Natural Disasters Topic 10 (Cyclones)
Overview of cyclonic storms for a GE-level course in natural disasters.
Published on: Mar 3, 2016
Transcripts - Natural Disasters Topic 10 (Cyclones)
Cyclones are centers of
by inward rotating
Tropical Cyclone Hamish (Australia), 2009
In North America, types
of cyclones include
tropical storms, and
Cyclones & the Coriolis Effect
Because of the Coriolis Effect, Northern Hemisphere storms track
clockwise with counterclockwise surface winds that converge inward
and clockwise high altitude winds that diverge outward. Those in the
Southern Hemisphere have the opposite relationship.
Cyclone Structure (wind)
Cyclone Structure (precipitation)
Rising air leads to the formation of rainbands (spiraled arms) that decrease
in magnitude outward.
“The Eye of the Storm”
The eye of the cyclone is an area of high pressure at the center of the
storm. It is characterized by clear, calm skies.
Tropical Cyclone Formation
1. A low-pressure system develops as air rises over
warm ocean water (at least 800 F to depths of 150 ft
with no vertical wind shear).
2. Because of the Coriolis Effect, air begins to spiral
toward the center of the low-pressure cell, increasing
in velocity as it approaches the eye-wall.
3. The incoming air rises and condenses as it loses heat,
creating an area of high pressure at the center, which
becomes the “eye” of the storm.
4. The storm continues to grow as it moves across warm
ocean surface water.
5. Upon landfall or passing over cold water, the cycle
breaks, and the storm begins to wane.
Stages of Development
• Tropical Disturbance:Winds weak and unorganized
• Tropical Depression:Winds less than 39 mph
• Tropical Storm:Winds 39 to 74 mph
• Cyclone:Winds greater than 74 mph
Hurricane Bill, 2009
Hurricane Dean, 2007
Because of the Coriolis Effect, most cyclones
track clockwise in the Northern Hemisphere
and counterclockwise in the southern, though
the amount of deflection can vary. A few have
relatively straight paths.
Types of Cyclones
Cyclones have regional names, such as hurricane and typhoon,
conforming to local traditions.
Hurricanes are cyclones that originate in the south Atlantic Ocean,
moving to the northwest toward the Gulf Coast. Hurricanes have wind
speeds of at least 75 mpg and can approach 200 mpg. They are named
for Hurican, the evil Caribbean god of winds and destruction.
Typhoons are the same as hurricanes but occur in the northern Pacific
Ocean. They come from Chinese for “scary wind” or “wind from four
The Saffir-Simpson Scale
Number 1 2 3 4 5
(mb) > 980 965–979 945–964 920–944 < 920
(mi/hr) 74–95 96–110 111–130 131–155 > 155
surge (ft) 4–5 6–8 9–12 13–18 > 18
damage minimal moderate extensive extreme catastrophic
Cyclones are classified on the basis of wind speed, as defined
by the Saffir-Simpson Scale.
• Largest recorded hurricane: Typhoon Tip, Northwest Pacific,
12 October 1979
• Smallest recorded hurricane: Tropical Cyclone Tracy, near
Australia, 24 December 1974
storms are classified
on the basis of wind
speed, not size.
Cyclone Frequency (annual)
Tropical cyclones rely on dissipation of rising heat as an energy source;
therefore, they are most frequent during time of the year when ocean
surface water is warm.
Cyclone Frequency (long-term)
Since 1851, when we began keeping records, cyclone frequency appears
to be cyclical, most likely tied to solar energy output.
The major effects of cyclones include high winds, heavy rains, storm
surges, and tornadoes.
Wind speeds range from 75 to close to 200 mph, causing a tremendous
amount of damage.
Tornadoes are commonly associated with cyclonic storms, particularly
along the leading edge.
(Graphic by Robert Simmon, NASA GSFC)
As a cyclone approaches land, sea-level rises (storm tide) due to low
pressure conditions. This combines with wind-driven surface waves to
produce high, surging waves along the coastline. The surge is greatest
along the “front right” margin.
Hurricane Eloise, 1975, Florida Panhandle
Storm surges are the greatest cause
of damage and death associated
with cyclonic storms.
Near the eye-wall, precipitation rates can exceed 10 in/hr, leading to
severe flooding, including far inland from coastal regions.
Nor-easters are extratropical cyclones that form in the North Atlantic
during cold weather when the jet stream drops southward bringing
arctic air into contact with warmer air masses. Such storms are
characterized over land by large snowstorms and blizzards. Wind
speeds can approach those of small hurricanes.