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  2. Chapter Eleven Hurricanes

  3. What we are going to learn? We are going to learn the answers to the following questions • What are hurricanes? • How do they form? • Why they strike the east coast of the US than the west coast of the US? • Conditions necessary for the hurricane development • Why are skies are clear in a hurricane’s eye? • What is the fuel that derives the hurricane? • What factors tend to weaken hurricane? • Why hurricanes are more likely to strike New Jersey than Oregon? • Why do hurricanes move westward over tropical waters?

  4. Anatomy of a hurricane • Hurricane: An intense storm of tropical origin, with sustained winds >64 knots (74 mi/hr) • Other Names: Typhoon: Western North Pacific • Beguio: In Phillipines • Cyclone: In India and Australia • International agreement: Tropical cyclone • Tropics: region between 23.5° north – 23.5° south; noon sun is always high in the sky, and so diurnal and seasonal changes in temperature are small; daily heating of the surface & high humidity favor the development of cumulus clouds & afternoon thunderstorms

  5. A tropical wave as shown by the bending of streamlines; the waves move westward, bringing fair weather on its western side & showers on the eastern side

  6. Tropical weather • Most of tropics are marked by seasonal differences in amount of precipitation, as temperature remains warm all year long • Greatest cloudiness & precipitation occur during high-sun period (when the ITCZ moves into the region) • Winds blow from the east, northeast or southeast – the trade winds • Streamlines: depict wind flow direction; useful to see where surface air converges and diverges • Tropical wave: A wavelike disturbance in the tropical easterlies; have wavelengths ~2,500 km & travel from east to west at speeds 10-20 knots

  7. Hurricane Elena over the Gulf of Mexico, about 130 km southwest of Apalachicola, FL (1985); winds flow counterclockwise about its center; central press. 955 mb

  8. Hurricane Anatomy • Average diameter of a hurricane ~500 km • Elena’s eye is ~ 40 km wide; within the eye, the winds are light and clouds are mainly broken • Surface winds increase in speed as they blow counter-clockwise and inward toward the center • Eye wall: A wall of dense thunderstorms that surrounds the eye of a hurricane; within the eye wall, heaviest precipitation & strongest winds are found; extend up to ~15 km above sea level • Observations when we cross a hurricane (W to E): • When we approach, sky is overcast with cirrostratus clouds; • Pressure drops slowly at first & then more rapidly as we move closer to the center • When we approach the eye, winds blow from the north and northwest with increasing speed; heavy rain showers

  9. Hurricane anatomy – contd. • Inside the eye: air temperature rises, winds slacken, rainfall ceases & sky brightens; lowest pressure reading (~50 mb lower than just outside the storm) • East side of the eye wall: heavy rain, strong southerly winds • As we move away from the eye wall, pressure rises, winds diminish, rain decreases, & eventually sky clears • Why no thunderstorm inside the eye: In the eyewall, severe thunderstorms lead to release of latent heat which in turns warms the eye --- leading to higher pressure which initiates a downward air motion (sinking air) --- sinking air warms by compression and absence of thunderstorms (so much heat is added to the air that surface air temp remains uniform in the hurricane)

  10. A model vertical view of air motions, clouds & precipitation in a typical hurricane

  11. Hurricane formation & dissipation • Hurricanes form over tropical waters (5-20 °) where winds are light, surface water temp is warm (>26.5°C) over a vast area, high humidity; warm surface water must extend >200 m in the water; 2/3 hurricanes in 10-20 ° • These conditions prevail over tropical and subtropical North Atlantic and North Pacific oceans during summer and early fall (June-November) • The surface winds must converge for a mass of unorganized thunderstorms to develop into a hurricane; surface winds along ITCZ; when a wave forms along the ITCZ, convection becomes organized leading to the formation of hurricanes

  12. Hurricane formation and dissipation • Hurricanes do not where the surface winds are strong • During El Nino event, strong winds aloft typically occurs over the tropical Atlantic --- fewer Atlantic hurricanes than normal • During El Nino event, warmer water of El Nino in the northern tropical Pacific favors the development of hurricanes in that region • Energy for a hurricane come from the direct transfer of sensible het from the warm water into the atmosphere and from the transfer of latent heat from the ocean surface; if thunderstorm start to organize along the ITCZ or along a tropical wave, and if the trade wind inversion is weak, conditions will lead to the formation of a hurricane

  13. Hurricane formation & dissipation - contd • Tropical disturbance: A thunderstorm with a slight wind circulation • Tropical depression (TD): When Tropical disturbance becomes TD when winds speeds reach 20-34 knots & several closed isobars appear about its center in a surface weather map • Tropical Storm: When TD wind speed reaches 35-64 knots & when the isobars are packed together • Hurricane: When tropical storm speed exceeds 64 knots • Stages: Tropical disturbance crossing over Panama; tropical disturbance speed 25 knots; tropical storm speed < 40 knots; hurricane >110 knots

  14. Visible satellite image of 4 tropical systems, each in a different stage of its life style

  15. Hurricane movement • Hurricanes form in tropical oceans (except in the S.Atlantic & eastern South Pacific: water temp is too cold); hurricanes formed over the N. Pacific & N. Atlantic are steered by easterly winds and move west or northwestward • Actual path of a hurricane is determined by the structure of the storm & the storm’s interaction with the environment • Off coast of Mexico over the N. Pacific spawns ~8/yr • Tropical North Atlantic ~ 6/yr • Hawaiian Islands (20-23°N) is in direct path of eastern Pacific hurricanes & tropical storms

  16. Regions where tropical storms form, their names and typical paths

  17. Some erratic paths taken by hurricanes

  18. Hurricane and middle latitude storms • Hurricane Middle latitude storms • Energy from warm water & horizontal temp gradients latent heat of condensation Hurricane weakens with height Intensifies with height Contains an eye where air is Characterized by centers of sinking rising air Strongest near the surface Strongest winds found aloft in the jet stream Around the hurricanes, isobars are more circular, pressure gradient is much steeper & winds are stronger

  19. Destruction & warning • A hurricane moving northward over the Atlantic will normally survive for a longer time than will the one from the same latitude over the eastern Pacific – surface water of Atlantic is warmer than the Pacific • When a hurricane is approaching from the east, its highest winds are usually on its north side (winds that push the storm along add to the winds on the north & subtract from the winds on the south) • When wind blows over water, the water beneath is set in motion; bending of water with depth (Ekman spiral) causes a net transport of water (Ekman transport) • High winds of hurricanes generate large waves (10-15 m in height); waves move outward in the form of swells – storm’s energy is carried; effects of storm is felt days before the hurricane arrives

  20. Destruction and warning – contd. • Region of low pressure allows the ocean level to rise (~50 cm); 1mb drop ~ 1 cm rise • Storm surge is produced by a combination of high winds, high waters and the net transport of water toward the coast – inundates low-lying areas and turns beachfront homes into piles of splinters • Hurricane watch: issued ~24-48 hrs before the storm arrives • Hurricane warning: When storm appears to strike an area within 24 hrs, warning is issued; issued for a large coastal area ~550 km (5 degrees) in length • After landfall, hurricanes become tropical storms with peak winds < 50 knots

  21. 12 Most intense hurricanes from 1900-1999 • .

  22. A composite IR images of Hurricane Georges (September 18-28, 1998); its trek across Caribbean and northward into US

  23. Hurricane Gloria on Sept. 27, 1985; moving northward @25 knots, winds of 100 knots on its right & 50 knots on left side; 945 mb at the central

  24. When a storm surge moves in at high tide it can inundate and destroy a wide swath of coastal lowlands

  25. Color-enhanced IR satellite image of Hurricane Hugo near Charleston, SC

  26. Saffir-Simpson Hurricane damage-potential scale • .

  27. Beach homes at Folly Beach, South Carolina before hurricane Hugo

  28. Beach homes at Folly Beach, South Carolina after Hurricane Hugo

  29. IR satellite image of Hurricane Andrew moving across South Florida on the morning of August 24, 1992; central pressure 932 mb and sustained winds of 126 knots

  30. Number of hurricanes (3,4,5 are major) in each category that made landfall along the coastal US from 1900-1999; all hurricanes struck the Gulf or Atlantic coasts

  31. Naming Hurricanes • A name is assigned when a storm reaches tropical storm strength • Beginning 1953 (-1977), the National Weather Service began using female names • Beginning 1978-1979, alternately male and female names were assigned

  32. Chapter Summary • What are streamlines • Hurricane-eye, eyewall, where the pressure is higher & lower; vertical structure, rotation of hurricanes, source of energy, conditions for the formation of hurricanes • Commonness between middle latitude cyclones & hurricanes • Various stages of hurricane, areas where more hurricanes • Why hurricanes don’t form in certain regions? • Saffir-Simpson scale • How hurricanes can be modified? • Damage by hurricanes • Other names of hurricanes