Glossary of Hurricane Terms
Advisories are formal messages issued each six hours concerning tropical storms and hurricanes. They give warning information on where the storm is located, how intense it is, where it is moving, and what precautions should be taken.
A measure of the reflectivity of a surface.
A dome of air with high atmospheric pressure. Same as a ‘high’.
Rotation that is counter-clockwise in the Southern Hemisphere and clockwise in the Northern Hemisphere. Opposite of cyclonic.
The furthest point of the moon’s orbit to the earth. Opposite of perigee.
An atmosphere in which differences in temperatures are significant enought that air density depends on both pressure and temperature. For most purposes, tropical cyclones are not considered baroclinic.
Standards and guidelines for construction of buildings to ensure a minimum level of safety for the occupants.
Cape Verde-type Hurricanes:
Cape Verde-type hurricanes are those 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.
Coastal Flood Warning:
A warning that significant wind-forced flooding is to be expected along low-lying coastal areas if weather patterns develop as forecast.
Coastal Flood Watch:
An alert that significant wind-forced flooding is to be expected along low-lying coastal areas if weather patterns develop as forecast.
Apparent effect of the earth’s rotation tending to turn the direction of any object or fluid toward the right in the northern hemisphere and to the left in the southern hemisphere. The Coriolis Force gives a tropical cyclone its spin. Without it, tropical cyclones would not form.
The process where water vapour is transformed into liquid water.
The transfer of heat or moisture in a medium by the movement of a mass or substance. When used to imply only upward vertical motion, it is then the opposite of subsidence.
Individual columns of updrafts produced by atmospheric convection.
Heaped or lumpy clouds that form in an unstable atmosphere.
A type of cloud that is exceptionally dense and vertically developed. These clouds appear as mountains or huge towers. If they reach the top of the troposphere (as in the picture below) the tops spread out and look like a blacksmith’s anvil. They are more commonly known as thunderstorms.
Rotation that is counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Opposite of anticyclonic.
The temperature at which a vapour begins to condense.
A sudden descent of cool or cold air to the ground, usually with precipitation, also associated with a thunderstorm or shower. The opposite of an updraft.
A warming of Pacific Ocean waters near the Equator that typically occurs every 3 to 7 years. Such an event dictates a shift in "normal" weather patterns.
Extratropical (ET) Cyclone:
A cyclone which attributes the majority of its energy to baroclinic processes. An ET cyclone has significant vertical wind shears, and a distinctive asymmetric temperature and moisture field. It may develop a cold core in its later stages.
The eye of the hurricane is the innermost portion of the storm. This zone is surprisingly calm with little or no wind. Within the eye, the skies are often clear, despite the fact that winds and clouds continue to rage around the edge of the eye. The eye is not always in the centre of the storm. Sometimes it turns or moves in various directions with the storm itself, which continues to move forward on its own course. Over the ocean the sea can be treacherous under the eye because high waves are converging.
The ring of thunderstorms that surrounds a storm's eye. The heaviest rain, strongest winds and worst turbulence are normally in the eye wall.
The expected severity of flooding (minor, moderate or major) as well as where and when the flooding will begin.
Hurricanes are cyclones of tropical origin with wind speeds of at least 118 kilometres per hour. A hurricane is a large, rotating storm, where the winds move around a relatively calm centre called the ‘eye’. These storms are known as ‘typhoons’ in the western Pacific, ‘cyclones’ in the Indian Ocean, and ‘baguios’ in the Philippines. Each storm usually has a life span of several days.
Hurricane Eye Landfall:
When the eye, or physical center of the hurricane, reaches the coastline from the hurricane's approach over water.
Line of movement (propagation) of the eye through an area.
The portion of the year having relatively high incidence of hurricanes. North American hurricane season is normally from June to November. The most active month in Atlantic Canada is September.
A hurricane warning is issued to coastal areas where winds of 118 kilometres per hour or higher are definitely expected to occur. A warning also may include coastal areas where dangerously high water or exceptionally high waves are predicted, even though the winds expected may be less than hurricane force. When the warning is issued, all precautions should be taken immediately. The warnings are seldom issued more than 24 hours in advance. If the hurricane path is unusual or erratic, the warnings may be issued only a few hours before the onset of hurricane conditions.
As the hurricane continues to approach the mainland and is considered a threat to coastal and inland regions, meteorologists issue a hurricane watch for the regions in the calculated path. This watch does not mean that a hurricane is definitely going to strike. It means that everyone in the area covered by the alert should watch more carefully for the hurricane and be prepared to act quickly if definite warnings are issued that a hurricane will strike.
Invisible rays of the spectrum, just beyond red, that have a penetrating heat effect.
A line drawn on a weather map connecting points of equal pressure.
High-speed airflow in narrow bands within the upper-air westerlies and along certain other zones at high levels.
A measurement of wind speed. 1 knot = 1.853 kilometres per hour.
Heat released or absorbed by a substance as it changes its state. When water vapour condenses into liquid, it releases this heat into the surrounding atmosphere. The atmosphere around this condensation then warms.
La Reunion Island:
Island in the Indian Ocean off the east coast of Africa.
Low Pressure System (low):
Area which has a central pressure lower than its surroundings. Storm systems are all low pressure systems.
A metric measurement of air pressure. 1 inch of mercury = 33.87 millibars.
The point in the moon’s orbit which is closest to the Earth. Opposite of apogee.
The time between two successive peaks of a wave.
The smallest unit of resolution on a computer screen or image.
A tropical storm or hurricane that moves beyond the tropics into the mid-latitudes and begins losing its tropical characteristics. The size of the circulation usually expands, the speed of the maximum wind decreases, and the distribution of winds, rainfall, and temperatures becomes more asymmetric.
The point when a tropical cyclone begins to take on characteristics apart from those of a pure tropical cyclone.
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 atmospheric effects such as wind or pressure.
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.
Severe Thunderstorm Warning:
Indicates that severe thunderstorms have been sighted or indicated on radar.
Severe Thunderstorm Watch:
Indicates that conditions are favourable for lightning, damaging winds greater than 90 kilometres per hour and hail and/or heavy rainfall.
Small Craft Advisory:
A warning of winds from 20 to 33 knots or for sea conditions either forecasted or occurring that are considered potentially hazardous to small boats in coastal waters.
A brief, violent windstorm usually but not necessarily associated with rain.
Spiral Rain Bands:
Bands of thunderstorms that wrap around a hurricane.
A non-convective state in the atmosphere, opposite of unstable. Occurs in a section of the atmosphere when the air temperature either decreases very slowly with height or even increases with height.
Uniform, featureless clouds that form in a stable atmosphere.
The high and forceful dome of wind-driven waters sweeping along the coastline near where the eye makes landfall or passes close to the coast.
A low pressure system, developing over waters which are neither in the tropics nor the mid-latitudes, and which initially contains few tropical characteristics. With time the subtropical storm can become tropical.
Subtropical high-pressure belts:
Belts of persistent high atmospheric pressure trending east-west and centered about latitude 30° N and S.
Deep high pressure systems, caused by the accumulation of air as a result of the convergence in the upper troposphere.
The configuration of a surface including it relief and the position of its natural and man-made features.
Surface winds in the low level air flow within the tropical easterlies.
The generic term for the class of tropical weather systems, including tropical depressions, tropical storms, and hurricanes.
Low-latitude wind system of persistent air flow from east to west between the two subtropical high-pressure belts.
A tropical cyclone with maximum sustained surface winds less than 63 kilometres per hour.
A tropical cyclone with maximum sustained surface winds 63 kilometres per hour to 117 kilometres per hour.
Tropical Storm Watch:
An announcement that a tropical storm or tropical storm conditions pose a threat to coastal areas generally within 36 hours. A watch should normally not be issued if the system is forecast to attain hurricane strength.
Tropical Storm Warning:
Same as a Hurricane Warning, except winds of 63 to 11 7 kilometres per hour are expected.
A kink or bend in the normally straight flow of surface air in the tropics which forms a low pressure trough, or pressure boundary, with showers and thunderstorms. Can develop into a tropical cyclone.
Lowermost layer of the atmosphere in which air temperature falls steadily with increasing altitude. All weather occurs in the troposphere.
An elongated area of low barometric pressure.
A hurricane in the north Pacific west of the International Date Line.
A turbulent, convective state in the atmosphere resulting from a rapid decrease in air temperature with height above the ground.
A small scale current of air with vertical motion. If there is enough moisture, then it may condense, forming a cumulus cloud and possibly thunderstorms. The opposite of a downdraft.
Vertical Wind Shear:
The magnitude of wind change with height. Vertical wind shear can weaken or destroy the cyclone by interfering with the organization of deep convection around the cyclone centre.
A measure of local rotation in a fluid flow; the spin of a fluid.
Surface winds blowing from a generally westerly direction in the midlatitude zone, but varying greatly in direction and intensity.