Tropical Storm A tropical cyclone is a storm system with a low-pressure

centre. However, while typical Canadian lows andstorm systems are fueled by a battle between cold andwarm air, tropical cyclones are fueled by a differentprocess. This process involves water being convertedto water vapour, which is then converted back to liquidwater

Warm, moist air moves over the ocean.

Water vapour rises into the atmosphere

As the water vapour rises, it cools and condenses into liquid droplets

Condensation releases heat into the atmosphere, making the air lighter

The warmed air continues to rise, with moist air from the ocean taking its place and creating more wind

Warm ocean waters to fuel the tropical cyclone. Studies have shown that sea surface temperature must be at least 26°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.

Conditions Needed For Tropical Cyclones To Form

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.

When and where tropical cyclones formThe 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.

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

Severe Cyclonic Storm

4. N Indian Ocean(includes Bay of Bengal, Arabian Sea)

April to June and October to December

May and November

Tropical Cyclone

Map Reference Ocean Basin Season Season Peak Name When Winds Exceed 118 km/h

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

All year February to early March

Severe Tropical Cyclone