1. Hurricanes and Extratropical Cyclones
Chapter 10
Hurricanes and Extratropical Cyclones

2. 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

3. 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

4. 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

5. Tropical and Extratropical 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

6. Classification Nor’easter Hurricanes Typhoons Cyclones
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

7. Table 10.1

8. 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)

9. 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

10. Tropical Depressions and Tropical Storms
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

11. 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.

12. Hurricane Structure Rain bands Eyewall Eye
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

13. 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

14. Figure 10.14

15. 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

16. 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

17. 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

18. 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

19. 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

20. Figure 10.20

21. Figure 10.21

22. 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

23. 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

24. Figure 10.24

25. 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

26. Figure 10.25

27. 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

28. 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

29. Figure 10.26

30. 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

31. 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

32. 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

33. Links to Other Natural Disasters
Coastal erosion
Flooding
Mass wasting
Other types of severe weather
Tornadoes, severe thunderstorms, snowstorms, and blizzards

34. 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

35. 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

36. 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

37. Forecasting Tools Weather satellites Aircraft Doppler radar
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

38. 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

39. Figure 10.32

40. 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

41. 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

42. 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

43. 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

44. Hurricanes and Extratropical Cyclones Chapter 10
End
Hurricanes and Extratropical Cyclones
Chapter 10