1. Hurricanes
(Tropical Cyclones)

2. Geography of storm names
Atlantic and eastern Pacific: “hurricanes” Indian Ocean and Australia: “cyclones”
Western Pacific, N. hemisphere: “typhoons”

3. Western Pacific typhoons, 2011
Season: June 1 – November 30

4. Eastern Pacific hurricanes
Hurricane Linda
Season: May 15 – November 30

5. Atlantic hurricanes
Season: June 1 – November 30

6. Australian cyclones
season
Season: November 1 – April 30

7. North Indian Ocean cyclones
Season: April – December

8. South West Indian Ocean cyclones
Season: November 15 – April 30

9. Origin regions: Requirements:
expansive surfaces of warm (>26⁰C), deep ocean waters
no upper level wind shear
Coriolis (therefore do not develop at the equator)

10. Trade wind Inversion
In an INVERSION, temperature increases with altitude
Instead of the normal decrease with altitude
Recall the Hadley Cell circulation:
Descending air in subtropical Highs
Creates a subsidence inversion
Subsiding air warms
Makes a layer of warm air over cold air

11. Subsidence inversion

12. Inversion is strongest at
Subtropical highs and
weakens towards equator
Inversion prevents
tall clouds from forming
Marine layer
(beneath inversion)
Trade winds
equator
subtropics

13. Inversion is also lowest along eastern margins of oceans ,
Where upwelling and cold, ocean currents keep the marine layer cool.
Inhibits vertical cloud growth.
On western side of ocean basins, warm surface waters heat marine layer and
it expands to a greater height . Tall cloud growth is possible.

14. Therefore, hurricanes form in western portions of ocean basins.

15. Ways that hurricanes begin:
1. as easterly waves
(Hurricane Georges is an example)
Requires spin
Most Atlantic hurricanes begin in Africa

16. Pacific High
Azores Bermuda High
Monsoonal Low

17. NE Trades
ITCZ
Turned SW by Coriolis deflection
equator
ocean
Africa
SE Trades

19. If storm moves off African continent towards Atlantic, becomes “easterly wave” (carried west by equatorial easterlies)
Of approx. 100 each year, 6 become hurricanes

20. 60% of the Atlantic tropical storms and minor hurricanes originate from easterly waves
nearly 85% of major hurricanes have their origins as easterly waves
nearly all of the tropical cyclones that occur in the Eastern Pacific Ocean can be traced back to Africa

21. 2. with wind shifts associated with fronts in the tropics
(Hurricane Arlene is an example)
NE trades
front
S winds

22. front

24. Easterly wave moves across Atlantic as a small bunch of thunderstorms;
May become a tropical disturbance : disorganized group of thunderstorms that spin
tropical disturbances
disorganized groups of t-storms; some spin
90% fizzle out
2. tropical depressions : wind speeds 23 mph
3. tropical storm : wind speeds 39 mph (named)
4. hurricane : wind speed 74 mph

25. Progression of storms to hurricane:
1. tropical disturbances
disorganized groups of t-storms; some spin
90% fizzle out
2. tropical depressions : wind speeds 23 mph
3. tropical storm : wind speeds 39 mph (named)
4. hurricane : wind speed 74 mph

26. Speed and direction in North Atlantic basin:
10 – 20 mph travel (NOT wind speed)
Originally steered by easterly winds (trades)
Are drawn into circulation around subtropical highs and are veered to the north (clockwise)
When north enough, are steered by westerlies
Details of paths are erratic
Due to storm structure and interaction with environment

31. Anatomy of a hurricane Spiral bands of thunderstorms
Strong central LOW
Convergence at surface
Moist warm air rising
Condensation
Upper level winds and
cirrus clouds at top

33. Eye and Eyewall Rising air causes crowding at top center
Causes a HIGH at top
center
HIGH at top and LOW
at bottom makes air
descend in center
Descending air can’t
make clouds
Therefore clear EYE
in center

35. High Low Eye is clear, calm
Average : 15 miles wide; experienced on ground for approx. 1 hour
High
Low

36. Eyewall: surrounds eye
High
Low

37. eyewall
Greatest wind speed, thickest cloud cover; heaviest rain (100 in/day)
Wind speeds strongest here due to conservation of angular momentum

40. Hurricane hunter air flight