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Hurricane Principles

Learn about the formation and conditions needed for hurricanes, the stages of their development, the structure of a hurricane, and the damage they can cause. Discover how hurricanes are measured and forecasted.

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Hurricane Principles

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  1. Hurricane Principles Hurricane Isabel, 2003

  2. Outline • Definitions • Formation and Conditions Needed • Growth and Structure of a Hurricane • Where do They Form? • Measurement • Damage • Forecasting

  3. 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

  4. 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

  5. Stages of hurricane development

  6. Beginnings • Low pressure with cluster of thunderstorms • Important for circulation • Tropical disturbance stage • Thunderstorms grow, winds strengthen

  7. 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)

  8. 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

  9. 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

  10. 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

  11. Eye Wall • Cylinder of upward winds • Strongest winds • Heaviest precipitation

  12. Eye and Eyewall structure

  13. 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

  14. Intensity • Controlling factors: • Temperature of water • Release of latent heat • Why? Higher temperature water can drive more t-storms

  15. 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

  16. 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

  17. General origin points and paths of hurricanes/cyclones

  18. Origins of Atlantic Hurricanes

  19. Measuring Size • Saffir-Simpson scale • Based on wind speed, pressure • Expected storm surge, possible damage • Category 1-5

  20. Saffir-Simpson Scale

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

  22. Damage from Hurricanes • High winds • Storm surge • Heavy rain • Mudslides

  23. 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

  24. Wind damage in Florida - Andrew 1992

  25. Storm Surge • Produce much of damage, fatalities • Abnormal rise in water level of few meters

  26. Storm surge is a big problem for low-lying areas Deep water coastlines not as much of a problem

  27. Effects of storm surge on Florida coastline

  28. 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

  29. 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

  30. Downtown Houston after Tropical Storm Allison, 2001

  31. Mudslides • Many examples of rain soaked hillslopes failing after hurricanes

  32. 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

  33. 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)

  34. General Path of Storms Probabilities of hurricanes hitting U.S. coastlines in any given year

  35. Short term prediction • Location, movement, intensity closely monitored • Ship reports • Satellites • Radar • Buoys • Aircraft

  36. 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

  37. Example of watches and warnings posted for Hurricane Erin, 1995

  38. Warnings and Watches • Significantly reduced loss of life from hurricanes • Not amount of damage

  39. 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

  40. 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

  41. Next Time • Hurricanes Part 2

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