Chapter 11 Hurricanes

What is a hurricane? Fig Winds ≥74 mph Winds ≥74 mph Typically 300 miles in diameter Typically 300 miles in diameter HurricanesHurricanes Atlantic & Eastern Pacific Atlantic & Eastern Pacific TyphoonsTyphoons Western Pacific Western Pacific CyclonesCyclones Indian Ocean Indian Ocean

Tropical Cyclone Categories Fig Tropical Cyclones refer to hurricanes and storms with winds < 74 mph Tropical Cyclones refer to hurricanes and storms with winds < 74 mph Tropical Disturbance: < 23 mph Tropical Disturbance: < 23 mph Tropical Depression: mph Tropical Depression: mph Tropical Storm: mph Tropical Storm: mph Receives nameReceives name

Tropical Cyclone Names There is a 6-year list of names for each basin (Atlantic, Eastern Pacific, Western Pacific) There is a 6-year list of names for each basin (Atlantic, Eastern Pacific, Western Pacific) If there is a significant storm, the name is retired and replaced (Hugo, Camille, Andrew, Katrina) If there is a significant storm, the name is retired and replaced (Hugo, Camille, Andrew, Katrina)

Tropical Cyclone Names Names Names Names

Where do hurricanes form? (Fig. 11.9) Form between Tropics of Cancer and Capricorn Form between Tropics of Cancer and Capricorn Do NOT form at equator Do NOT form at equator

When do they form? Fig Atlantic Hurricanes Atlantic Hurricanes June 1 – Nov. 30June 1 – Nov. 30 Eastern North Pacific Eastern North Pacific May – DecemberMay – December Northwest Pacific Northwest Pacific All year (mostly Jul – Dec)All year (mostly Jul – Dec) South Indian and SW Pacific South Indian and SW Pacific All year (mostly Dec – Jun)All year (mostly Dec – Jun) North Indian North Indian All yearAll year

Tropical Weather Towering cumulus clouds and precipitation increase closer to the ITCZ Towering cumulus clouds and precipitation increase closer to the ITCZ Stable conditions are prevalent near the subtropical high Stable conditions are prevalent near the subtropical high

Tropical Cyclone Formation Pre-existing large scale flow pattern Pre-existing large scale flow pattern Moist, unstable air Moist, unstable air Absence of strong wind shear Absence of strong wind shear Warm ocean surface water Warm ocean surface water Nonequatorial location Nonequatorial location

Hurricane Growth Only one tropical depression in five reach tropical storm status Only one tropical depression in five reach tropical storm status Only half of those reach hurricane status Only half of those reach hurricane status Hurricanes are relatively rare because the previously mentioned conditions must be sustained over several days Hurricanes are relatively rare because the previously mentioned conditions must be sustained over several days

Hurricane Paths While in the tropics, the storm is between the subtropical high and the ITCZ While in the tropics, the storm is between the subtropical high and the ITCZ Storms usually follow the easterly trade winds Storms usually follow the easterly trade winds Once the storm travels far enough north it is then “captured” in the westerlies and starts to recurve Once the storm travels far enough north it is then “captured” in the westerlies and starts to recurve

Structure of Mature Hurricanes The eye is the distinctive region of clear or nearly clear skies, subsiding air, and light winds—It is the center of circulation The eye is the distinctive region of clear or nearly clear skies, subsiding air, and light winds—It is the center of circulation The eyewall is the ring of towering thunderstorms that typically contain the storm’s heaviest rain and strongest winds The eyewall is the ring of towering thunderstorms that typically contain the storm’s heaviest rain and strongest winds

Satellite picture of Hurricane

The Hurricane’s Eye Sinking air Sinking air Warms and causes clouds to evaporateWarms and causes clouds to evaporate By the conservation of angular momentum, as the eye’s radius decreases, the rotational speed increases By the conservation of angular momentum, as the eye’s radius decreases, the rotational speed increases

What Sustains the Hurricane? Latent heat release sustains a hurricane Latent heat release sustains a hurricane Warm ocean water is the source Warm ocean water is the source More upper level divergence than surface convergence will lower the pressure More upper level divergence than surface convergence will lower the pressure

Hurricane Conditions Winds Winds Storm Surges Storm Surges Heavy Rainfall Heavy Rainfall

Hurricane Winds Even though hurricanes are often circular, winds are not the same in all directions Even though hurricanes are often circular, winds are not the same in all directions

Storm Surges The stronger winds usually to the right of the storm motion push water forward The stronger winds usually to the right of the storm motion push water forward The lower pressure at the center of the hurricane causes the sea level to rise The lower pressure at the center of the hurricane causes the sea level to rise Can be 25 feet high Can be 25 feet high

Heavy Rainfall Dependent upon speed of storm Dependent upon speed of storm Typical rainfall totals are generally 4 – 6 inches, but they are greater for slow moving storms Typical rainfall totals are generally 4 – 6 inches, but they are greater for slow moving storms Tropical Storm Alberto dumped 24 inches in Georgia (1994) Tropical Storm Alberto dumped 24 inches in Georgia (1994) A typhoon in the Philippines (1911) produces 88 inches of rain over 4 days A typhoon in the Philippines (1911) produces 88 inches of rain over 4 days

The Saffir-Simpson Scale Classifies hurricane intensity Classifies hurricane intensity Although it classifies according to central pressure, maximum sustained winds, storm surge, and damage, it mainly uses maximum sustained winds to classify Although it classifies according to central pressure, maximum sustained winds, storm surge, and damage, it mainly uses maximum sustained winds to classify 1 (weakest) to 5 (strongest) 1 (weakest) to 5 (strongest)

Table 11.2

Hurricane Dissipation Landfall Landfall No warm water to provide the “fuel”No warm water to provide the “fuel” Physical barriers (Increase friction)Physical barriers (Increase friction) Reduce Wind Speed Reduce Wind Speed The center of the hurricane begins to cool (cool air being more dense will raise the pressure) and the hurricane dissipates The center of the hurricane begins to cool (cool air being more dense will raise the pressure) and the hurricane dissipates In addition to landfall, passing over colder water and strong wind shear also can also weaken hurricanes In addition to landfall, passing over colder water and strong wind shear also can also weaken hurricanes

Hurricane Katrina Track (2005)

New Orleans Damage

New Orleans (Before/After)

Biloxi, MS (Before)

Biloxi, MS (After)

Worst Hurricanes in U.S. History