Overview
This lecture covers the formation, structure, classification, and impacts of hurricanes, including notable historical events and trends in hurricane activity.
Hurricane Basics
- Hurricanes are the most powerful storms on Earth, also called typhoons (western Pacific) and cyclones (Indian Ocean).
- Most hurricanes form in late summer to early fall when ocean temperatures exceed 27°C (80°F).
- Hurricanes do not form within 5° of the equator due to the weak Coriolis effect, which is needed for storm rotation.
- Hurricanes rarely form poleward of 20° latitude because the ocean waters are too cold.
Hurricane Formation & Development
- Hurricanes begin as cloud clusters moving westward in trade winds, often associated with easterly or tropical waves.
- As low pressure deepens, clusters form a tropical disturbance, which can become a tropical depression.
- A tropical depression with sustained winds ≥39 mph becomes a tropical storm and receives a name.
- When sustained winds reach ≥74 mph, it is classified as a hurricane (Atlantic/Eastern Pacific), typhoon, or cyclone.
- Hurricanes are fueled by heat and moisture from warm ocean water, becoming self-sustaining systems.
Structure and Mechanics
- A hurricane is an intense low-pressure center with the lowest pressure in the eye.
- The eyewall surrounds the eye and contains the highest winds and heaviest rain.
- Air spirals inward at the surface, rises in the eyewall, and outflows at the storm top.
- Pressure gradients determine wind speed: steeper gradients produce stronger winds.
Classification: Saffir-Simpson Scale
- Category 1: 74–95 mph, minimal damage, storm surge 4–5 ft, pressure >980 mb.
- Category 2: 96–110 mph, moderate damage, storm surge 6–8 ft, pressure 965–979 mb.
- Category 3: 111–130 mph, extensive damage, storm surge 9–12 ft, pressure 945–965 mb.
- Category 4: 131–155 mph, extreme damage, storm surge 13–18 ft, pressure 920–944 mb.
- Category 5: >155 mph, catastrophic damage, storm surge >20 ft, pressure <920 mb.
Hurricane Hazards and Impacts
- Storm surge is the deadliest hazard: a dome of water flooding coastal areas, worst on the right side of the eye (NH).
- Wind damage affects a large area; hurricanes can also spawn tornadoes.
- Heavy rains cause inland flooding far from the coast.
- Damage severity depends on storm strength, area size, population density, and coastal geology.
Detection, Tracking, and Recent Trends
- Satellite and forecasting advances allow for warnings and evacuations days in advance.
- Atlantic hurricanes form in warm waters off Africa and move westward, threatening the US east coast.
- The Atlantic hurricane season peaks in September.
- Hurricane frequency and intensity have increased since 1995, with major hurricanes (Category 3+) now averaging 3.6 per year.
- 2005 and 2020 were record-setting years for storm activity.
Notable Hurricanes
- Hurricane Katrina (2005) and Hurricane Maria (2017) are among the deadliest US storms.
- Typhoon Tip (1979) was the largest and most intense tropical cyclone ever recorded.
Key Terms & Definitions
- Hurricane — Intense tropical cyclone with sustained winds ≥74 mph, called typhoon or cyclone in other regions.
- Coriolis Effect — Force caused by Earth's rotation; causes storm spin.
- Saffir-Simpson Scale — 1–5 scale measuring hurricane intensity by wind speed and expected damage.
- Storm Surge — Dome of water pushed ashore by hurricane winds, causing major coastal flooding.
- Eyewall — Ring of intense storms and winds around the eye of a hurricane.
- Tropical Disturbance/Depression/Storm — Stages of hurricane development, classified by wind speed.
Action Items / Next Steps
- Review the Saffir-Simpson scale for each category's wind speeds, pressure, and storm surge.
- Prepare for the next lecture on historic hurricanes, focusing on Galveston (1900) and Katrina (2005).