Summary

Tropical cyclones are intense, rotating, low-pressure storms that develop over warm ocean waters. They are accompanied by intense winds and thunderstorms and heavy rainfall. (2)

A Tropical Cyclone can be broken down into three parts; the Eye, the Eyewall, and the Rainbands:
The eye or center of a cyclone is the calmest part of the storm. The eye is usually about 60km wide. (10)
The Eyewall is the closest part to the Eye, as it contains the winds rotating directly around the Eye. This is where the thunderstorms occur that contain heavy rains and the strongest winds. (10) Rainbands consist of curved bands of thunderstorms and clouds that spiral away from the eyewall. The rainband produces tornadoes, rain, and bursts of wind. (10)

There are three main types of Tropical Cyclones; tropical storms, tropical depressions, which progress into the most severe, a cyclone.
-A tropical depression has winds less than 39 m/h (2)
-A tropical storm has winds between 39 m/h and 73 m/h
-A cyclone in the Southern Hemisphere and in the Northern Hemisphere; a typhoon in the west and a hurricane in the east has winds of at least 74 m/h. (10)

Over warm oceans there is moist, humid air. (4) The warm air moves along the surface, converges and rises rapidly creating a low pressure system. (7) As the air rises, it cools and condenses forming thunderclouds. Air moves in at the surface taking the place of the rising air. This intensifies convergence and creates wind. (4) The Coriolis Effect causes winds to spin faster – counter-clockwise in the northern hemisphere and clockwise in the southern hemisphere. (7)

In order for tropical cyclones to develop, ocean temperatures must be 26.5oC, air must be cool and moist at higher levels, the region must be within 500km from the equator, warm air must converge at the surface and occurs only where wind speed/direction remain constant (no wind shear). (9)

Case Study: The 1970 Bhola Cyclone

The 1970 Bhola cyclone was the deadliest tropical cyclone ever recorded. It developed over the Bay of Bengal. It hit East Pakistan (now known as Bangladesh) and the West Bengal of India on November 12, 1970. With wind speeds of up to 115 m/h the cyclone had the strength of a category 3 hurricane. Most of the damage was caused by a 33-foot high storm surge that flooded the low-lying islands of Ganges Delta, wiped out crops, destroyed whole villages, and demolished 85% of the homes in the area. The storm caused 86.4 million dollars in damage and left 300,000 to 500,000 people dead. (8)

The Top Ten Key Points


1. Cyclones develop over warm oceans (at least 26°C) (7)

2. Developmental progression: depression-->storm-->cyclone(2)

3. To be classified as a cyclone, wind speeds must be 74 m/h (2)

4. Tropical cyclones are intense, rotating, low-pressure storms (2)

5. Cyclones produce powerful thunderstorms, heavy rainfall and massive storm surges which result in flooding (2)

6. Warm, moist air rises over oceans causing strong updrafts, cools, condenses and forms clouds (4)

7. Air quickly moves in at the surface to take the place of the rising air, creating strong winds (4)

8. Rising air spirals around the center due to the coriolis effect (7)

9. Tropical cyclones generally occur within the tropics and do not form over the equator where the coriolis effect is greatly reduced (4)

10. The 1970 Bhola Cyclone was the deadliest cyclone recorded with 300,000 – 500,000 fatalities (8)

Bibliography

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

(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

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

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

(9) National Oceanic and Atmospheric Administration. (2007). Frequently Asked Questions: How do tropical cyclones form? Retrieved November 22, 2009, from http://www.aoml.noaa.gov/hrd/tcfaq/A15.html

(11) National Weather Service: National Hurricane Center. (2009). Glossary of NHC Terms. Retrieved November 22, 2009, from http://www.nhc.noaa.gov/aboutgloss.shtml

(10) National Weather Service. (2009). Tropical Cyclone Structure. Retrieved November 21, 2009, from http://www.srh.noaa.gov/jetstream/tropics/tc_structure.htm

(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

(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

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

(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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