Four key conditions are necessary for hurricane formation: warm sea surface temperatures above 26.5°C over a large area, instability in the atmosphere allowing air to rise, little wind shear through the troposphere, and location within 5-10 degrees latitude of the equator. Hurricanes are fueled by the release of latent heat as water vapor condenses, powered by a feedback loop between warm ocean waters and rising moist air. As hurricanes approach land, they lose their energy source and weaken due to disruption of this cycle from colder waters and increased wind shear.

1. Hurricane Formation:
Ocean and Atmosphere Systems
Interact to Create Hurricanes
This work is supported by a National Science Foundation (NSF) collaboration between the
Directorates for Education and Human Resources (EHR) and Geociences (GEO) under grant DUE - 1125331

2. Questions
• What is a hurricane?
• What four conditions are necessary for hurricane formation?
• What happens as hurricanes approach land?

3. What is a Hurricane?
• Make a list of hurricane characteristics:

4. What is a Hurricane?
• Formal definition:
An intense low pressure system with sustained
winds >74 mph

5. What Makes a Hurricane?
• Four essential ingredients

6. What Makes a Hurricane?
1) Warm SST >26.5°C (80°F) over large area

7. What Makes a Hurricane?
1) Warm SST (>26.5 C) over large area
2) Instability in atmosphere (rising air mass, beneath which surface
winds converge)

8. What Makes a Hurricane?
1) Warm SST (>26.5 C) over large area
2) Instability in atmosphere (rising air mass)
3) Little/no vertical wind shear through atmosphere to tropopause
(tropopause)
(atmosphere)
(ocean)
Which wind profile has
too much vertical shear
for hurricanes to form?

9. What Makes a Hurricane?
1) Warm SST (>26.5 C) over
large area
2) Instability in atmosphere
(rising air mass)
3) Little/no vertical wind shear
through troposphere
4) Sufficient latitude >5-10o off
equator

10. Hurricanes are powered by:
• latent heat stored in water vapor
– released when water condenses
1. Warm water supplies sensible heat and
humidity to overlying air
2. Air decreases density; rises
3. Air cools; H2Ovapor condenses
4. Latent heat released
-- Heat warms air; rises faster
5. P gradient increases
-- Faster winds converge at the low pressure center
-- More water vapor into system!
Feedback
between the
ocean and
atmosphere
systems!

11. Can Hurricanes Cross the Equator?
• Discuss with the person next to you.
• Why or why not?

12. Can Hurricanes Cross the Equator?
– Discuss with the person next to you.
– Why or why not?
(Hurricane tracks 1985-2005; NASA)

13. North Atlantic Hurricane Season is
June-November
Most storms occur in which month?
4
3.5
3
2.5
2
1.5
1
0.5
0
Average number per month, 1851-
2011
Tropical Storms
Hurricanes
Hurricanes w/US Landfall

14. Which of these things would cause
a hurricane to lose energy?
a) Moving over colder water
b) Moving over warmer water
c) Making landfall
d) Crossing the ocean

15. As a hurricane approaches land

16. Wind and Rain
(NOAA)

17. Storm Surge
(NASA)

18. As hurricanes make landfall, they
decrease in strength
Why?
Remember
what fuels
hurricanes
…
Warm
water!

19. Draw:
• The relationships between the ocean,
atmosphere, and people in a hurricane.
Make sure you can now answer these:
What is a hurricane?
- What four conditions are necessary for hurricane
formation?
- What happens as hurricanes approach land?

20. References
• Slides 4 and 5. Public Domain Image of Hurricane Rita as a Category 5 hurricane.
http://www.nnvl.noaa.gov/hurseas2005/Rita2045zD-050921-1kg12.jpg
• Slide 6. Public Domain Image of SST.
http://www.osdpd.noaa.gov/data/sst/fields/FS_km10000.gif
• Slide 7. Public Domain illustration of tropical waves.
http://www.aoml.noaa.gov/hrd/tcfaq/Twaves.jpg
• Slide 9. Creative Commons illustration of hurricane forces:
http://en.wikipedia.org/wiki/File:Hurricane_isabel_and_coriolis_force.jpg
• Slide 12. Public Domain image of hurricane tracks:
http://www.nasa.gov/images/content/313775main_global_hurr_tracks_HI.jpg
• Slide 15. Public Domain image of Hurricane Eloise approaching
shore.http://www.photolib.noaa.gov/historic/nws/images/big/wea00416.jpg
• Slide 17. Public Domain illustration of storm
surge.http://earthobservatory.nasa.gov/Library/Hurricanes/Images/storm_surge.gif
• Slide 18. Public Domain image of 2012 hurricane tracks.
http://www.nhc.noaa.gov/2012atlan.shtml
• All other illustrations and text were created by Lisa Gilbert under a Creative Commons Attribution-NonCommercial-
ShareAlike license http://creativecommons.org/licenses/by-nc-sa/3.0/ You may reuse this item for non-commercial
purposes as long as you provide attribution and offer any derivative works under a similar license.