Hurricane Study Notes

Hurricanes Overview

  • Presentation Date: 27 Oct 2025

  • Source: NOAA/NESDIS/STAR GOES-19 - GEOCOLOR Composite - Hurricane Melissa

How a Hurricane Forms

  • Key Conditions for Formation (Source: US Department of Commerce, 1971):

    • Surface Water Temperature: > 27˚ C

    • Warm Moist Air: Essential for uplift and development of the storm.

    • Weak Upper-Level Winds: Allow for vertical development without shear.

    • Coriolis Effect: Occurs approximately 500 km from the equator, crucial for cyclone rotation.

Characteristics of Hurricanes

Hurricane Dorian Example

  • Stalled over warm water in the Bahamas

  • Critical Temperature: Surface water > 27˚ C

Hurricane Katrina Example

  • Strengthened over warm water

  • Impact of Temperature: Warm water significantly contributes to hurricane energy.

Understanding Low-Pressure Zones

  1. Air Movement: Air flows upward and counterclockwise, creating a cyclone effect.

  2. Components of Hurricanes:

    • Warm moist air contributes to storm development.

    • The weak upper-level winds support the maintenance of the hurricane structure.

Mechanism of Hurricane Formation

  • Winds flow into the center of the updraft.

  • Formation of the Eye: Once wind speeds reach 119 km/h, not all winds can reach the center, resulting in the formation of an eye.

  • Distinction of Storm Types: Wind speed threshold of 119 km/h is critical to distinguish between a tropical storm and a hurricane.

Structure of Hurricanes

  • Key Features:

    • Eye: The calm center of the hurricane.

    • Eye Wall: Surrounds the eye, composed of the most intense winds and rainfall.

    • Overshooting Top: Represents severe convective activity in the storm.

Positive Feedback Mechanisms in Hurricanes

  • Energy Generation:

    • Sea Spray: Increases evaporation by 100–1,000 times.

    • Updrafts: Carry vapour molecules into the dry upper troposphere, leading to enhanced evaporation and latent heat release.

  • These mechanisms provide hurricanes with more energy, perpetuating their strength and development.

Energy Dynamics in Hurricanes

  • Energy Sources:

    • Heat is released during the condensation of water vapor to liquid.

    • New moist air continuously adds energy to the system.

    • The energy released by condensation is 200 times the world-wide capacity to generate electricity.

    • Kinetic energy of the winds is approximately half of the global electrical generation capacity.

  • Total Energy Calculation:

    • Released energy from cloud and rain formation is estimated to be 400 times greater than hurricane wind energy.

Hurricane Geography

  • Hurricanes occur predominantly in certain geographic regions:

    • Atlantic Ocean = Hurricanes

    • Pacific Ocean = Typhoons

    • Indian Ocean = Cyclones

  • Coriolis Effect: Formation limitations exist within 500 km of the equator near tropical regions.

Tracks of Tropical Cyclones (1985-2005)

  • Analysis shows the absence of hurricanes at the equator and in polar regions due to the Coriolis effect.

Seasonal Patterns of North Atlantic Hurricanes

  • Bermuda High Influence: Hurricanes typically flow around the seasonal Bermuda high pressure zone.

Hurricane Probability and Return Frequency in North America

  • Annual Probability Map: Utilizes historical data to predict hurricane occurrences across various longitudes and latitudes in North America.

  • Return Period Data:

    • Frequencies are assessed based on past hurricane impacts along the U.S. coast, highlighting areas with increased risk of hurricanes.

Atlantic Multidecadal Variability

  • Explanation: Natural cycles in Atlantic temperature that influence hurricane frequency.

  • Notable Increase in Frequency: Observed peaks in the 1940s and 2000s.

Saffir-Simpson Scale for Hurricane Intensity

  • Categories of Damage:

    • Category 1:

    • Wind Speed: 119–153 km/h (74–95 mph)

    • Damage: Light damage to well-constructed homes; minor flooding.

    • Category 2:

    • Wind Speed: 154–177 km/h (96–110 mph)

    • Damage: Extensive damage to residential structures; significant power outages.

    • Category 3:

    • Wind Speed: 178–208 km/h (111–129 mph)

    • Damage: Devastating; major roof and wall loss, prolonged power outages.

    • Category 4:

    • Wind Speed: 209–251 km/h (130–156 mph)

    • Damage: Catastrophic; severe damage to buildings, widespread destruction and isolation.

    • Category 5:

    • Wind Speed: 252 km/h or higher (157 mph or higher)

    • Damage: Total catastrophic damage; only well-reinforced buildings remain standing.

Consideration of Scale Updates

  • Potential for Updates: Discussion about whether the Saffir-Simpson scale needs revisions based on recent storm data including Typhoon Haiyan and Hurricane Patricia.

Historical Examples of Damage (Categories 1-5)

  • Visual documentation of damages incurred from hurricanes across the Saffir-Simpson scale.

Safety Protocols if Trapped in a Hurricane

  1. Stay away from glassed areas.

  2. Listen to emergency information on a battery-powered radio.

  3. Evacuate if required to designated shelters and follow evacuation routes.

  4. Abandon mobile homes for stronger structures.

  5. Unplug major appliances if power is lost.

  6. If outdoors, take cover avoiding bridges and overpasses.

  7. Abandon vehicles for stronger shelters.

Building Near the Coastline

  • Construction Technique: Use of pilings driven deep into the ground to prevent complete wash-away during flooding.

The People Factor: Survey Results from Hurricane Sandy Warnings

  • Preparation Response:

    • 90% made some preparations (extra water, fuel in cars).

    • 50% of homeowners with storm shutters installed them.

    • 20% of those under evacuation orders planned to evacuate.

  • Common Misconceptions: Many believed that winds would cause the majority of the damage, rather than flooding.