Chapter 11: Hurricanes Tropical weather Tropical weather Anatomy of a hurricane Anatomy of a hurricane Hurricane formation and dissipation Hurricane formation and dissipation Some notable hurricanes Some notable hurricanes Hurricane watches, warnings and forecasts Hurricane watches, warnings and forecasts Modifying hurricanes Modifying hurricanes

Tropical Weather Streamlines – lines that show wind flow Streamlines – lines that show wind flow Tropical wave – many hurricanes start as open waves Tropical wave – many hurricanes start as open waves The tropics are close to the equator, where the Coriolis force is too small to balance the pressure gradient force. Thus winds are not geostrophic.The tropics are close to the equator, where the Coriolis force is too small to balance the pressure gradient force. Thus winds are not geostrophic.

Tropical Weather Convergence on east side of wave Convergence on east side of wave (thunderstorms) (thunderstorms) Divergence on west side of wave Divergence on west side of wave The tropics are close to the equator, where the Coriolis force is too small to balance the pressure gradient force. Thus winds are not geostrophic.The tropics are close to the equator, where the Coriolis force is too small to balance the pressure gradient force. Thus winds are not geostrophic.

Anatomy of a Hurricane Tropical cyclone – any hurricane-like storm that forms in the tropics Tropical cyclone – any hurricane-like storm that forms in the tropics Eye – light winds Eye – light winds little precipitation lowest pressure Eyewall – heaviest Eyewall – heaviestprecipitation strongest winds Spiral rainband – Spiral rainband – precip bands that flow toward storm center

Fig. 11-3, p. 302 Stepped Art

The Right Environment Hurricanes need convergence to form Hurricanes need convergence to form Convergence happens in low pressure systems away from the equator Convergence happens in low pressure systems away from the equator Trade wind inversion Trade wind inversion Inversion brought on by sinking air from subtropical highs (bad for development) Inversion brought on by sinking air from subtropical highs (bad for development) Hurricanes need weak wind shear to develop Hurricanes need weak wind shear to develop

The Developing Storm Latent heat released in thunderstorms causes air to be warmer in thunderstorms than air around it Latent heat released in thunderstorms causes air to be warmer in thunderstorms than air around it This causes high pressure aloft and low pressure at surface (surface pressure drops) This causes high pressure aloft and low pressure at surface (surface pressure drops) Operates like a heat engine. Warm air taken in from ocean, energy is used, then radiation is lost to space at top of thunderstorms Operates like a heat engine. Warm air taken in from ocean, energy is used, then radiation is lost to space at top of thunderstorms The whole thing is driven by sensible and latent heat from where? The whole thing is driven by sensible and latent heat from where?

The Storm Dies Out Hurricanes could last several weeks, but most last less than a week Hurricanes could last several weeks, but most last less than a week Three ways for hurricane to weaken Three ways for hurricane to weaken When hurricanes travel over cold water, heat source is cut off When hurricanes travel over cold water, heat source is cut off Shallow warm water/hurricane stirs it up Shallow warm water/hurricane stirs it up Landfall. Land causes friction that tears the hurricane apart Landfall. Land causes friction that tears the hurricane apart Even though making landfall cuts off a hurricane’s energy supply, the storm is often still quite strong and can even produce tornadoes.Even though making landfall cuts off a hurricane’s energy supply, the storm is often still quite strong and can even produce tornadoes.

Hurricane Stages of Development Tropical disturbance – Mass of thunderstorms/only slight circulation Tropical disturbance – Mass of thunderstorms/only slight circulation Tropical depression – Winds between 20 and 34 knots Tropical depression – Winds between 20 and 34 knots Tropical storm – Winds between 34 and 64 knots. Storm gets a name Tropical storm – Winds between 34 and 64 knots. Storm gets a name Hurricane – Winds above 64 knots Hurricane – Winds above 64 knots This progression of stages is followed in reverse order as a storm weakens.This progression of stages is followed in reverse order as a storm weakens.

Fig. 11-9, p. 306

Hurricane Movement Most hurricanes move west/northwestward Most hurricanes move west/northwestward Are steered by the subtropical highs Are steered by the subtropical highs Eventually curve north/northeastward by westerlies Eventually curve north/northeastward by westerlies No hurricanes in South Atlantic/eastern South Pacific (why?) No hurricanes in South Atlantic/eastern South Pacific (why?) Hurricanes in eastern Pacific take more west track Hurricanes in eastern Pacific take more west track Because of the Bermuda High, westward-moving North Atlantic hurricanes often take a turn towards the north as they approach North America.Because of the Bermuda High, westward-moving North Atlantic hurricanes often take a turn towards the north as they approach North America.

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Naming Hurricanes and Tropical Storms Past practices Past practices Named by location (latitude, longitude) Named by location (latitude, longitude) Named by radio code in WWII Named by radio code in WWII In 1953, female names by alphabet In 1953, female names by alphabet In 1978, eastern Pacific used male/female In 1978, eastern Pacific used male/female In 1979, north Atlantic used male/female In 1979, north Atlantic used male/female Whenever a hurricane has had a major impact, any country affected by the storm can request that the name of the hurricane be retired by the World Meteorological Organization.

Devastating Winds and the Storm Surge Where are the strongest winds? Where are the strongest winds? Most destruction caused by flooding Most destruction caused by flooding Storm surge – a “bubble” of water like sucking from a straw. Extremely dangerous Storm surge – a “bubble” of water like sucking from a straw. Extremely dangerous Saffir-Simpson scale Saffir-Simpson scale Category 1 – mph…4-5 foot Category 1 – mph…4-5 foot Category 2 – mph…6-8 foot Category 2 – mph…6-8 foot Category 3 – mph…9- 12 foot Category 3 – mph…9- 12 foot Category 4 – mph…13-18 foot Category 4 – mph…13-18 foot Category 5 - >155 mph…>18 foot Category 5 - >155 mph…>18 foot

Some Notable Hurricanes Camille, 1969 Camille, 1969 Hugo, 1989 Hugo, 1989 Andrew, 1992 Andrew, 1992 Ivan, 2004 Ivan, 2004 Katrina, 2005 Katrina, 2005

Other Devastating Hurricanes October, 1893 Gulf coast hurricane October, 1893 Gulf coast hurricane August, 1893 Charleston hurricane August, 1893 Charleston hurricane 1970: killer cyclone in Bangladesh 1970: killer cyclone in Bangladesh

Hurricane Watches, Warnings and Forecasts Hurricane watch Hurricane watch Issued when hurricane is direct threat (24 to 48 hours before) Issued when hurricane is direct threat (24 to 48 hours before) Hurricane warning Hurricane warning Issued when it appears the storm may strike an area (comes with probability) Issued when it appears the storm may strike an area (comes with probability)

Hurricane Watches, Warnings and Forecasts Models use information from satellites, buoys, reconnaissance aircraft Models use information from satellites, buoys, reconnaissance aircraft

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Modifying Hurricanes Cloud seeding Cloud seeding Silver iodide used to seed just outside eyewall Silver iodide used to seed just outside eyewall Project STORMFURY Project STORMFURY In 1963, NOAA seeded several hurricanes with some success (????) In 1963, NOAA seeded several hurricanes with some success (????) Seeding has stopped since the 1970s Seeding has stopped since the 1970s Monomolecular films Monomolecular films Place film over water to prevent evaporation Place film over water to prevent evaporation