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11. Wind Systems
11. WIND SYSTEMS
A&B: Ch 8 (p 214-238)
Concepts:
I. Scale
II. Differential heating
III. Wind direction
1. Scales: Three major divisions
Space
Micro
meters
Meso
kilometers
Macro Synoptic
100 - 1000 km
Planetary 1000+km - Global
Time
seconds - minutes
seconds - hours
days
days - weeks
2. Differential Heating
Spatially - get differences in surface heating
e.g. Micro: grass - concrete - Lab 5
Meso: land - lake
Macro: equator - poles
• T difference
3. Wind direction
Based on where the wind is coming from
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e.g. sea breeze - air coming from the sea
valley breeze - air coming from the valley
Micro-scale
Example: Turbulent eddies
- small whirls of air
- dust devils
- gusts
Meso-scale
a. Land-sea breeze (land - lake breeze)
Dail y T difference between land and sea
Daytime:
• land heated more intensively compared to water
(remember temperature section of course)
http://www.indiana.edu/~geog109/topics/temp/temp.htm
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•
• Pressure gradient force: H ⇒ L
Nighttime:
• Reverse
• Land cools more rapidly than water
• Warmer over the water
• Land breeze
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11. Wind Systems
Sea breeze - can have a significant modifying effect on
the T in coastal areas
e.g. Lake breeze - Chicago
Size of breeze varies.
b. Mountain - Valley Breeze
Day:
• slopes of mountains
get more intense
heating than air at the
same elevation over
the valley floor
• Valley breeze
• Most common in
summer
Sunset/Night
• Rapid cooling of
slopes
• Mountain breeze
• Most common in
winter
•
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⇒ seasonal preference for winds
c. Foehn, Chinook, (Santa Ana) winds
Chinook – Rockies (Montana and Wyoming)
Foehn - Alps, N.Z.
Santa Ana – California –special case (high pressure)
• Migration of surface
and upper level
weather systems
• Low pressure system
on the lee side of a
mountain barrier
⇒ Pulls the air across
• Adiabatically warms
as it comes down
mountain
• Usually occur winter
/spring
When T cold, melts
snow
• T can rise by 20 C
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Special cases of ⇒
d. Katabatic Winds --- from local cooling over highelevation plateau
• downslope wind
• winter time
•
Bora - Adriatic sea
Mistral - France
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Bora--Adriatic Sea: Cold, north to northeast katabatic wind flowing through mountain
passages into the Adriatic Sea east of Italy. Bora--Adriatic Sea may be associated with
stormy weather, with winds reaching 100 kts or more. Bora-Adriatic Sea develops in the
winter when cold polar air builds over the Balkans and flows katabatically through the
valleys of the Dinaric Alps and into the Adriatic Sea.
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Mistral -- France
Mistral: Cold, northerly katabatic wind flowing into the Gulf of Lion from the southern
coast of France. Mistrals are most common during winter and spring. In the winter,
wind speeds can reach over 100 knots off the southern coast of France. Gale-force
Mistrals often develop when cyclogenesis occurs over the Gulf of Genoa with the
passage of the 500 mb trough (see forces and pressure notes) through eastern France.
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Synoptic and Planetary Scale Wind systems
Larger scale circulations influence the smaller scale
winds
i.e. it is important what the synoptic situation is
Global Circulation
• Differential heating between equator and poles ⇒
pressure differences
Three cell circulation model
• Realistic model – base on earth rotation and
energy gradient between the equator and the
poles
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• Hadley Cell : Equator and Subtropics --thermally
driven circulation—intense heating upward expansion
o ITCZ – Very strong zone of low pressure at
the equator --Convergence--( small
deflection -- Tradewinds)
• Ferrel Cell : Midlatitudes (NH-deflection to right—
SH deflection to left--westerlies)
• Polar Cell : Poles –thermally driven circulation—
Very cold condition--high surface pressure--sinking air—
strong deflection
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Close approximation to real world patterns
Reality
9 ITCZ
9 Trade winds
9 Hadley Circulation Cell
Break down of Ferrel and Polar cells
Surface winds
9 Westerlies esp. SH
• Real world --- alternating semi-permanent high
and low pressure cells
January
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Jet stream
• An area of increased wind speeds
o Narrow band: 100 - 500 km wide
o 9 – 12 km above sea level
o Speeds: 200 - 500 km h-1
40F
45F
75F
80F
•
•
•
•
•
•
• Most powerful jetstream: polar jet stream
o Moves north and south with the seasons
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World Jet stream positions 11/14/05
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US Jet Stream Position 11/21/05
http://www.weatherimages.org/data/imag192.html
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US Jet Stream Position 11/05/2007
http://www
.nws.noaa.
gov/outloo
k_tab.php