The Physical Characteristics of Tropical Cyclones

Case Study: The 1970 Bhola Cyclone

Tropical Cyclones�

• A tropical cyclone is a storm system characterized by a large low-pressure center and numerous thunderstorms
• Tropical Cyclones: Grounds of rotating, low-pressure storms that form around the equator.

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Cyclones, Hurricanes, Typhoons

• Named according to region where they develops
• Cyclones – Develop over the Indian Ocean on worm waters
• Hurricanes - Develop over Atlantic and eastern Pacific Oceans
• Typhoons – Develop over western Pacific Oceans
• All produce similar effects

Typhoon Odessa, Philippines

Eye

Fact.

The strongest of these cyclones storms are known in the US and other parts of the Atlantic ocean

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Developmental Stages

• Progression: depression storm cyclone
• Classified according to wind speeds
• Tropical Depression - less than 39 m/h
• Tropical Storm - between 39 m/h and 73 m/h
• Tropical Cyclone - at least 74 m/h

Tropical Cyclones Introduction

• Tropical cyclones are intense, rotating, low-pressure storms
• Develop over warm, tropical oceans
• Produce strong winds and heavy rainfall
• Can lead to immense flooding

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Destructive Winds

Formation of tropical cyclones

• Tropical Cyclones require to basic conditions to form: an abundant supply of very warm ocean water and some sort of disturbance to lift warm air and keep it rising.

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Formation of Tropical Cyclones

• Warm water generates formation of tropical cyclones
• Ocean temperatures must be at least 26.5°C which is worm
• Over warm oceans moist, humid air (water vapour)
• Warm air moves along the surface
• This air converges and rises rapidly
• Intense rising of air forms low pressure systems

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Formation of Tropical Cyclones

• As air rises, it cools, becomes saturated and condenses
• Water vapour condenses into liquid droplets
• Formation of thunderclouds
• Condensation releases heat stored in water vapour
• This latent heat warms the atmosphere
• Air becomes lighter causing strong updrafts

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Formation of Tropical Cyclones

• Air expands and diverges at higher levels
• Occurs only where wind speed/direction remain constant (no wind shear)
• Air moves in at surface taking the place of rising air
• This intensifies convergence
• Creates wind and increases rising movement
• Builds and progresses into a mature cyclone

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Coriolis Effect

• Spinning of earth on its axis produces wind deflections (Coriolis Effect)
• Causes rising air to spiral around center (core)
• Spirals with great force
• Winds are now rotating, rising and moving in to fill spaces
• Wind speeds increase and cyclone grows

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Conditions

• These conditions exist in all tropical oceans except the south Atlantic ocean and the pacific, west of the South American coast ocean waters in these areas are somewhat cooler.

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Coriolis Effect

• Northern hemisphere winds are blowing counter-clockwise around center core
• Southern hemisphere winds are blowing clockwise around center core

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The Eye, Eyewall, Rainbands

• The Eye - the clear, calm center of storm
• Cool air descends into this center (downdrafts)
• Eyewall – ring of thunderstorms close to eye
• Eyewall produces the most devastating winds
• Rainbands – curved groups of clouds and thunderstorms
• Rainbands travel away from eye in a spiral motion

Storm Surges

• Occur when tropical cyclones reach land
• Storm surge – raised swell of water
• 60 to 80 km across
• Around 2 to 5 meters higher than normal tides
• Created by heavy winds and cause flooding

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Locations of Tropical Cyclones

• Between 10 to 30° North and South of equator
• Within the tropic of Cancer and Capricorn Poleward side of the Intertropical Convergence Zone
• Zone of low pressure (ITCZ)
• Coriolis effect greatly reduced at equator No tropical cyclones occur at equator

Tropical Cyclone Appearances

ITCZ

Tropical Cyclone Categories

Case Study: The 1970 Bhola Cyclone

Storm Survivors

The 1970 Bhola Cyclone

• November 12, 1970 The deadliest tropical cyclone recorded Developed over Bay of Bengal Gathered speed as it moved northward Devastated East Pakistan (Bangladesh) and West Bengal of India

Storm Path

India

Bay of Bengal

The Bhola Cyclone Characteristics

• Wind speeds reached 115 m/h Strength of a category 3 hurricane 33 foot high storm surge Massive storm surge caused flooding and fatalities

Bhola Cyclone on November 11, 1970

The Bhola Cyclone - Damage

• Storm surge:
• Flooded low-lying islands
• Wiped out crops
• Destroyed whole villages
• Demolished 85% of homes in the area Estimated damage at 86.4 million dollars 300,000 - 500,000 people were killed

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Great Bhola Cyclone

Mid-latitude Storms

• Unlike mid latitude storms that derive their energy from the contrast between warm and cold air masses. Tropical Cyclones oceans as water evaporates from the ocean surface.

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Ocean Surface

• The ocean surface latent heat is later released when the air begins to rise and water vapor condenses into clouds and rain

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References

• ^{Ashley Proctor Samone Proctor Chelsea Addison}

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• ⁽³⁾ Australian Government Bureau of Meteorology. (2009). About Tropical Cyclones. Retrieved November 22, 2009, from http://www.bom.gov.au/weather/cyclone/about/about-tropical-cyclones.shtml

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• ⁽⁸⁾ 1970 Bhola cyclone. (2009, November 22). In Wikipedia, The Free Encyclopedia. Retrieved November 23, 2009, from http://en.wikipedia.org/wiki/1970_Bhola_cyclone

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• ⁽¹⁾ Catto, N., Hyndman, D., Hyndman, D. (2009). Natural Hazards and Disasters. Toronto, ON: Nelson Education

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• ⁽⁶⁾ Earth Science Australia. (2009). Tropical Cyclones. Retrieved November 22, 2009, from http://earthsci.org/flood/J_Flood04/cyclone/cyclone.html

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• ⁽⁴⁾ National Weather Service. (2008, October 8). Tropical Weather. Retrieved November 22, 2009, from http://oceanservice.noaa.gov/education/yos/resource/JetStream/tropics/tc.htm

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• ⁽⁵⁾ Nelson, S.A. (2009, October 30). Exceptional Weather – Tropical Cyclones. Retrieved November 22, 2009, from http://www.tulane.edu/~sanelson/geol204/tropical_cyclones.htm

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• ⁽²⁾ Tropical cyclone. (2009, November 22). In Wikipedia, The Free Encyclopedia. Retrieved November 20, 2009, from http://en.wikipedia.org/wiki/Tropical_cyclone

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• ⁽⁷⁾ WikiAnswers. (2009). How do cyclones form? Retrieved November 23, 2009, from http://wiki.answers.com/Q/How_do_cyclones_form&alreadyAsked=1&rtitle=What_causes_cyclones

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