Announcements

1
Announcements
Quiz 3 take home - Available in class today. Due
a week from today - Tuesday, December 6.
Quiz 4 (last one!) - This Thursday. Covers
lectures for Ch. 8, book and lectures for Ch. 10.
I am accepting exam questions through today -
hand them in at the end of lecture.

• Final Exam Review Session
• Review session Wed. 7 Dec, 600 - 700 pm, room
  ILC 140.
• Will be cumulative.
• I have put together a set of questions from
  throughout the semester for you to study from and
  will hand it out next week.

2
Review
Tornadoes
Basic facts and occurrence statistics
Rotational winds/Fujita Scale
Formation of supercell and nonsupercell tornadoes
Waterspouts
Basic definition
Some similarities to tornadoes over land
3
Tornadoes
Tornado facts
100 - 600 m (300 - 2000 ft) in diameter
Move at 20 - 40 knots (23 - 46 mph) (forward
speed)
Last only a few minutes, traveling about 7 km (4
miles)
Tornado occurrence
Where do most tornadoes occur in the world?
What region of the US see the most tornadoes?
During what season do most tornadoes occur?
4
Tornadoes
Formation of supercell tornadoes
What is special here? Look at mid-levels to get a
clue.
Wind speed increasing with height
Cold, dry air is above warm, moist air ?
conditionally unstable!
How does warm, moist air rise?
5
Tornadoes
Nonsupercell tornado formation (landspouts)
Favorable environment Surface winds flowing in
opposite directions and converging along a
boundary
Why do clouds form on boundary?
Developing cloud moves over region of rotating air
Why does surface rotating air get drawn into
cloud?
6
Key Concepts
Tropical Weather
Where are the tropics?
What are the temperature ranges and prevailing
wind patterns in the tropics?
What marks the seasons in the tropics?
What are easterly waves and how are they related
to hurricanes?
Hurricanes
When does a tropical storm become a hurricane?
Hurricane facts (time and space scales, typical
wind speeds, etc)
Naming hurricanes
Anatomy of a hurricane (terminology used to
describe storm)
7
The Tropics
Where are the tropics located?
Between 231/2 north and south latitudes or
between the Tropic of Cancer and the Tropic of
Capricorn
Doldrums
Southeasterly trades
8
Tropical Climate
What are the temperature variations like in the
tropics?
Diurnal (daily) and seasonal temperature changes
are very small
Monthly Averaged Surface Air Temperature
January
June
9
Tropical Climate
What are the precipitation variations like in the
tropics?
Precipitation in the ITCZ defines the seasons in
the tropics more so than temperature variations
Monthly Averaged Precipitation
January
June
September
March
10
Tropical Climate
Easterly waves - waves in the easterly trade
winds, most active when ITCZ is active in a
region, many hurricanes form from these waves
Prevailing winds
source
L
L
L
L
Courtesy of http//www.planearthscience.org.
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Tropical Climate
Easterly waves
Recall that pressure is lowest at the equator in
the tropical regions, so a trough looks different
in the tropics than at mid-latitudes
Mid-latitude Trough
Tropical Trough
EOM, Fig. 5, p. 225
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Tropical Climate
Easterly waves
Convergence and divergence regions also change
sides
Mid-latitude Trough
Tropical Trough
Courtesy of http//www.planearthscience.org.
13
Tropical Climate
Easterly waves
Clouds form on east side of low pressure region
in area of convergence
Courtesy of http//www.planearthscience.org.
14
Tropical Climate
Easterly waves and hurricanes
Low pressure region can close off, creating a
tropical storm and eventually a hurricane if
conditions are right
Visible Satellite Images
Aug 8
Aug 5
25N
15N
5N
5S
Aug 6
Aug 9
Aug 7
Aug 10
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Hurricanes
Where do hurricanes form?
hurricanes (Atlantic), a.k.a. typhoons (west
Pacific), cyclones (Australia) - all are tropical
cyclones
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Hurricanes
Stages of Development
Tropical disturbance - initial mass of
thunderstorms, could be in convergence region of
a tropical wave
Tropical depression - low pressure closes off,
winds from 20-34 knots
Tropical storm - winds from 35-64 knots
Hurricane - winds exceed 64 knots (74 mph)
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Hurricanes
Hurricane Facts

• Average about 500 km (310 miles) in diameter

• Rotational winds 64 knots (74 mph), can be 200
  mph!

• Forward motion in trade winds averages 10 knots

• Form over water ? 26.5C (80F)

• Rotate counter-clockwise in NH, clockwise in SH

What atmospheric scale are hurricanes?
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Hurricanes

• Triggers for hurricanes (or any tropical system)
  include
• Easterly waves
• Large-scale upper level divergence
• Mid-latitude fronts moving into tropics

What do all these triggers provide?
19
Hurricanes
Naming Hurricanes

• Assigned when the storm reaches tropical storm
  strength

• From 1953-1977 only female names were used, where
  names were given in alphabetical order each season

• 1978 female and male names were used alternately
  in the east Pacific, the same was done for
  Atlantic storms starting in 1979

• Six lists are used in rotation. So the list for
  2005 will not be used again until 2011.

• Similar lists exist for the central and west
  Pacific regions.

• Names of storms which caused great damage are
  retired for at least 10 years

20
Hurricanes
Anatomy of Hurricanes
Counter-clockwise rotation (NH)
Eye
Eye wall
Rain free area
Spiral rain band
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Hurricanes
Anatomy of Hurricanes
Eye Light winds Broken clouds 25 km wide Lowest
pressure within the storm
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Hurricanes
Anatomy of Hurricanes
Eye wall Surrounds the eye Intense thunderstorms
to 15 km high Strongest winds Heaviest
precipitation (25 cm per hour) Large storm surge
beneath this region, waves can be 10 m (33 ft) or
more!
Hurricane Katrina, 2005
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Hurricanes
Anatomy of Hurricanes
Inflow - warm, moist air evaporated from the warm
surface waters flowing counter-clockwise into the
storm base Outflow - clockwise rotating winds of
dry air flowing out of the storm at the top of
the clouds
Hurricane Katrina, 2005
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Hurricanes
Anatomy of Hurricanes
Outflow
25 km
Eye wall
15 km
Eye
surface
inflow
500 km