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Tropical Cyclone Classification (Stages of Development)

Tropical cyclone “opportunities”

One of the conditions needed for a tropical cyclone to develop is a pre-existing near-surface disturbance, low-pressure area or region of convergence. Tropical cyclones cannot suddenly appear on their own. They need a trigger mechanism to begin drawing air inwards at the lowest levels of the atmosphere. These triggers can include old weather fronts, lines of convergence or disturbed weather, and tropical easterly waves, to name a few.

Tropical cyclone development lines of convergence. Photo: NASA
Tropical cyclone development lines of convergence. Photo: NASA

Tropical waves are particularly important in the creation of tropical cyclones--60% of Atlantic cyclones (85% of the intense ones) form from these weather disturbers. Tropical waves, or easterly waves, move westward across the tropical Atlantic under the easterly trade winds. When other conditions are just right, these waves can form a tropical disturbance, the embryo of a tropical cyclone.

Tropical cyclone waves formation. Photo: Environment Canada © 2009
Tropical cyclone waves formation. © Environment Canada, 2009

Tropical disturbance

When an area of disturbed weather in the tropics causes a moving area of thunderstorms, everyone pays a bit closer attention. In the Atlantic, when that area of thunderstorms continues for 24 hours or more, the U.S. National Hurricane Center (NHC) calls the weather system a tropical disturbance and starts a formal watch.

Satellite image of a tropical disturbance. Photo: NOAA
Image of a tropical disturbance. Photo: NOAA
Satellite image of a tropical disturbance taken August 27, 2004. Photo: NOAA
The tropical disturbance in this image was taken August 27, 2004. Photo: NOAA

Tropical depression

If the area of thunderstorms in a tropical disturbance organizes in such a way that a rotation develops and winds become strong (20 knots or 37 km/h), the system is upgraded to a tropical depression and it is given a number. Tropical Depression 10 from 2004 is shown in the image. This storm never strengthened beyond the tropical depression stage.

Tropical storm

If the winds continue to increase to sustained gale strength (34 knots or 63 km/h) the NHC upgrades the system to a tropical storm. While the winds may be tame in comparison with a hurricane, rainfalls of 100-200 mm are not uncommon.  The storm is now given a name.

Shown here is Alex, as a tropical storm on both August 1 and August 2, 2004. Alex eventually strengthened to a Category 3 hurricane as it moved through Canadian waters.

Hurricane Alex, August 1, 2004. Photo: NASA
Hurricane Alex, August 1, 2004. Photo: NASA
Hurricane Alex, August 2, 2004. Photo: NASA
Hurricane Alex, August 2, 2004. Photo: NASA

Hurricane

Should the winds reach 64 knots (119 km/h) or more, a hurricane is born. It is at the hurricane stage that the “eye of the storm” is often visible. There are 5 classes of hurricanes on the Saffir-Simpson hurricane scale. Pictured here is Hurricane Alex in 2004.

Hurricane Alex, August 3, 2004. Category 1 hurricane near the Carolinas. Photo: NASA
Hurricane Alex, August 3, 2004. Category 1 hurricane near the Carolinas. Photo: NASA
Hurricane Alex, August 4, 2004. Category 1 hurricane south of Cape Cod. Photo: NASA
Hurricane Alex, August 4, 2004. Category 1 hurricane south of Cape Cod. Photo: NASA

Hurricane Alex, August 5, 2004

Hurricane Alex, the first major hurricane to enter Canadian waters in over 20 years*, was a Category 3 storm as it moved into Canadian waters. It was the first hurricane on record in the Atlantic to reach the Category 3 level at such a northern latitude. Alex’s sustained winds in Canadian waters were estimated at 180 km/h. This is about 50% more destructive than the winds of Hurricane Juan (2003). 

* Debbie (1982) had been the last major hurricane to affect Canadian waters before the arrival of Alex, more than 20 years later.

Hurricane Alex, August 5, 2004. Eye of the storm. Photo: NOAA
Hurricane Alex, August 5, 2004. Eye of the storm. Photo: NOAA
Hurricane Alex storm track, August 5, 2004.
Hurricane Alex storm track, August 5, 2004

Saffir-Simpson Hurricane Scale

The Saffir-Simpson Hurricane Scale is a five-point rating scale used to describe a hurricane's present intensity. This scale is used to estimate the potential property damage and flooding expected along the coast from a hurricane landfall. Wind speed is the determining factor in the scale, as storm surge values are highly dependent on the slope of the continental shelf and the shape of the coastline in the landfall region. (Note: this scale is intended to be used in southern latitudes and has not proven to be that useful for Canada. For example, during Hurricane Juan (2003), the scale was not very helpful because the winds were marginal Category 2 strength yet the damage to trees was at the Category 3 level.)

A full description of the Saffir-Simpson Hurricane Scale is available on the National Hurricane Center website. (http://www.nhc.noaa.gov/aboutsshs.shtml)

Types of tropical cyclone categories. Photo: NOAA
Types of tropical cyclone categories. Photo: NOAA
International Standard for Surface Winds Relating to Tropical Cyclones
 TCTypeWind (km/h)Surge (m)Rain (mm)Waves (m)
Depression> 370.5200+depends
Tropical Storm> 631.0200+depends
Hurricane
Category 1
> 1181.2+200+ depends
Hurricane
Category 2
>1521.8+200+ depends
Hurricane
Category 3
> 1762.7+ 200+ depends
Hurricane
Category 4
>2094.0+200+ depends
Hurricane
Category 5
> 2515.5+200+ depends

Surface winds

Surface winds reported in tropical cyclones follow an international standard: they are maximum sustained winds averaged over 1 minute, taken at the 10-m level (33 feet) above the surface away from buildings, structures and friction of the ground.