How and Where Hurricanes Form

A tropical cyclone is a storm system with a low-pressure centre. However, while typical Canadian lows and storm systems are fueled by a battle between cold and warm air, tropical cyclones are fueled by a different process.  This process involves water being converted to water vapour, which is then converted back to liquid water.

Most people know that heat is required to evaporate water. For example, a pot of water is put on a stove and heated. 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. And like the trapped water vapour in the air in the pot, the water from the ocean surface rises up through the atmosphere.  An atmospheric disturbance has begun! 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.

While most people know that it takes heat to evaporate water and turn it into water vapour, many don’t realize that when water vapour is cooled back into liquid water, the heat is released. Even more heat is given back if the process is continued to the point of freezing. The condensation process essentially extracts heat from the air as water vapour is condensed back into liquid water. This is referred to as the latent heat of condensation, where latent heat literally means “stored heat.”

The heat that was “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 accelerate 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, and the result is a potential “seed” for a tropical cyclone. The eventual tropical cyclone that forms will be referred to as 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); and tropical cyclones.

Conditions needed for tropical cyclones to form

Warm ocean water is not the only ingredient needed for tropical cyclones to form. Each of the following conditions must be in place:

• Warm ocean waters to fuel the tropical cyclone. Studies have shown that sea surface temperature must be at least 26.5°C, and this temperature is actually required to a depth of at least 50 m. That’s why tropical cyclones can’t form outside of the tropics--water temperatures are too cold.
• A warm, moist tropical atmosphere that encourages thunderstorm development. Thunderstorm development is the foundation of the latent heat release process, the driving mechanism of tropical cyclones.
• More than 500 km (about 5° latitude) away from the equator. This is important because the Coriolis force--the apparent force of the rotating earth--is necessary to generate the rotation of the growing disturbance, and without it a low pressure cannot be maintained. The Coriolis force is slight near the equator and gets stronger towards the poles.
• A pre-existing near-surface disturbance, low-pressure area or region of convergence. This is necessary because 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 upper part of the atmosphere where weather occurs, just below the stratosphere). Vertical wind shear is simply 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. If a tropical cyclone has already formed, large vertical wind shear can weaken or destroy it by interfering with the processes of deep convection (overturning air) around the cyclone centre by things such as tilting them over and poking holes in the warm core.

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.

As well, while it is not required, a high-pressure area located at the top of the troposphere above the storm or growing disturbance can greatly help a tropical cyclone to form. This acts as a “chimney” for the storm, and it does at least two important things: 1) 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, and 2) 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 don’t form near the equator (the Coriolis force is too weak to initiate rotation) and they don’t form far away from the equator (water temperatures are too cold). Therefore, tropical cyclones typically form within a band of latitudes.

Canada’s west coast does not get hit directly by Pacific tropical cyclones. However, once or twice a year, remnants of West Pacific typhoons can travel across the ocean, undergo extratropical transition and impact British Columbia as significant extratropical 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.