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Lecture 3

GEOL 106 Lecture Notes - Lecture 3: Andrea Gail, Freezing Rain, Landfall

Geological Sciences and Geological Engineering
Course Code
GEOL 106
John Hanes

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Week 3: Hurricanes
Cyclones: What are They?
An area of low atmospheric pressure characterized by rotating winds
Rotation is:
o Counterclockwise in the northern hemisphere
o Clockwise in the southern hemisphere
Rotation deflection is a result of the Coriolis Effect
Coriolis Effect
A phenomenon that causes water and air to curve as they travel across or above the earth's
Earth rotates at an effectively constant angular velocity, different latitudes have different
linear speeds
o Equator: ca. 1970 km/h
o 60 degree latitude: ca. 800 km/h
Influences big, slow moving things (global weather patterns and storms) NOT small fast things
(toilet & sink water draining)
Classifying Clones
Scientific classifications:
o Extratropical cyclones- cyclones in the mid-latitudes; have their own classifications
o Hurricanes- tropical cyclones in the Atlantic and Eastern Pacific Oceans
o Typhoons- tropical cyclones in Pacific Ocean west of International Dateline and north of
o Cyclones- tropical cyclones in the Indian Ocean
Tropical Cyclones
We call them hurricanes (also called typhoons, cyclones)
Have warm central cores
Derive energy from warm ocean water and latent heat
Must have:
o Warm ocean waters (above 26 degrees Celsius)
o Coriolis force
o Sustained wind speed of at least 119 km/h
Hurricane Structure
Rain bands
o Areas of clouds and rain containing numerous thunderstorms that spiral around a
o Thunderstorms and surface winds increase in intensity towards the centre of the storm
o Innermost band of clouds and rain
o Contains the most intense winds and rainfall
o A circular area of calm conditions and broken clouds in the centre
o Low barometric pressure
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Formation of Hurricanes
Most start as tropical disturbance- an organized mass of thunderstorms persisting for over 24
hours (200-600 km in diameter_
Associated with elongated areas of low pressure (trough)
Has week rotation (Coriolis effect)
In the Atlantic ocean, most tropical disturbances start from easterly waves that form over
western Africa
Tropical depression:
o Winds begin to increase in speed and spiral
o Formation of a low-pressure centre
Tropical storm:
o Winds increase to 63 km/h
o Storm is given a name
o Wind speeds are not yet hurricane strength, but rainfall can be intense
o Not all tropical storms develop into hurricanes
Classified when winds reach 119 km/h
Environmental conditions:
o Thick layer of warm ocean water
o Steep vertical temp gradient (atmosphere cools quickly with increasing altitude)
o Weak vertical wind shear (strong winds aloft prevent hurricane development)
Naming Hurricanes
Only a small percentage are given names
Tropical storms and hurricanes given names established by international agreement through
the World Meteorological Organization
o Named once winds exceed 63 km/h
o Names are assigned sequentially each year from a list for each origin (21 names on each
o Male/female names alternated
o Names are re-used every 6 years
o Names of major storms are retired (Katrina)
Hurricane Season
Late summer/early fall
o On land, highest temp occur ca. 4-6 weeks after summer solstice (June 21)
o Oceans have higher heat capacity, reach highest temperature ca. 3 months after the
summer solstice
Northern hemisphere: July- October
Southern Hemisphere: January-April
Tropical Cyclone Berguitta
Formed into a tropical cyclone on January 14, 2018
Sustained wind speed measured at 157 km/h on January 16 (cat. 2)
Rainfall rate >176 mm/h
Tracking towards the island of Mauritius
Warm ocean temperatures in the Indian ocean
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Veers west and towards the south pole, quickly looses power as Ocean temperature cools
Saffir-Simpson Hurricane Scale
Hurricane Movement
Atlantic Ocean: (check power point for paths)
o Starts in the central Atlantic Ocean (east of this map_
o All 3 common paths threaten the Caribbean Islands
o Path 1 can affect Bermuda
o Path 2 can affect the Gulf Coast
o Path 3 can affect the East Coast of USA and Canada
Geographical Regions At Risk For Tropical Cyclones
Most serious threat in North America is to:
o Eastern contiguous United States
o Caribbean Islands
o Gulf Coast
Lesser threat to:
o Hawaii
o Atlantic Canada
o Baja California and west coast of Mexican mainland
Tropical Cyclones: Hazards and Risks
Main threats are wind, rainfall and storm surge
Wind Damage
Sustained winds can be as high as 310 km/h with gusts exceeding 320 km/h
Pressure exerted on a surface is roughly proportional to the square of the wind speed
(exponential relationship)
Partial vacuums are created, increasing damage to structures
Average hurricane produces a trillion gallons of water every day
Rainfall is heaviest when a hurricane passes over (or close to) a mountainous island
o L Reunion Island in the Indian Ocean has set five rainfall records during tropical
cyclones: 1144 mm in 12 hours; 1825 mm in 24 hours
Rainfall can cause inland flooding severity depends on:
o Speed of storm
o Changes in land elevation
o Interactions with other weather systems
o Amount of water in the soil, streams and lakes before storm arrives
Rainfall: Hurricane Ivan
Hurricane Ivan hit the East Coast of the US in 2004
Widespread inland flooding from Georgia to Ohio, Pennsylvania and southern New York State
Caused, in part, by:
o Slow moving storm (Ivan)
o Hilly and mountainous terrain of the Appalachian mountains
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