How and where tropical cyclones form

A tropical cyclone is a storm system with a low-pressure centre. This can take different forms, including a tropical depression, a tropical storm or a hurricane. However, while typical Canadian lows and storm systems are fueled by a battle between cold and warm air, a different process fuels tropical cyclones. This process involves water converting to water vapour, then converting back to liquid water.

Graphic showing the 5 steps of tropical cyclone formation.
Figure 1: steps showing how a tropical cyclone forms. 
Long description
Figure 1: steps showing how a tropical cyclone forms

Step number
Description of event
1 Warm, moist air moves over the ocean.
2 Water vapour rises into the atmosphere.
3 As the water vapour rises, it cools and condenses into liquid droplets.
4 Condensation releases heat into the atmosphere, making the air lighter.
5 The warmed air continues to rise, with moist air from the ocean taking its place and creating more wind.

Most people know that water needs heat to evaporate into vapour. For example, a pot of water is heated on a stove. The heat is what evaporates the water, turning it into water vapour (steam) that gets trapped in the air. The water vapour just above the water in the pot is now hotter than the surrounding air, and it rises. The warm tropical atmosphere is like the stove: it heats up the water at the ocean surface and begins to evaporate it. Like the trapped water vapour in the air in the pot, the water from the ocean surface rises up through the atmosphere, beginning an atmospheric disturbance. The rising air then meets lower pressures and temperatures at higher altitudes, and water vapour begins to condense back into liquid water. We see this as clouds. This is when the interesting physics of tropical cyclone formation really begins.

Many people do not realize that when water vapour cools back into liquid water, the heat is released. Even more heat is given back if the process continues to freezing point. The condensation process essentially extracts heat from the air as water vapour condenses back into liquid water. This is called the latent heat of condensation, where latent heat literally means, “stored heat.”

The heat “stored” in the air at the ocean surface is released back into the atmosphere at higher altitudes when the rising air cools and the water vapour condenses into liquid water. This warming effect at higher altitudes causes the air to move upward because it is now hotter than its surroundings. This then draws air up from below and speeds up the rate of rising air near the surface. Surface air around the growing disturbance rushes in to replace it. This motion of surface air is the wind we feel in a tropical cyclone. Rising columns of air quickly form tropical thunderstorms, resulting in a potential “seed” for a tropical cyclone. The eventual tropical cyclone that forms is called a warm-air or warm-core storm because of the process just described.

The term “cyclone” is a meteorological term and generally refers to any low-pressure area. In the Northern Hemisphere, cyclones rotate counterclockwise; in the Southern Hemisphere, they rotate clockwise.

There are different kinds of cyclones, such as:

  • extratropical cyclones (referring more to the process of formation rather than geographic origin)
  • mesocyclones (the large and intense thunderstorms that spawn tornadoes)
  • tropical cyclones

Conditions needed for tropical cyclones to form

Warm ocean waters to fuel the tropical cyclone

  • Studies have shown that sea surface temperature must be at least 26.5°C
  • This temperature is required to a depth of at least 50 metres
  • Tropical cyclones cannot form outside of the tropics because the water temperatures are too cold

A warm, moist tropical atmosphere

  • Encourages thunderstorm development, the foundation of the latent heat release process that drives tropical cyclones

More than 500 kilometres (about 5° latitude) away from the equator

  • The apparent force of the rotating earth, called the Coriolis force, is necessary to generate the rotation of the growing disturbance
  • The Coriolis force is slight near the equator and gets stronger towards the poles
    • Without it a low pressure cannot be maintained

A pre-existing near-surface disturbance, low-pressure area, or region of convergence

  • Tropical cyclones cannot generate spontaneously and they require a trigger mechanism to begin drawing air inwards at the lowest levels of the atmosphere

Little to no vertical wind shear between the surface and the upper troposphere

  • The troposphere is the upper part of the atmosphere where weather occurs, just below the stratosphere
  • Vertical wind shear is a change of wind speed or direction with increasing altitude
  • Large vertical wind shear disrupts a growing disturbance and can prevent a tropical cyclone from forming
  • Large vertical wind shear can weaken or destroy an already-formed tropical cyclone by tilting it over and poking holes in the warm core
    • This interferes with the processes of deep convection (overturning air) around the cyclone centre

While these conditions are needed to create a tropical cyclone, it does not mean that they are enough to create one. Often all of these conditions exist, yet a tropical cyclone does not form. This is part of the challenge in forecasting when a tropical cyclone will develop--a process known as tropical cyclogenesis.

A high-pressure area located at the top of the troposphere, above the storm or growing disturbance, can also help a tropical cyclone to form. This acts as a “chimney” for the storm that does at least two important things:

  • It takes the rising air away from the storm centre so that it doesn’t pile up above the storm and cause the storm to collapse on itself
  • It draws air up through the storm to help the air rise and keep the low pressure at the surface

When and where tropical cyclones form

The table below outlines the seven basic tropical cyclone “basins,” the times of year when each basin is active, and the names given to the strongest tropical cyclone. Note on the map that tropical cyclones do not form near the equator (the Coriolis force is too weak to initiate rotation) and they do not form far away from the equator (water temperatures are too cold). Therefore, tropical cyclones typically form within a band of latitudes.

Map of tropical cyclones
Figure 2: Tropical cyclones - names and seasons
Long description
Figure 2: Tropical cyclones - names and seasons
Map reference Ocean basin Season Season peak Name when winds exceed 118 km/h
1 N Atlantic Ocean
(includes Caribbean and Gulf of Mexico)
June to November September Hurricane
2 NE Pacific
(east of dateline)
mid May to mid November Late August to early September Hurricane
3 NW Pacific Ocean
(west of dateline, includes S China Sea)
All year Late August to early September Typhoon
4 N Indian Ocean
(includes Bay of Bengal, Arabian Sea)
April to June and October to December May and November Severe Cyclonic Storm
5 SW Indian Ocean October to May mid January to early March Tropical Cyclone
6 SE Indian Ocean
(north of Australia)
October to May mid January to early March Severe Tropical Cyclone
7 SW Pacific Ocean November to April February to early March Severe Tropical Cyclone
Aerial map tracking tropical cyclones around the world
Aerial map of the North Atlantic displaying tropical cyclone tracks
Aerial map of the Northeast Pacific displaying tropical cyclone tracks

Canada’s west coast is not directly hit by Pacific tropical cyclones. However, once or twice a year, remnants of West Pacific typhoons can travel across the ocean, undergo post-tropical transition and impact British Columbia as significant post-tropical cyclones (typical mid-latitude storm systems). The most famous of these storms was ex-Typhoon Freda, in 1962, which wreaked havoc along the west coast of North America. It became known as the infamous Columbus Day Storm.

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