1. Tropical Cyclones and Hurricanes Read Chapter 11

2. Charley
Ivan
Katrina's landfall

4. The Hurricane
A low pressure system of tropical origin that produces sustained surface winds >=74mph
Organized convection around center
Tropical cyclone is generic name given low-pressure systems forming over warm, tropical seas (typhoon in western pacific, super typhoon if winds >150 mph)

5. The Hurricane: Birth Regions
Not over land
Not high latitudes, but not at the equator either
(Graphic by Robert Simmon, NASA GSFC)

6. Active Season last year, Inactive this year
2006 8 5 2

7. A Recipe for Hurricane Formation
1) Sea surface temperature (SST) >80°F (26.5°C) & deep layer of warm water
2) Conditional/absolute instability through a deep layer of troposphere- Important for thunderstorms!
3) Moist air in mid-troposphere
4) Weak vertical wind shear
5) Genesis region at least 5° away from equator
6) Source of low-level cyclonic spin

8. Ingredient #1: Warm water
Warm water ensures high evaporation rates, thus high low level dew points…promotes instability & convection!
Tropical cyclones can rapidly strengthen if they move over patch of warm water

9. Atlantic Hurricane Season Peaks ~ Sept
Atlantic Hurricane Season Peaks ~ Sept.10 – around when ocean is warmest

10. Why is a relatively deep layer of warm water needed?
Strong hurricane wind can lead to upwelling of colder water from below as
high winds at surface mix the ocean water
Cooler surface waters lead to less evaporation, more stabilized atmosphere

11. Ingredient # 2: Conditional Instability- How convection leads to pressure falls
Animation on CD – let’s take a look

12. Ingredient #3: Mid-Tropospheric Moisture
Dry mid-level air promotes hurricane decay by leading to precipitation evaporating => downdrafts => cooler, more stable air near ground => less convection

13. #4) Vertical wind shear separates low level circulation from tall thunderstorms
Less wind shear => better environment for hurricane formation

14. Ingredient #5: Cyclonic vorticity (spin)
Earth vorticity is increased as one head toward poles, it’s zero at equator (related to Coriolis force)
Earth vorticity needed for cluster of t-storms to obtain cyclonic spin
Tropical cyclone formation at least 5° from equator

15. Ingredient #6: Tropical Waves
Emerge every 3-4 days off coast of Africa from June-October, about 60/year
When waves spawn hurricanes in east-central Atlantic, storm is termed “Cape Verde” storm…Cape Verde season is from August to October

16. Intertropical Convergence Zone (ITCZ): The source for tropical waves

17. Stages of Development I
Tropical disturbance: disorganized blob of thunderstorms, often forms within the ITCZ
If atmosphere and ocean conditions favor development, we enter a positive feedback loop
Latent heat release in thunderstorms through condensation
Increased pressure aloft
Upper-level divergence, latent heating & compressional warming lead to lower surface pressure
Increased inflow deflected to right and cyclonic circulation enhanced, more evaporation into air =>more fuel for t-storms…
Animation of this process on CD

18. …->Tropical Depression
organized thunderstorms, circular wind flow around center, wind speed less than 39 mph
Given number-letter tag (i.e. Tropical Depression 5-A)

19. Stages of Development II
Tropical Storm: sustained wind speeds from 39-73mph with higher gusts
Storm gets a name
Six-year cycle (
Names reused or retired if storm is historic
If all names used in a season, storms named with Greek alphabet
If conditions remain favorable, positive feedback loop continues, leading to…

20. Stages of Development: Hurricane
Wind speed (sustained) at least 74 mph
An eye forms
Spiral bands
Conservation of angular momentum
Strength dictated by SSTs

21. Strength of tropical cyclone related to number & intensity of thunderstorms around eye

22. Winds fastest near surrounding calm eye of hurricane…known as eye wall

23. Relative Velocity
Strongest winds in a tropical cyclone are in the right front quadrant

24. How does the eye form?
As air spirals into center of storm, outward acting centrifugal force increases…eventually, centrifugal force equals PGF and air stops spiraling inward….leads to increased convergence and rising air motion
Diameter ~ 25 mi
Smaller eye-stronger hurricane

25. Loop of Hurricane Katrina showing cyclonic flow

26. Importance of anticyclonic flow aloft above a hurricane…increased upper-level divergence

27. Hurricane Felix

28. Vertical Wind Profile in a Hurricane
Speedy ring of air at surface in eye wall expands outward as it rises due to upper-level divergence
Wind speed decreases with height conserving angular momentum

29. Hurricane: Saffir-Simpson Scale (in need of revision?)
This graphic was created by Lynn A. Dombrowski, Ed. D.

30. Storm Surge
Water, not wind, is by far the biggest killer (includes both storm surge and fresh water flooding)
Right front quadrant is most dangerous (N.Hemisphere)
Strongest winds (rotation + translation velocities)
Winds blow on-shore which piles up water

31. Apartment Complex before Camille

32. Apartment Complex after Camille

33. Demise Loss of fuel Wind shear Increased friction (landfall)
Moves into colder water
Moves over land
Wind shear
Tilts the thunderstorms
Lowest pressure not concentrated, pressure gradient relaxes and winds weaken
Increased friction (landfall)

34. Symbols on Weather Maps
Tropical Storm Symbol
Hurricane Symbol

35. Hurricane Watches/Warnings
Watch - Issued when there is a possibility that a part of the coastline could experience hurricane conditions within 36 hours
Warning – Issued when hurricane conditions are expected for a part of a coastline within 24 hours

36. Understanding National Hurricane Center (NHC) Forecasts