sustained surface winds of at least 118 kilometres per hour. Hurricanes are known as typhoons in the western Pacific, very severe cyclonic storms in the North Indian Ocean, and severe tropical cyclones in Australia. There are 5 classes of hurricane intensity as outlined by the Saffir-Simpson Scale.
Hurricane Season
Hurricane season in the North Atlantic runs officially from June 1 to November 30 when almost 95% of all tropical cyclones occur. About 85% of "landfalling" tropical cyclones in Canada occur during August through October.
Hurricane Warning
An announcement that one or both of the following dangerous effects of a hurricane are expected in a specified area in 24 hours or less: (a) average sustained winds 118 kilometres per hour or higher; (b) dangerously high water or a combination of dangerously high water and exceptionally high waves, even though winds expected may be less than hurricane force. By nature a hurricane also implies the threat of local flooding from heavy rainfall.
Hurricane Watch
An announcement for a specific area that a hurricane or an incipient hurricane condition poses a possible threat within 36 hours. This watch does not mean that a hurricane is definitely going to strike. It means that everyone in the area covered by the watch should watch more carefully for the hurricane and be prepared to act quickly if definite warnings are issued that a hurricane will strike.

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Landfall
When the eye, or physical centre of the hurricane, reaches a coastline it is said to "make landfall." The term "landfall" is generally not used when post-tropical storms move inland because they typically do not have an eyewall and the worst impacts of the storm have already spread out at a distance from the storm centre.
Latent Heat
Heat that is stored in water vapour in the atmosphere. When water vapour rises, cools and condenses into liquid water, it releases this heat into the surrounding atmosphere. This is the driving mechanism for tropical cyclones.

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Perigee (lunar)
The point in the moon’s orbit that is closest to the Earth. Opposite of apogee.
Period
The time between two successive crests or troughs of a wave.
Post-Tropical Storm
A storm system that used to be tropical but has since lost most of its tropical characteristics. Since the remnant system may or may not be an "extratropical cyclone" (a term set aside for a specific breed of cyclone) the term "post-tropical" is a convenient communication term to permit the ongoing use of the storm name. This is important because storms with a tropical pedigree seldom lose all of their tropical characteristics, such as intense rainfalls and high wind gusts.
Predicted Astronomical Tide
The height of tides at various locations and times based solely on astronomical calculations. These are the values printed in tide tables. These values do not account for effects such as wind, atmospheric pressure or waves.

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Saffir-Simpson Scale
The scale used to give public safety officials an assessment of the potential wind and storm surge damage from a hurricane. Scale numbers are available to public safety officials when a hurricane is within 72 hours of landfall. Scale assessments are revised regularly as new observations are made. Public safety organizations are kept informed of new estimates of the hurricane's disaster potential. Scale numbers range from 1 to 5.
Spiral Rain Bands
Bands of thunderstorms and intense rain that wrap around a hurricane.
Storm Surge
The positive or negative difference in sea level from the predicted astronomical tide, due to the forces of the atmosphere. The two main atmospheric components that contribute to storm surge are air pressure and wind. Deep low pressure systems can create a dome of water under the storm (much like the low pressure in a vacuum on a carpet). High winds along a coastline can also elevate the water levels at the shore, depending on the direction of the wind with respect to the coast. For powerful storms like hurricanes, the abnormally high water levels are due mostly to the high winds and high waves at the coast.
Storm Surge Warning
A warning that significant wind-forced flooding is to be expected along low-lying coastal areas if weather patterns develop as forecast.
Storm Track
The line of movement (propagation) of the storm's centre through an area.
Subtropical Ridge
A large belt of high pressure located around 30 degrees latitude in both hemispheres and characterized by mostly calm winds. Air flows out from the ridge toward the upper and lower latitudes of each hemisphere, creating both the trade winds and the westerlies. A change in the ridge position of the Atlantic subtropical ridge (often referred to as the "Bermuda high," or "Azores high") has a direct impact on the tracks of tropical cyclones which typically form to its southern and western periphery.
Subtropical Storm
A subtropical storm is a cyclone that has characteristics of both a tropical storm and an extratropical cyclone. Subtropical cyclones can form in waters normally too cool for tropical cyclones ... therefore they are not as restricted to the hurricane season months. With time the subtropical storm can become tropical.

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Trade Winds (also called Tropical Easterlies)
The belts of winds on either side of the equator, blowing from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere. In both hemispheres the winds become more easterly closer to the equator.
Tropical Cyclone
The generic term for the class of tropical low pressure systems, including tropical depressions, tropical storms, and hurricanes. These non-frontal cyclones originate over tropical or sub-tropical waters and are identified by organized showers and thundershowers and a cyclonic surface wind circulation. They are unlike extratropical cyclones in that they are generally symmetric in their temperature, rain and wind patterns. Rainfall with tropical cyclones is not dependent on the intensity of the storm. The driving mechanism for a tropical cyclone is the release of latent heat. These systems typically last a week or more.
Tropical Cyclone Information Statement
Routine bulletins issued by the Canadian Hurricane Centre when a tropical cyclone could bring high winds or seas to Canadian waters, or high winds, heavy rain, dangerous coastal waves or coastal storm surge to Canadian territory within the next 72 waters.
Tropical Depression
A tropical cyclone with maximum sustained surface winds of 37 to 62 kilometres per hour. It does not typically have the organization or the spiral shape of the more powerful storms. When a tropical cyclone reaches this stage it is given a number.
Tropical Disturbance
An organized region of showers and thunderstorms in the tropics - generally 200 to 600 kilometres in diameter - that maintains its identity for at least 24 hours but does not have a closed wind circulation.
Tropical Easterlies (also called Trade Winds)
The belts of winds on either side of the equator, blowing from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere. In both hemispheres the winds become more easterly closer to the equator.
Tropical Storm
A tropical cyclone with maximum sustained surface winds of 63 to 117 kilometres per hour. When a tropical cyclone reaches this stage it is given a name.
Tropical Storm Warning
An announcement that tropical storm conditions, including possible sustained winds within the range 63-117 kilometres per hour, are expected in specified areas within 24 hours or less. By nature a tropical storm also implies the threat of local flooding from heavy rainfall.
Tropical Storm Watch
An announcement for a specific area that a tropical storm or an incipient tropical storm condition poses a possible threat within 36 hours.
Tropical Wave (also called Easterly Wave)
Also known as easterly waves or African easterly waves, they are a type of atmospheric trough, oriented north to south, which move from east to west across the tropics causing areas of cloudiness and thunderstorms. A tropical wave can develop into a tropical cyclone.
Typhoon
The name given to hurricanes in the western North Pacific Ocean, west of the International Date Line.

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Vertical Wind Shear
The condition produced by a change in wind velocity (speed and/or direction) with height. Vertical wind shear can weaken or destroy a tropical cyclone by interfering with the symmetric nature and organization of the deep convection around the cyclone centre.

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Wavelength
The distance between two successive crests or troughs of a wave.
Westerlies
The dominant west-to-east motion of the atmosphere, centered over the middle latitudes of both hemispheres.