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

2. Tropical Weather Tropics: 23.5N-23.5S streamlines tropical wave
easterly wave,
2500km wavelength,
10-20 knots speed
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.

3. Anatomy of a Hurricane hurricane (typhoon, cyclone): > 64 knots
eye; eyewall
spiral rainband
Hurricanes are quite similar to, yet also quite different from mid-latitude storms.

4. Sinking air and clear sky in the eye due to high pressure aloft
Figure 11.3: A model that shows a vertical view of air motions and clouds in a typical hurricane in the Northern Hemisphere. The diagram is exaggerated in the vertical.
Watch this Active Figure on ThomsonNow website at
Fig. 11-3, p. 302

5. Hurricane Formation and Dissipation
Hurricane forms over tropical
waters where winds are light,
humidity is high in a deep layer,
and surface temperature is warm,
typically 26.5C (80F) or greater,
over a vast area
SST > 28C
Over Atlantic

6. The Right Environment
Convergence trigger
ITCZ, easterly waves, midlatitude fronts to tropics
Weak trade wind inversion
Weak upper wind
during El Nino over Atlantic
Some Coriolis force: 5-20deg latitude

7. The Developing Storm
Heat engine: heat taken at high T, converted into work, then ejected at low T.
Hurricane: heat taken from warm ocean, converted into kinetic energy (wind), lost at its top through radiational cooling
Maximum wind depends on surface and tropopause temperature difference and the potential of sea surface evaporation

8. The Storm Dies Out
cutting off the storm’s energy supply by moving over cooler ocean
Landfall: lose energy sourse and increased friction to reduce wind

9. Hurricane Stages of Development
tropical disturbance: unorganized mass of thunderstorms, weak wind
tropical depression: knots, closed isobars
tropical storm: knots, with a name
hurricane: > 64 knots, with a name
This progression of stages is followed in reverse order as a storm weakens.

10. Hurricane vs midlattitude storms
Hurricane: 1) derive energy from warm ocean and latent heat of condensation; 2) warm core low; 3) high center aloft; 4) sinking air in the eye; 5) strongest wind near surface; 6) stronger wind; 7) smaller size
Midlatitude storms: 1) derive energy from horizontal temperature difference; 2) cold core low; 3) intensifies with height; 4) rising air at center; 5) strongest wind aloft in the jet stream; 6) wind not as strong; 7) larger size
Both are low pressure systems with counterclockwise motions

11. Hurricane Movement role of the ITCZ
northward movement due to subtropical highs
influence of the westerlies
Because of the Bermuda High, westward-moving North Atlantic hurricanes often take a turn towards the north as they approach North America.

12. Lack of hurricanes adjacent to South America: cooler water
vertical wind shear, unfavorable ITCZ position
Figure 11.10: Regions where tropical storms form (red shading), the names given to storms, and the typical paths they take (red arrows).
Fig , p. 307

13. Naming Hurricanes and Tropical Storms
past practices: female names
current protocol: female and male names; in alphabetic order so that the first hurricane starts with the letter A; then in Greek alphabet
Letters Q, U, X, Y not used over north Atlantic
Letters Q, U not used over north Pacific
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.

14. Devastating Winds and the Storm Surge
location of strongest winds
Ekman transport
10-15 m high waves
High ocean due to low pressure
storm surge: several
meter rise of sea level
Flooding: due to heavy
rain and storm surge; cause
most human casualties

15. Table 11-2, p. 313

16. Table 11-3, p. 316

17. High winds, large waves, and large storm surge caused
Katrina, 2005:
$75B damage;
>1200 deaths;
High winds, large waves, and
large storm surge caused
disastrous breeches in the levee
system
Figure 4: The paths of eight hurricanes that impacted Florida during 2004 and Notice that in 2004 hurricanes Frances and Jeanne made landfall at just about the same spot along Florida’s southeast coast. The date under the hurricane’s name indicates the date the hurricane made landfall.
Figure 4, p. 319

18. Hurricane Watches, Warnings and Forecasts
24-48 hr before landing
hurricane warning:
storm will strike an area
Forecasts:
improvement in path;
not in strength
Wrong forecasts
also cause Economic loss

19. Modifying Hurricanes cloud seeding to reduce maximum wind
monomolecular films