Hurricane Principles

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Hurricane Isabel, 2003

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Outline

• Definitions

• Formation and Conditions Needed

• Growth and Structure of a Hurricane

• Where do They Form?

• Measurement

• Damage

• Forecasting

What are hurricanes?

• Intense storm of tropical origin– Typhoon in North Pacific– Cyclone in India, Australia

• Winds sustained at 75+ mph

• ~80 form every year– Usually only few hit land/U.S.– Average ~30-40 fatalities/year

Formation Requirements

• Shallow ocean water 80°F or more

• Warm humid air above water

• Weak upper level winds, blowing in direction of developing storm

• Timing: Summer, early Fall – Season in U.S. is June-November

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Stages of hurricane development

Beginnings

• Low pressure with cluster of thunderstorms– Important for circulation– Tropical disturbance stage

• Thunderstorms grow, winds strengthen

Rotation

• Surface winds converge, thunderstorms become more organized– Converge in CCW fashion due to Coriolis force– Only form ~5-20° latitude, not at equator

(Coriolis force = 0)

Strengthening

• Organization - means more warm moist air rising– Cools, releases massive amount of latent heat

during condensation– Warms surrounding air, causes more updrafts,

brings more air from below• FUEL for more t-storms

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Into a Hurricane

• More fuel, more rotation, more speed

• Tropical storm: surface wind speeds between 39-74 mph

• Hurricane: surface winds sustained at 74+ mph– Also development of eye

Eye of Hurricane

• Develops at ~74 mph wind speed– Why? Harder for rotating winds to reach

surface

• Calm area, clear and cloud free– Why? Cold air sinking in eye, as descends,

warms and absorbs moisture

Eye Wall

• Cylinder of upward winds

• Strongest winds

• Heaviest precipitation

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Eye and Eyewall structure

Conditions in Storm from West to East

• Approach: overcast sky, pressure begins to drop• Towards eye: increasing wind speeds, huge waves

(up to 30 ft), heavy rain• In eye: air temperature increases, low wind, no

rain, bright sky, lowest pressure• East of eye: heavy rain, strong winds• Move away from eye: pressure rises, winds and

rain decrease

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Intensity

• Controlling factors: – Temperature of water– Release of latent heat– Why? Higher temperature water can drive more

t-storms

Duration of Storm

• Most last ~1 week

• Longer if they stay over warm water

• Shorter if they move over cold water or land

• Why?– Energy source gone– More friction for winds

Where do they form?

• Not at equator (no Coriolis force)

• Subtropics ~5-20° latitude– Then move to higher latitudes– Path can vary based on details of high/low

pressure systems it encounters

• Majority form SE Asia, India, Australia

General origin points and paths of hurricanes/cyclones

Origins of Atlantic Hurricanes

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Measuring Size

• Saffir-Simpson scale

• Based on wind speed, pressure– Expected storm surge, possible damage– Category 1-5

Saffir-Simpson Scale

Identification

• Names assigned at tropical storm strength

• Currently alternate male and female names alphabetically– System started in 1979

– 6 lists are repeated

– Names retired if storm is very costly/deadly

Atlantic List 2005 Arlene Bret Cindy Dennis Emily Franklin Gert Harvey Irene Jose Katrina Lee Maria Nate Ophelia Philippe Rita Stan Tammy Vince Wilma

Damage from Hurricanes

• High winds

• Storm surge

• Heavy rain

• Mudslides

Winds• Can be over 155 mph

(Category 5)• Impact can depend on which

side of storm hits– Wind speed can be additive in

direction of storm

• Can also generate large (10-15 m) waves that hit shorelines in advance of hurricane

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Wind damage in Florida - Andrew 1992

Storm Surge

• Produce much of damage, fatalities

• Abnormal rise in water level of few meters

Storm surge is a big problem for low-lying areas

Deep water coastlines not as much of a problem

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Effects of storm surge on Florida coastline

Storm Surge

• Why? – Eye is low pressure zone, local sea level rises– Winds near eye push water into mound, leads to

big surge of water hit shore, move far inland

• Surge can be 20-30 ft– Think about New Orleans, much below sea

level

Heavy Rain

• After moving inland, no more water vapor to add fuel

• But there is a lot in the cloud already!– Will fall as rain– Can be significant (measured in ft)– Leads to flooding

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Downtown Houston after Tropical Storm Allison, 2001

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Mudslides

• Many examples of rain soaked hillslopes failing after hurricanes

Forecasting: Seasonal

• General observations for Atlantic hurricanes

• More frequent if:– Wet summer in west Africa– Warmer sea temperatures– Low atmospheric pressure in Caribbean– No El Nino

• Weather phenomena of high level east-blowing winds in Pacific

Why?

West Africa wet summer: more t-storms, more hurricane potential

Warm sea temps: more energy for storms

Low pressure in Caribbean: requirement for storms

No El Nino: weaker upper level winds (also condition for formation)

General Path of Storms

Probabilities of hurricanes hitting U.S. coastlines in any given year

Short term prediction

• Location, movement, intensity closely monitored– Ship reports– Satellites– Radar– Buoys– Aircraft QuickTime™ and a

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Warnings and Watches

• Watch: issued for large areas that might be affected– Usually issued a few days before landfall

• Warning: usually issued within 24 hours of storm striking area (+probability of striking)– Usually issued for large area (over 300 miles) to

compensate for wide swath, variations in landfall

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Example of watches and warnings posted for Hurricane Erin, 1995

Warnings and Watches

• Significantly reduced loss of life from hurricanes

• Not amount of damage

Evacuation

• Can be a problem as more people move into coastal areas

• Evacuation time estimates for areas– 72 hours New Orleans– 50-60 hours Ft. Myers, FL– 30-39 hours Miami– Problem: usually don’t have detailed

knowledge of landfall during these periods

Mitigating Damage

• Similar to earthquake planning• Building codes

– Withstand winds– Keep roofs on– Protecting windows

• Land use– Low-lying areas for parks, golf courses, not

houses

Next Time

• Hurricanes Part 2