Valley Breeze Example
©1997 Prentice-Hall, Inc.
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Valley Breeze Clouds
J.M. Moran -- ©1997 Prentice-Hall, Inc.
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Heat Island -- Washington D.C.
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©1997 Prentice-Hall, Inc.
Sea Breeze Example
©1998 Wadsworth Publishing Company
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Hadley Cell
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Smooth Earth
No Rotation
©1998 Prentice-Hall, Inc.
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Add Rotation
• We now allow the
earth to rotate. As
expected, air travelling
southward from the
north pole will be
deflected to the right.
• Air travelling
northward from the
south pole will be
deflected to the left.
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Three-Cell Conceptual Model
©1998 Prentice-Hall, Inc.
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Three-Cell Model
• By looking at the actual winds, even
after averaging them over a long period
of time, we find that we do not observe
this type of motion.
• In the 1920’s a new conceptual model
was devised that had three cells instead
of the single Hadley cell.
• These three cells better represent the
typical wind flow around the globe.
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Planetary Scale Circulations
• A very complicated pattern that we shall
examine only in a very simple form.
• To begin, imagine the earth as a nonrotating sphere with uniform smooth
surface characteristics.
• Assume that the sun heats the
equatorial regions much more than the
polar regions.
• In response to this, two huge convective
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cells develop.
Horse Latitudes
• Around 30oN we see a region of
subsiding (sinking) air.
• Sinking air is typically dry and free of
substantial precipitation.
• Many of the major desert regions of the
N.H. are found near 30o latitude.
– Sahara
– Middle East
– SW United States
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Intertropical Convergence Zone
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ITCZ -- Moves south in January
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Intertropical Convergence Zone
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ITCZ -- Moves north in July
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Monsoon
• Wind systems that exhibit a pronounced
seasonal reversal in direction.
• Best known monsoon is found in India
and southeast Asia.
• Winter -- Flow is predominantly off the
continent keeping the continent dry.
• Summer -- Flow is predominantly off the
oceans keeping the continent wet.
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Monsoon -- Winter
Continents are
dry -- dry
season.
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Monsoon -- Summer
Continents are
wet -- rainy
season.
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Air Masses
• Air Mass
– A huge volume of air that is relatively
uniform horizontally in temperature
and water vapor concentration.
– Properties of an air mass are usually
defined by the type of surface it
develops over ---- the source region.
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Air Masses
• Source Regions
– Relatively flat
– Uniform Surface Compositions
• Oceans
• Great expanse of snow covered
ground
• Air needs to be in contact with the
source region for a long period of time
to develop uniform characteristics.
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Classification of Air Masses
• Temperature
– Cold
Polar
(P)
– Warm
Tropical (T)
• Moisture
– Dry
– Moist
Continental (c)
Maritime (m)
• Results in four basic air mass types.
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Classification of Air Masses
• Continental Tropical (cT)
– Hot, Dry
– Develops over the deserts of Mexico and
the Southwestern United States
• Maritime Tropical (mT)
– Warm, Humid
– Develops over the tropical and subtropical
oceans and the Gulf of Mexico
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Classification of Air Masses
• Continental Polar (cP)
– Cold, Dry
– Develops over the northern interior of
North America -- Central Canada
• Maritime Polar (mP)
– Cold, Moist
– Develops over the cold ocean waters of the
North Pacific and North Atlantic
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Classification of Air Masses
• Arctic Air [(A) or (cA)]
– Very Cold and Usually Dry
– Develops over the snow or ice covered
regions of continents
– Similar to polar air masses but usually
develops in regions north of 60N where
there is little or no insolation during the
winter.
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Classification of Air Masses
Arctic
mP
cP
mP
cT
mT
mT
mT
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Air Mass Modification
• Air masses do not remain over their
source region.
• If the air moves over a region that is
different from where it originated, the air
mass will be modified, or changed, by
the land that the air is travelling over.
• Changes include: warming, cooling,
adding or reducing moisture
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Air Mass Modification
cP
The cP air mass will
be warmed by the
warmer land that it
passes over.
Warmer
Land
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Air Mass Modification -Changes in Stability
Cold air mass moves over a warmer surface.
cP
The lower layers are
warmed and may
become unstable.
Warm, Moist
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Air Mass Modification -- Changes in Stability
Cold air mass moves over a warmer surface.
7000
7000
6000
6000
5000
5000
4000 Altitude
4000 Altitude
3000
-10
0
10
20
(m)
3000
2000
2000
1000
1000
0
0
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Temperature (C)
Sounding Before Modification:
Conditionally Unstable
-10
0
10
20
30
Temperature (C)
After modification:
Absolutely Unstable
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(m)
Air Mass Modification -- Changes in Stability
Warm air mass moves over a colder surface.
mT
Warm, Moist
The lower layers are
cooled and tend to
stablize.
Cold
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Air Mass Modification -- Changes in Stability
Warm air mass moves over a colder surface.
7000
7000
6000
6000
5000
5000
4000 Altitude
4000 Altitude
3000
-10
0
10
20
(m)
3000
2000
2000
1000
1000
0
0
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Temperature (C)
Sounding Before Modification:
Conditionally Unstable
-10
0
10
20
30
Temperature (C)
After modification:
Absolutely stable
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(m)
Air Mass Modification -- Orographic
Effects
Pacific Air is warm but not as
dry as continental air.
“Pacific”
Air
mP
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Fronts
• Front
– A narrow transition zone between air
masses of differing densities.
– The density differences usually arise
from temperature differences.
– Density differences may be a result of
humidity differences (summer).
• A front is the boundary or transition
zone between different air masses.
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Fronts
• The transition zone is usually narrow
(15 to 200 km in width).
• Fronts may be over 1000 km long.
• On maps, we draw the frontal boundary
on the warm side of the transition zone.
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Idealized Cold Front
Isotherms
COLD
FRONT
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0C
5
100 km
75
101520
25
30
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Frontal Zone
Idealized Cold Front
• Cold Front
– A boundary that moves in such a way
that the colder (more dense) air
advances and displaces the warmer
(less dense) air.
– The largest temperature differences
are normally associated with cold
fronts.
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