Download Atmospheric Circulation Density of Air

Atmospheric Circulation
Today s topics:
•  Atmospheric
circulation: –  generation of wind
patterns on a rotating
Earth
–  Seasonal patterns of
climate: Monsoons and
Sea Breezes
•  Tropical Cyclones:
Hurricanes and
typhoons
Density of Air
•  Varies with T, P & H2O
•  T effect: hot air expands due to collisions between molecules
–  Thus, as air warms it becomes less dense and rises
–  Cold air has lower volume due to fewer collisions. Thus, denser and sinks
Density of Air: H2O and Pressure effects
•  H2O effect: Moist Air is less dense
than dry air! –  Density of H2O molecule is less
than the density of the air
molecules (N2 + O2) it displaces.
–  Moist air rises
•  P effect: Atmospheric Pressure ~
weight of atmosphere (~14.7 lb/in2
at sea level)
–  P is greatest at the surface
(compresses air) and decreases
with elevation (air expands).
•  Compressed air warms up (due to
colliding molecules)
•  Expansion of air causes cooling
(reduced number of collisions
between molecules)
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Vertical Circulation
• 
Near Surface Atmosphere
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Heating from surface causes expansion Moisture from surface absorbed by air
Combined effect: decrease density, and Thus, warm, moist air rises
Rising air experiences lower pressure
–  Thus, air expands and cools
–  Cool air cannot hold moisture
–  Thus, condensation of droplets, releases
heat of vaporization to the air
•  Causes warming, resulting in…
–  Continued rising and cooling and
condensation
•  If rapid enough = storm
• 
Eventually, at high altitude vapor water
is lost and air mass is cooled –  Causes air mass to sink
–  Compresses under pressure and Warms
Draw vertical circulation of the atmosphere due to surface heating
Which way does the wind usually blow in Illinois?
A. 
B. 
C. 
D. 
North to South
East to West
West to East
South to North
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True or False: The direction of rotation (clockwise vs. counterclockwise) of
tropical storm depends on the hemisphere.
A.  True
B.  False
Why?
• 
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Take a minute to
consider this. You may
talk with your neighbor
I will call on you
True or False: The direction of rotation (clockwise vs. counterclockwise) of
water down a draining sink depends on the hemisphere.
A.  True
B.  False
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Remember, Earth Rotates! •  Rotation toward East
•  Angular velocity of Earths
rotation = 15º/hr
•  But arc-length is greatest at
low latitude, thus faster
rotational velocity at low
latitude
–  This results in Coriolis effect
–  Which in combination with
solar heating of surface results
in atm. circulation pattern
15º/hr
Coriolis Effect
•  Plane leaves equator with a
northward velocity
–  But also, initial eastward velocity
due to rotation of the Earth (1674
km/h at equator)
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This initial eastward velocity is greater
than the rotation speed of Earth s surface
at higher latitudes.
So, relative to Earth s surface the plane
veers eastward as it travels northward
Coriolis Effect deflects flow paths to the
right in N. hemisphere and to the left in S.
hemisphere
Note: this deflection is relative to Earth s
surface. •  An observer from space would see
the plane flow due north without
deflection
Show animation Coriolis 1
Draw atm circulation with rotation
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Atm. Circulation & the Coriolis effect
•  Vertical circulation model combined with Coriolis effect produces the pattern of
observed circulation cells.
• 
Equatorial heating
–  Near surface air masses heat, rise, expand, cool, and precipitate over tropics
–  Air flows over surface to replace rising air mass
–  Easterlies = surface flow deflected by Coriolis
Circulation at Polar cells
Polar cooling
•  Air sinks
•  Flow over surface toward lower latitude, deflected westward by Coriolis –  Polar easterlies = deflection
•  Heats as it flow over surface and rises at border with temperate cell
Temperate or Mid-latitude (30-60º ) cells
Flow deflected by
Coriolis =
westerlies
•  Result from neighboring cells:
–  sinks at subtropic high pressure belt due to sinking of subtropical cell
–  Flows over surface, heating and Rises at 60º due to meeting air mass from polar cell.
Circulation Animation
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Can you predict Wet vs Arid regions based on the pattern of circulation cells?
Weather animation
Effect of land on Circulation:
Sea Breezes
•  During day - on-shore breeze
–  Heat land (low heat capacity)
–  Rising air mass draws in
cooler air from over the ocean
•  During night = off-shore
breeze
–  Warm ocean relative to land
(heat capacity again)
–  Warm air over ocean rises as
cool air over land sinks and
flows out to ocean
Monsoon ~ season long onshore breeze
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Heat continent over summer
Continuously rising air mass
Replaced by moist ocean air
Continuous rain
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Tropical Cyclones (wind speeds > 120 km/hr)
•  Hurricane (Atlantic) = Typhoon (Pacific) = Cyclone (Indian)
Wind Speeds
•  Tropical Depression
< 61 km/hr winds
•  Tropical Storm
between 61 & 120km/h winds
Saffir-Simpson Intensity Scale
1: 120-153 km/hr winds (74-95 mph), minor bldg. damage
2. 154-177 km/hr winds (96-110 mph), damage to roofs, windows, trees downed
3. 178-209 km/hr winds (111-130 mph), some structural damage, wall failures, large trees downed
4. 210-249 km/hr winds (131-155 mph), extensive structural damage, most signs and trees downed
5. > 250 km/hr (>150 mph), catastrophic damage to entire buildings common
Tropical Cyclones
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Atmospheric disturbance (in Atlantic, usually a wave from the Sahara)
Low pressure cell along tropical convergence zone breaks away and flows into N
or S hemisphere
–  Warm surface water (>27ºC, ~81ºF) evaporates readily and in great quantities.
–  Warm, moist air rises, expands, cools & Precipitates rain releasing huge amounts of E
to the atm (latent heat of vaporization) and supplies additional energy for WIND
–  Near surface flow rushes in to replace rising air - deflected by Coriolis resulting in
spinning storm
–  Wind speed > 119 km/h (74 m/h)
Tropical Cyclone Structure
•  In one day, a large cyclone
releases 2.4 trillion kilowatthours of power –  Equal to the annual electrical
power needs of the US
•  Why do they spin?
–  We need to draw this….
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Draw N hemisphere cyclonic spin
Hurricanes + land = devastation
Damage from Wind (exceeding 120 mph)
Heavy rain (flooding)
Storm surge along the coast (up to 40
feet high)
• Low pressure of the storm causes a
bulge or hill to form on the surface
of the ocean. When the storm
comes ashore, this hill does too
(storm surge)
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Hurricanes produce huge waves:
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Hurricane Ivan (09/04)
Track (green path) of storm directly
over wave buoy (red dot)
Measured increasing average storm
wave height, H (red circles) and
maximum height, Hmax (blue x s)
as eye approaches (green square).
Hmax = 27.7 m
24 waves with H >15 m
The measured Hmax is not true Hmax
of storm. NE quad of the storm
passed E of buoy, thus the wave
with the true Hmax past NE of buoy.
Hurricanes = Major source of sediment to the coast
[sufficient to account for all (in-org) sediment in healthy saltmarsh wetlands]
Rita
Katrina
Damage From Katrina: Much of it avoidable
• 
Nov. 2009, Federal Judge Ruled Army Corps of
Engineers culpable in flooding of Lower 9th Ward and
neighboring St. Bernard Parish
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Corps failed to maintain the Mississippi River-Gulf Outlet
navigation channel linking river to the gulf
Corp failed to maintain banks of channel, allowing widening
of channel and a wave of flood water to impact the levee
causing it to fail and flood.
Corps inaction cited as "monumental negligence by the
judge and a clear case of talk is cheep .
First time Corps ever found liable for disaster.
Government now open to 100,000 s lawsuits
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Review Questions
•  What causes the circulation patterns in the atmosphere?
–  How are these patterns affected by the Coriolis effect?
•  Describe the Coriolis effect and how it works.
•  What causes sea breezes and monsoons?
•  Tropical Cyclones
–  What conditions are necessary for their formation?
–  Why do cyclones in the northern hemisphere spin counter
clockwise?
–  How do cyclones do damage and why are they beneficial?
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