How and Where Hurricanes Form

In Canada, our storm systems are fueled by a battle between cold and warm air. Tropical cyclones, however, are fueled by a different process. This process involves water being converted to water vapour, which is then converted back to liquid water. This process releases a lot of heat, which powers the storm. How a Hurricane Forms:

Number Description of Event 1 Warm, moist air moves over the ocean.

2 Water vapour rises into the atmosphere. As the water vapour rises, it cools and condenses into liquid droplets. 3 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 4 creating more wind.

As the process continues, the air moves faster and faster. Winds caused by the Coriolis Effect cause the rising air to begin to spin. The longer the system travels 5 over the water and warmer the water, the larger and faster the storm becomes. Without the warm water, the storm dies out. Conditions needed for tropical cyclones to form

Warm ocean water is not the only ingredient needed for tropical cyclones (hurricanes) 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 process that releases heat higher up in the atmosphere, the engine that powers tropical cyclones.  More than 500 km (about 5° latitude) away from the equator. This is important because the Coriolis force--the force created by the rotating earth--is necessary to generate the rotation of the developing storm. The Coriolis force is small near the equator and gets stronger towards the poles.  A pre-existing weather disturbance (storm conditions). This is necessary because tropical cyclones cannot generate spontaneously and they require a trigger mechanism to begin drawing air inwards.

As well, while it is not required, a high-pressure area with fast moving winds located above the storm can greatly help a tropical cyclone to form. This acts as a “chimney” for the storm, and 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

Hurricanes are intense tropical cyclones, capable of delivering winds of extreme velocity and causing massive damage to a large area. The name hurricane is used specifically for tropical cyclones in the Atlantic Ocean, but other names are used in other part of the world. Typhoons affect the China Sea and the coasts of Japan, Philippines and China, willy-willies start in the south western Pacific and track towards Queensland and Papua New Guinea, Cyclones in the Indian Ocean affect the Bay of Bengal, Madagascar and the eastern coast of Africa. Most tropical cyclones form in a worldwide band of thunderstorm activity known as the Intertropical convergence zone (ITCZ). Worldwide, an average of 80 tropical cyclones form each year.

Once the hurricane moves over land, or over colder water, it loses its source of latent heat energy and rapidly decays. Atlantic hurricanes track across the Caribbean islands, towards the USA, crossing Florida and into the Gulf of Mexico or swinging north to the coast of Carolina. As the hurricane makes landfall it rapidly loses energy and wind speeds drop - the hurricane is downgraded to a tropical storm and then to a tropical depression.

StormThe surge table isbelow water outlines from the sevenocean basic that tropical is pushed cyclone “zones,” the times of year when each area is active, and the towardnames the given shore to bythe the storm force type. of Note the windson the swirlingmap that tropical cyclones don’t form near the equator (the Coriolis force is too aroundweak the to hurricane.begin the rotation) This advancing and they don’tsurge form combines far away from the equator (water temperatures are too cold). Therefore, withtropical the normal cyclones tides typically and can form increase within thea band water of latitudes. level by 9 m or more. Storm surge combined with waves can cause extensive damage.

Tropical Cyclones – Names and Seasons, from Figure 2

Ocean Basin Season Season Peak Name When Winds Map Ref. Exceed 118 km/h the storm is called a: 1 N Atlantic Ocean June to November September Hurricane (Caribbean & Gulf of Mexico) NE Pacific mid May to mid November August to Sept Hurricane 2 3 NW Pacific Ocean All year August to Sept Typhoon April - June & 4 N Indian Ocean May & November Cyclone October - December

5 & 6 SW Indian Ocean October to May Jan - early March Cyclone

7 SW Pacific Ocean November to April Feb - early March Cyclone

The eye is a region of mostly calm weather at the center of strong tropical cyclones. The eye of a storm is a roughly circular area, typically 30–65 km in diameter. It is surrounded by the eyewall, a ring of towering thunderstorms where the most severe weather occurs. As a hurricane crosses over land or cold water, it begins to dissipate and reduce in strength. This is because it is no longer over the warm ocean water that it needs for energy. At this point, a hurricane can still cause a lot of damage because of high winds, rain, and flooding, but it is downgraded from a hurricane back to a tropical storm. Saffir-Simpson Hurricane Wind Scale Category Maximum Wind Storm surge Damage prediction speed (kph) (ft)

Depression less than 60 Storm 60 - 119 Hurricane 1 119- 150 4 - 5 Minimal: damage to roof, shingles, siding and gutters. Large branches might break. Trees with shallow roots may be uprooted. Hurricane 2 150 - 175 6 - 8 Moderate: homes could sustain major roof and siding damage. Many shallowly rooted trees will be snapped or uprooted. Power loss is expected with outages that could last from several days to weeks. Hurricane 3 175 - 210 9 - 12 Extensive: homes may incur major damage or removal of roof decking and gable ends. Many trees will be snapped or uprooted. Electricity and water will be unavailable for several days to weeks after the storm passes. Hurricane 4 210 - 250 13 - 18 Extreme: homes can sustain severe damage with loss of most of the roof structure and/or some exterior walls. Most trees will be snapped or uprooted and power poles downed. Power outages will last weeks to possibly months. Most of the area will be uninhabitable for weeks or months. Hurricane 5 more than 250 more than 18 Catastrophic: A high percentage of framed homes will be destroyed, with total roof failure and wall collapse. Fallen trees and power poles will isolate residential areas. Power outages will last for weeks to possibly months. Most of the area will be uninhabitable for weeks or months.

To Recap 1) The sun heats the water near the equator most between the months of June to November. As the water heats up, it also heats the air above it and a lot of water evaporates into the air.

2) As you have learned, heated air particles will begin to move faster and further apart. The density of this air decreases and so the warmed air rises.

4) As this moist air rises into the atmosphere its moisture begins to condense. The process of condensation releases a lot of heat. This heat further increases the rate at which the air rises. This produces strong convection currents. These rapidly rising air particles begin to turn to the right due to the rotation of the Earth. The air mass begins to spin in a counter-clockwise rotation.

5) Back down at the surface of the water, a low pressure is created due to the decrease in number of air particles.

6) The faster this upward rotation occurs; the faster the surface particles rush into the center of the storm (the low pressure).

7) This pattern of increasingly faster pull of the surface air particles into the low and up out of the high pressures area at the top cause the speed of the rotation of the air particles (wind) until it reaches incredibly high rates.

9) Wind speeds of 62 km/hr are called tropical storms. Wind speeds of 119 km/hr are called HURRICANES. Hurricanes last until their source of energy is gone. Though the wind speeds in tornadoes are higher than in hurricanes, hurricanes are much larger (100’s of km’s across). They also last for many days. Tornadoes are much narrower and last only minutes – though their affects can be devastating. Hurricane Question Sheet

Name: ______Date: ______

1) When is the peak season for hurricane in the North Atlantic?

2) What is the importance of the Coriolis Effect? Why can't hurricanes form just off the coast of Nova Scotia?

3) How many cyclones happen in an average year? ______

4) Where do most hurricanes occur (in which "zone" of the earth)?

5) What are the winds speeds of a Category 5 hurricane? What type of damage could occur during a Category 5 hurricane?

6) How is the formation of a Canadian storm different from a the formation of a hurricane?

7) Where would you find the strongest winds in a hurricane? 8) What is the eye of a hurricane?

9) What do they call a hurricane in the North Western Pacific? ______

10) What is storm surge?

11) Why can't hurricanes form directly over the equator?

12) What happens to the strength of a hurricanes makes land fall? Why?

13) Which is worse, a hurricane or a tornado?

14) Draw a diagram that shows how a hurricane forms: