Hurricane Principles Hurricane Isabel, 2003

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

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

Into a Hurricane More fuel, more rotation, more speed Tropical storm: surface wind speeds between 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

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

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

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

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

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 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

Downtown Houston after Tropical Storm Allison, 2001

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

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

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