© 2012 Pearson Education, Inc.
Lecture Presentation
Chapter 10
Hurricanes and Extratropical
Cyclones
© 2012 Pearson Education, Inc.
Learning Objectives
Understand the weather conditions that create, maintain, and dissipate cyclones
Understand the difficulties in forecasting cyclone behavior
Know what geographic regions are at risk for hurricanes and extratropical cyclones
Understand the effects of cyclones in coastal and inland areas
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Learning Objectives, cont.
Recognize linkages between cyclones and other natural hazards
Know the benefits derived from cyclones
Understand adjustments that can minimize damage and personal injury from coastal cyclones
Know the prudent actions to take for hurricane or extratropical cyclone watches and warnings
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Introduction to Cyclones
An area or center of low pressure with rotating winds Counter-clockwise in Northern Hemisphere Clockwise in Southern Hemisphere
Tropical or extratropical Based on origin and core temperature
Characterized by intensity Sustained wind speeds and lowest atmospheric
temperature
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Tropical and Extratropical Cyclones
Tropical Cyclones Form over warm tropical or subtropical ocean water (5°–20°) Have warm central cores Tropical depressions, tropical storms, hurricanes High winds, heavy rain, surges, and tornadoes Derive energy from warm ocean water and latent heat
Extratropical Cyclones Form over land or water in temperate regions (30°–70°) Associated with fronts and cool central cores Strong windstorms, heavy rains, surges, snowstorms,
blizzards Most do not produce severe weather Derive energy from temperature contrasts along fronts
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Classification
Nor’easter Extratropical cyclone that moves along northward along East
Coast U.S.
Hurricanes Tropical cyclones in Atlantic and eastern Pacific Oceans
Typhoons Tropical cyclones in Pacific Ocean west of International
Dateline and north of the equator
Cyclones Tropical cyclones in Indian Ocean
Saffir-Simpson Scale classifies hurricanes based on wind speed
© 2012 Pearson Education, Inc.Table 10.1
© 2012 Pearson Education, Inc.
Naming
Extratropical storms are sometimes named after their origins Example: Alberta Clipper
Hurricanes named by international agreement through World Meteorological Organization Named once winds exceed 63 km (39 mi.) per hour Names assigned sequentially each year from list for each
origin Male/Female names alternated Names are reused every 6 years Names of big storms are retired (example: Katrina)
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Cyclone Development: Tropical Disturbance
A organized mass of thunderstorms persisting for > 24 hours
Typically 200 to 600 km (120 to 370 mi.)
Has a weak rotation due to Coriolis effect
Formed by Lines of convection Upper-level low pressure troughs Cold front remnants Easterly waves of converging and diverging winds
Atlantic Ocean hurricanes
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Tropical Depressions and Tropical Storms
Tropical Depression Tropical disturbance wind speeds increase and
begins to spin A low pressure center is formed
Tropical Storm Winds increase to 63 km (39 mi.) ph Storm is given a name Wind speeds are not at hurricane strength, but
rainfall can be intense
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Hurricanes
Not all tropical storms develop into hurricanes Classified when winds reach 119 km (74 mi.) per hour
Environmental conditions Thick layer of warm ocean water
Water must be warm and there must be deep
Steep vertical temperature gradient Atmosphere must cool quickly with increasing altitude
Weak vertical wind shear Strong winds aloft prevent hurricane development.
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Hurricane Structure
Rain bands Clouds that spiral inward around center
Counterclockwise in Northern Hemisphere Increase in intensity towards the center of the hurricane
Eyewall Innermost band of clouds Contain the greatest winds and rainfall
Eye Area of calm at center of the hurricane Narrow at surface and wider at top
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Hurricane Structure, cont.
Warm, moist air spirals upward around eyewall Air rises, it loses moisture
Upward rotation draws air from eye, causing dry air to sink back into center
Upward rotation also causes air to flow out the top of the storm concentrated in exhaust jets Allows additional warm air to feed bottom of the
storm
© 2012 Pearson Education, Inc.Figure 10.14
© 2012 Pearson Education, Inc.
Hurricane Paths and Demise
Movement is controlled by the Coriolis effect and steering winds In Northern Hemisphere storms deflect to the right Track west in trade winds and curve northwest and
then northeast Hurricanes can make a loop In North Atlantic, steered by Bermuda High
As hurricane moves over land, it loses energy (warm water) Can become extratropical cyclone
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Extratropical Cyclones
Necessary conditions Strong temperature
gradient at surface usually along cold, warm or stationary fronts
Strong upper level winds provided by jet stream
Polar jet stream Subtropical jet stream
Figure 10.18
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Polar and Tropical Jet Stream
Polar jet stream shifts from crossing the United States in the winter to crossing southern Canada in the summer Subtropical jet stream crosses Mexico and Florida and is
strongest in the winter
Large high-pressure ridges and low-pressure troughs cause jet streams to bend and producing waves or meanders May also split in two around isolated high-pressures and
reunite
Extratropical cyclones often develop in curves or divergences in jet streams
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Polar and Tropical Jet Stream, cont.
Bending or splitting cause the polar jet stream to dip south and the subtropical jet stream to flow northeast
The southern branch of a split polar jet stream in the Pacific Ocean brings warm moist air out of the tropics West Coast forecasters refer to the flow of warm moist
air as the Pineapple Express, because of its origin near Hawai’i
Nor’easters form when bends of the polar and subtropical jet streams begin to merge off the southeastern coast of the United States
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Extratropical Cyclone Development
Low-pressure center develops along frontal boundary Cold front on southwest, warm front on east Conveyor belt of cold air circulates counter-clockwise
Warm air is wedged to the east Conveyor belt of warm air rises on the southeast side creating
a comma Conveyor belt of dry air aloft feeds the cyclone from behind
the cold front
Cold front wraps around the warm front, causing an occluded front develop trapping warm air aloft
Cold air completely displaces the warm air, pressure gradient weakens and storm dissipates
© 2012 Pearson Education, Inc.Figure 10.20
© 2012 Pearson Education, Inc.Figure 10.21
© 2012 Pearson Education, Inc.
Geographic Regions at Risk for Cyclones: North America Hurricanes threaten
contiguous United States, Puerto Rico, the Virgin Islands, and U.S. territories in the Pacific Ocean
They are a lesser threat to Hawai’i and Atlantic Canada
On the Pacific coast, hurricanes strike Baja California and the west coast of the Mexican mainland
Figure 10.22
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Atlantic Hurricane Paths
West toward East coast of Florida, sometimes passing over Caribbean Move out into the Atlantic
Ocean to the northeast
Westward over Cuba and into the Gulf of Mexico to strike the Gulf Coast
Westward to the Caribbean and then northeastward skirting the East Coast May strike the continent
from central Florida to New York
Figure 10.17
© 2012 Pearson Education, Inc.Figure 10.24
© 2012 Pearson Education, Inc.
Geographic Regions at Risk for Cyclones: Worldwide
Northwest Pacific is much more active than North Atlantic
Indian Ocean is also a very active hurricane zone
South Atlantic and southeast Pacific, rarely have hurricanes because of cold ocean water
Hurricanes do not form close to the equator because of the absence of the Coriolis effect
© 2012 Pearson Education, Inc.Figure 10.25
© 2012 Pearson Education, Inc.
Geographic Regions at Risk from Cyclones, Summary
Tropical cyclones East and Gulf Coasts Hawaii and Atlantic Canada Baja California and West Coast Mexico
Extratropical cyclones Winter windstorms in Pacific Coast Winter snow Sierra Nevada, Rocky Mountains and
east Spring and summer thunderstorms and tornadoes in
United States and Canada
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Cyclone Effects: Storm Surge
Local rise in sea level resulting from storm winds
Can be > 3 m (10 ft.)
Because of spinning, surge is greatest in right quadrant of storm as it makes landfall
Height is greatest near time of maximum winds
Height is also greater if landfall coincides with high tide
© 2012 Pearson Education, Inc.Figure 10.26
© 2012 Pearson Education, Inc.
Effects on Storm Surge Magnitude
Largest effect from stress exerted by wind on water Fetch refers to the area over which the wind blows Larger fetch results in larger storm surge
Smaller effect from low atmospheric pressure in storm pulling up on water surface
Also depends on shape of coastline
Water level tends to increase continually as storm approaches
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Cyclone Effects: High Winds
Described by Saffir-Simpson Scale
Decrease exponentially with landfall
Strongest recorded winds in United States from extratropical cyclone Responsible for strong winds in blizzards and
tornadoes
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Cyclone Effects: Heavy Rains
Average hurricane produces trillion gallons of water
Rainfall from cyclones can cause inland flooding
Flooding affected by: Storm’s speed Land elevation over which the storm moves Interaction with other weather systems Amount of water in soil, streams and lakes prior to
storm
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Links to Other Natural Disasters
Coastal erosion
Flooding
Mass wasting
Other types of severe weather Tornadoes, severe thunderstorms, snowstorms,
and blizzards
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Natural Service Functions of Cyclones
Source of precipitation
Redistribute warm air from tropics
Maintain ecosystems Winds carry plants, animals, and
microorganisms Waves stir up deeper, nutrient-rich waters Winds topple weak and diseased trees in forests Waves break apart some corals
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Human Interaction with Cyclones
Urbanization of vulnerable coastlines increases magnitude of the effect of cyclones
Destruction of sand dunes makes areas more susceptible to hurricane winds
Construction of seawalls and bulkheads reflect waves and contribute to beach erosion
Poor building materials and practices can make hurricanes more dangerous to people
Global warming may contribute to higher intensity and frequency of hurricanes in the future
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Minimizing the Effects of Cyclones: Forecasting and Warnings Forecast includes:
If it will make landfall Where and when Wind strength Rainfall amount Storm surge
Monitored by U.S. Hurricane Center, Canadian Hurricane Center
Hurricane watch means likely hurricane in 36 hours
Hurricane warning given when hurricane is likely within 24 hours or less
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Forecasting Tools
Weather satellites Detect early warning signs Can not show wind speed
Aircraft U.S. Air Force, NOAA airplanes fly into the storm to
collect data
Doppler radar Give information on rainfall, wind speed, and
direction of the storm
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Forecasting Tools, cont.
Weather buoys Automated weather stations that give
information at their locations
Computer models Predict and track hurricane progress Have vastly improved hurricane information Still lacking in predicting storm intensity
© 2012 Pearson Education, Inc.Figure 10.32
© 2012 Pearson Education, Inc.
Storm Surge Predictions
Predict the time and elevation of surge Forecasters use wind speed, fetch and average
water depth
Need detailed information on topography Different elevations on land affect the storm
surge
Computer models use central pressure, size, forward speed, track, wind speed, and seafloor topography
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Hurricane Prediction and the Future
Deaths have decreased dramatically because of better forecasting, improved education, and greater public awareness
However, coastal populations are increasing, increasing risk
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Perception of and Adjustment to Cyclones
Perception of hazard depends on personal experience More experienced people may take hazard more
seriously More seasoned people may also take less precautions
Community adjustments to cyclone hazard Warning systems Evacuation plans and shelters Insurance Building design
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Perception of and Adjustment to Cyclones, cont.
Personal adjustments to cyclone hazard Be aware of hurricane season Prepare homes and property for hazard Obtain flood insurance Install heavy shutters that can be latched Learn evacuation route Make a family emergency plan Collect emergency supplies
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End
Hurricanes and Extratropical Cyclones
Chapter 10
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