Tropical cyclones develop over warm tropical oceans and are low pressure storm systems characterized by strong winds and heavy rainfall. They form when moist air rises rapidly over warm ocean waters, condenses to form thunderstorms, and the Coriolis effect causes the storms to rotate. Tropical cyclones are categorized based on their wind speeds and can cause extensive damage through high winds, storm surges, and flooding. A notable example is the devastating 1970 Bhola cyclone in Bangladesh, which had wind speeds up to 115 mph and a 33 foot storm surge, resulting in 300,000 to 500,000 fatalities.

1. The Physical Characteristics of
Tropical Cyclones
Case Study: The 1970 Bhola Cyclone

2. 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.

3. Cyclones, Hurricanes, Typhoons
 Named according to region
where they develops
 Cyclones – Develop over
the Indian Ocean on worm
waters
Eye
 Hurricanes - Develop over
Atlantic and eastern Pacific
Oceans
 Typhoons – Develop over
western Pacific Oceans
 All produce similar effects
Typhoon Odessa, Philippines

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

5. 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

6. Tropical Cyclones Introduction
 Tropical cyclones are
intense, rotating, lowpressure storms
 Develop over warm,
tropical oceans
 Produce strong winds
and heavy rainfall
 Can lead to immense
flooding
Destructive Winds

7. 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.

8. 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

9. 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

10. 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

11. 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

12. 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.

13. Coriolis Effect
 Northern hemisphere winds are blowing counterclockwise around center core
 Southern hemisphere winds are blowing clockwise
around center core

14. 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

15. 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

16. 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
ITCZ
Tropical Cyclone
Appearances

17. Tropical Cyclone Categories

18. Case Study: The 1970 Bhola Cyclone
Storm Survivors

19. 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
India
Bay of Bengal
Storm Path

20. 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

21. 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
Great Bhola Cyclone

22. 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.

23. 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
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
1970 Bhola cyclone. (2009, November 22). In Wikipedia, The Free Encyclopedia. Retrieved November 23, 2009, from
http://en.wikipedia.org/wiki/1970_Bhola_cyclone
Catto, N., Hyndman, D., Hyndman, D. (2009). Natural Hazards and Disasters. Toronto, ON:
Nelson Education
Earth Science Australia. (2009). Tropical Cyclones. Retrieved November 22, 2009, from
http://earthsci.org/flood/J_Flood04/cyclone/cyclone.html
National Weather Service. (2008, October 8). Tropical Weather. Retrieved November 22, 2009, from
http://oceanservice.noaa.gov/education/yos/resource/JetStream/tropics/tc.htm
Nelson, S.A. (2009, October 30). Exceptional Weather – Tropical Cyclones. Retrieved November 22, 2009, from
http://www.tulane.edu/~sanelson/geol204/tropical_cyclones.htm
Tropical cyclone. (2009, November 22). In Wikipedia, The Free Encyclopedia. Retrieved November 20, 2009, from
http://en.wikipedia.org/wiki/Tropical_cyclone
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