Tropical Storms
Overview
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during the summer and fall, the tropics experience conditions ideal for the formation of large, rotating,
low-pressure tropical storms called tropical cyclones.
o In different parts of the world, the largest parts of these storms are known as hurricanes,
typhoons and cyclones.
Cyclone Location
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Favourable conditions exist in all tropical oceans except the South Atlantic Ocean and the Pacific
Ocean off the west coast of South America.
Cyclone Formation
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Require two basic conditions  an abundant supply of warm ocean water and a mechanism to lift
warm air and keep it rising.
Thrive on the tremendous amount of energy in the warm, tropical oceans.
As water evaporates from the ocean surface, latent heat is stored in the water vapour. This is later
released when the air rises and the water vapour condenses.
The air usually rises because of some sort of existing weather disturbance moving across the tropics.
Only a small percentage of tropical disturbances develop into cyclones.
Three Stages of Development
Formative Stage
 first indication of a building tropical cyclone is a moving tropical disturbance.
 As these disturbances produce more precipitation, more latent heat is released
 As more warm, dense air moves toward the low pressure centre to replace the air that has risen, the
Coriolis effect causes the moving air to turn counterclockwise in the N. hemisphere.
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This produces the cyclonic (counterclockwise) rotation of a tropical cyclone.
This is known as a tropical depression
Mature Stage
 As the moving air approaches the centre of the growing storm, it rises, rotates and increases in speed.
 In the process, air pressure in the center of the system continues to decrease
 As long as warm air is fed into the system at the surface and removed in the upper atmosphere, the
storm will continue to build and the winds of rotation will increase as the air pressure drops.
 When wind speeds around the low-pressure center of a tropical depression exceed 65km/hr, the system
is called a tropical storm.
 If the storm air pressure continues to fall and winds around the centre reach 120 km/hr, the storm is
classified as a cyclone.
 At this point the development of a calm centre of the storm occurs – the eye
 The eye is usually a span of 30 to 60km of calm weather.
 The strongest winds in a hurricane are usually concentrated around the eyewall.
Dissipation Stage
 A hurricane will last until it can no longer produce enough energy to sustain itself.
 Usually happens when storm moves over land or over colder water.
 Hurricanes can undergo several fluctuations in intensity over its lifespan
Hurricane Hazards
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The Saffir-Simpson hurricane scale classifies hurricanes according to wind speed, potential for
flooding and potential for property damage (p. 275 in text).
Damage
o extensive
o along coastal areas
Winds
o most damage associated with violent winds
o strongest winds usually located at eyewall
o winds taper off as distance from the centre increases
Storm Surge
o occurs when hurricane-force winds drive a mound of ocean water toward coastal areas
o can sometimes reach 6 m above normal sea level.
o Hurricanes produce great amounts of rain because of their continuous uptake of warm, moist
ocean water. Therefore, floods are an additional hazard.
Hurricane Advisories and Safety
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The Canadian Hurricane Centre provides Canadians with meteorological information on hurricanes,
tropical storms and post-tropical storms.
When a storm is approaching it will issue an Information Statement, a Watch or a Warning bulletin.
Awareness, combined with proper safety precautions, has greatly reduced death tolls in recent years.