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,
2500 km wavelength,
10-20 knots speed
Q1: Why is there a convergence east of the trough? A: because SE flow converges with NE trade wind.
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, and instead of isobars, streamlines are used

3. Anatomy of a Hurricane hurricane (typhoon, cyclone): > 64 knots
eye; eyewall
spiral rain band
Hurricanes are quite similar to, yet also quite different from mid-latitude storms.
Q2: 1 knot equals
a) 1 km/hr
b) 1 mile/hr
c) 1 nautical mile/hr

4. Q3: What would be your experience if you cross a hurricane
Q3: What would be your experience if you cross a hurricane? A: heavy rain, followed by no rain; very heavy rain and very strong wind near eyewall, followed by clear sky in the eye due to high pressure aloft; very heavy rain near the eyewall again with the reverse of wind direction
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

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;
Peak in early September

6. The Right Environment Convergence trigger
ITCZ, easterly waves, midlatitude fronts to tropics
Weak trade wind inversion
Weak upper wind
e.g., during a La Nina event (for more hurricanes over Atlantic)
Some Coriolis force: 5-20deg latitude
Q4: Why are relatively strong temperature inversion and strong wind shear required for supercell? A: weak inversion would lead to ordinary thunderstorms only and weak wind shear would self-destroy the storm by downdraft.
Q5: Why are weak inversion and weak upper wind are required for hurricanes? A: warm ocean surface is the energy source that is not much affected by downdraft; strong inversion prevents the formation of thunderstorms; strong wind disrupts the organized pattern of convection.

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
Q6: With global warming, increased ocean surface evaporation would tend to
a) increase hurricane strength,
b) decrease hurricane strength,
c) not change hurricane strength

8. The Storm Dies Out
cutting off the storm’s energy supply by moving over cooler ocean
Landfall: lose energy source and increased friction to reduce wind A B C D
Q7: In the figure, which storm is dying out?
a) storm A, b) storm B, c) storm C, d) storm D
Q8: How to draw the vertical distribution of isobars associated with a hurricane? A: minimum surface pressure at hurricane center; high pressure near tropopause at center (see the drawing in class).

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.
Q9: Where does the strongest wind occur in a hurricane?
a) in the hurricane eye,
b) around the eyewall,
c) in the spiral rain band

10. Q10: Hurricanes can form over oceans with surface temperature
a) > 17 C, b) > 22 C, c) > 27 C
Q11: Do usually hurricanes form at the equator?
a) yes, b) no
Q12: Hurricane wind is stronger than
a) 34 knots, b) 44 knots, c) 54 knots, d) 64 knots
Q13: Favorable conditions for hurricane development include
a) strong inversion and strong upper wind
b) strong inversion and weak upper wind
c) weak inversion and strong upper wind
d) weak inversion and weak upper wind

11. Hurricane vs midlattitude storms
Q14: What do hurricanes and midlatitude storms have in common? A: both are low pressure systems with counterclockwise motions (in Northern Hemisphere).
Q15: What do they differ in energy source?
A: hurricanes derive energy from warm ocean and latent heat of condensation; while midlatitude storms derive energy from horizontal temperature difference
Q16: What do they differ in other aspects? A:
1) warm core low, high center aloft, and sinking air in the eye for hurricanes (versus cold core low, intensifies with height, and rising air at center);
2) strongest wind near surface in hurricanes (versus strongest wind aloft in the jet stream);
3) stronger wind and smaller size in hurricanes

12. Hurricane Movement role of the ITCZ
northward movement due to subtropical highs
influence of the westerlies
Q17: Westward-moving North Atlantic hurricanes often take a turn towards the north as they approach North America, because of
a) the Bermuda High, b) easterly wave, c) westerly wind

13. Q18: Why is there a lack of hurricanes adjacent to South America
Q18: Why is there a lack of hurricanes adjacent to South America? A: because of cooler water, vertical wind shear, and 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

14. January
Fig (a)
July
Fig. 7.22(b)

15. 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.

16. Devastating Winds and the Storm Surge
Q19: The strongest hurricane wind is located at the place where the hurricane rotational wind and the hurricane movement wind are: a) in the same direction, b) in opposite direction, c) perpendicular to each other
Q20: Does the Ekman transport increase or decrease the coastal sea level in the figure? a) increase, b) decrease, c) no effect
Q21: What would cause high ocean at the center? a) surface low pressure, b) high pressure

17. Devastating Winds and the Storm Surge
High waves would raise sea level
Ocean tides (due to gravity of sun and moon) also raise sea level
Storm surge (i.e., sea level rise by several meters) could be caused by the combination of Ekman transport, low pressure, high waves, and tides
Flooding: due to heavy
rain and storm surge; cause
most human casualties

18. defined primarily based on wind speed
Table 11-2, p. 313

19. Table 11-3, p. 316

20. High winds, large waves, and large storm surge caused
Katrina, 2005:
$75B damage;
>1300 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

21. 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

22. Modifying Hurricanes cloud seeding to reduce maximum wind
attempt to increase ordinary thunderstorms (rather than severe thunderstorm in a hurricane;
difficult to verify the effect
monomolecular films
attempt to reduce surface evaporation;