Review of adiabatic cooling and atmospheric stability:

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Review of adiabatic cooling and atmospheric
stability Adiabatic temperature change that
occurs without exchanging heat with its
surroundings What does that mean? As a parcel
rises, air expands, causing the temp. and density
to decrease helps the parcel to rise even
further As a parcel sinks, air compresses, the
temp and density of the parcel increase helps
the parcel sink further
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If the environmental lapse rate is lower than
both the dry and moist adiabatic rates
(temperature decreases faster) Then the air
parcel continues to rise because it is warmer
than its surroundings It is unstable
3000
DAR 10oC/km
MAR 6oC/km
2000
Altitude (m)
1000
Eventually if the parcel continues lifting and
cooling it may reach saturation temperatures and
condense
Temperature (oC)
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If the environmental lapse rate is higher than
both the DAR and MAR (temperature decreases
slower) Then the air parcel is always cooler than
the surroundings and will settle back down to the
surface It is stable
Eventually if the parcel continues lifting and
cooling it may reach saturation temperatures and
condense
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If the environmental lapse rate is between the
DAR and MAR, then it is known as conditionally
unstable If the air parcel is less than
saturated it follows the DAR and will be cooler
than its surroundings and remain stable
If the parcel is saturated it follows the MAR and
will be warmer than its surroundings, be unstable
and continue to rise and possibly make rain clouds
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Atmospheric lifting mechanisms Recall - A rising
air mass expands and cools adiabatically until it
reaches the dew-point temperature and condenses
to form clouds and eventually rain But why does
the air rise in the first place?
Convergence
Convection
Orographic lifting
Fronts
Figure 8.6
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Convergent lifting Air flows in toward an area
of low pressure When this occurs in tropics, may
create a disturbance that leads to development of
a tropical storm Commonly occurs along the
equator where the SE and NE tradewinds converge
to form the ITCZ Results in towering
cumulonimbus clouds and high average annual
precipitation
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Convectional lifting Local surface heating
causes warm air to rise Example air moving from
the ocean over the continent (the continent is
much warmer) air moving over a city or an area
of dark plowed field surrounded by lighter
planted fields
If atmosphere is unstable in that location, then
clouds formation occurs Peninsula of FL - land
heats up during the day relative the surrounding
ocean, convectional lifting begins and forms
cloud cover
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Orographic Lifting
Orographic lifting - air is forced into the air
over a barrier such as a mountain range If stable
air is forced up over a mountain, then it
produces stratiform clouds
Figure 8.9
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Unstable or conditionally unstable air mass forms
a line of cumulus or cumulonimbus clouds Barrier
enhances convection and causes additional lifting
as air passes extracts more moisture from
passing air masses
Figure 8.9
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As air moves up the mountain (windward slope),
adiabatic cooling causes moisture to condense On
the far side of the mountain (leeward slope),
descending air is heated by compression and any
remaining water evaporates Chinook winds are warm
downslope airflows that can bring a large
increase in temperature and decrease in relative
humidity on the lee side of mountains
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Orographic Lifting
Figure 8.10
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Frontal Lifting

• One air mass is forced over the top of another
  contrasting air mass
• named by Vilhelm Bjerknes, a Norwegian
  meteorologist during WWI
• Front is a place of atmospheric discontinuity - a
  narrow zone or line between two air masses of
  different temp, pressure, humidity, wind speed
  and direction, and cloud development
• Leading edge of cold air mass cold front
• Cold air forces warm air aloft
• 400 km wide (250 mi)
• Leading edge of warm air mass warm front
• Warm air moves up and over cold air
• 1000 km wide (600 mi)

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Polar Jet stream
Subtropical Jet stream
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Names of air masses in N America
Figure 8.2
Classified according to moisture and temperature
characteristics Depends on latitude
moisture mmaritime (wet), ccontinental
(dry) temperature AArctic, PPolar, TTropical,
EEquatorial, AAAntarctic
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cP - continental polar air mass only in N
hemisphere dominant in winter area covered in a
cP air mass experiences cold, stable air, clear
skies, high pressure, anticyclonic wind flow S
hemisphere lacks landmasses (continentality) at
high latitudes to create this air mass
Figure 8.2
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mT - maritime tropical Gulf/Atlantic air is
unstable and creates humid air in the East and
Midwest from late spring to early fall Pacific
air mass is stable or conditionally unstable with
lower moisture content and less energy affecting
the western US (less rainfall than rest of
country)
mP - maritime polar over northern oceans W and E
of N America area under this air mass experiences
cool, humid, low pressure, unstable air all year
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mT - maritime tropical air mass subtropical high
pressure cell in Atlantic winds in W Atlantic
(East US) blow onshore passing over warm Gulf
Stream - create moist unstable conditions
compared to Pacific winds in E Pacific (West US)
blow offshore passing over cold California
Current create drier stable conditions
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Clouds and Fog

• Cloud Formation Processes  
• Cloud Types and Identification  
• Fog  

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Cloud Formation
Clouds can indicate stability of air masses,
moisture content in the air, and weather
patterns Clouds form as air becomes saturated
with moisture and condenses around a nuclei of a
microscopic particle (these are common throughout
the atmosphere)
A cloud is an aggregate of tiny moisture droplets
and ice crystals suspended in air that are in a
large enough volume and concentration to be
visible Fog is a cloud in contact with the ground
Figure 7.20
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Cloud types organized by altitude and shape
(see table 7.2)
Figure 7.22
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• Low clouds (0-2000m) formed from water
• Stratus
• uniform, featureless,
• gray clouds with no rain

• Nimbostratus
• gray, dark clouds that produce drizzling rain
• usually begin as stratus clouds

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• Low clouds (0-2000m)
• Cumulus
• puffy, cottonball-like, with
• flat bottoms and billowing tops

• sometimes these can extend up to 13,000m
• Stratocumulus
• soft gray lumpy clouds in lines, groups or waves
  having heavy rolls
• sometimes indicate clearing weather

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• You'll see recurring pieces of terms referring to
  cloud form and development
• stratiform
• flat and
• layered clouds,
• develop
• horizontally
• cumuliform
• puffy and globular, develop vertically
• cirroform
• wispy clouds at high altitudes, made of ice
  crystals

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• Middle clouds (2000-6000m) formed from water and
  ice
• Altostratus
• may be thin or thick
• blanket of clouds, Suns'
• outline is just visible
• makes for a gray day

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• Altocumulus
• patchy clouds in cotton ball shapes that may be
  arranged in lines, waves or groups
• also includes lenticular (lens-shaped) clouds
  associated with mountains

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• High clouds (6000-13,000m) formed from ice
• Cirrus
• also called mare's
• tails, wispy, feathery
• streaks or plumes
• usually indicate a
• coming storm especially when thicker
• Cirrostratus
• veil of fused sheets of ice crystals, looks milky
  and Sun or Moon make halos

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• High clouds (6000-13,000m)
• Cirrocumulus
• dappled or "mackerel sky
• sometimes in lines or groups or ripples

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• Cumulonimbus
• tall towering dense
• thunderhead cloud creating
• thunderstorms with heavy
• rains
• may have a cirrus top
• blown by wind into an
• anvil head

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Cumulonimbus Development
Figure 7.23
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Effect of clouds on temperature In general,
cloud cover moderates temperature Specifically,
it depends on the type of clouds, their height
and density Moisture in clouds reflects, absorbs
and liberates large amounts of energy reduces
the insolation that reaches the surface
remember water is the most powerful greenhouse
gas This process is known as cloud-greenhouse
forcing
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Effect of clouds on temperature At night, clouds
act as insulation, preventing rapid loss of
energy During the day, clouds reflect insolation
(because of their high albedo) This process is
called cloud-albedo forcing
Clouds are the most variable and unpredictable
factor influencing Earth's radiation budget and
therefore climate Scientists resort to computer
models to try and predict climate change
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Fog is a cloud layer on the ground where
visibility is restricted to less than 1 km (3300
ft) Fog occurs when the air temperature near the
ground is at the dew-point temperature Air is
saturated with moisture Characterized by an
inversion layer Temperature can be up to 22?C
different between the cool air under the fog and
the warm air above the fog layer
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Advection Fog air migrates up or down and
becomes saturated with water vapor
Air moves to a new place where conditions are
right to make fog
Figure 7.24
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Example Warm moist air moves out over cold water
(ocean, lake, snow) and air becomes chilled to
the dew point Common off subtropical west coasts
around the world Example Cold air moves out over
warm water (lake, ocean, swimming pool) causing
evaporation until air becomes saturated
Sometimes happens out at sea (called sea smoke)
and becomes a shipping hazard
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Example Cool air moves down slope into a valley
causing the moist air in the valley to cool to
the dew point Called valley fog
Figure 7.25
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Advection Fog upslope Fog
Example Moist air moves up the side of mountain
to higher elevation, adiabatically cooling by
expansion until it reaches the dew point Called
upslope fog, it forms a stratus cloud at the
altitude of condensation Common in winter and
spring in Rocky Mountains and Appalachians
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Radiation fog Radiative cooling of a surface
chills the air above it to the dew point

• Forms over moist ground commonly on clear cool
  nights
• Cannot happen over water because water's latent
  heat prevents it from cooling enough at night

Figure 7.26
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Harvesting fog Taking our cue from certain
desert animals, humans have started trying to
harvest fog for a water source in desert
regions Example Sand beetles in the Namib
Desert (southwest Africa) harvest fog in the
early morning by holding up their wings to force
condensation of fog, letting the water drip into
their mouths Example in Atacama Desert (Chile
and Peru), people stretch large nets to intercept
fog, forcing condensation on the netting, which
drips into pans and then into pipes the feed a
reservoir (100,000L)
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Project in Chile started in 1993 There are 30
countries around the world that have conditions
suitable for this kind of technology
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Cold Front
cold air mass has a steep face and hugs the
ground because of high density and uniform
characteristics warm moist air in front of the
cold front is lifted up and over the cold air
mass, causing adiabatic cooling
Figure 8.11
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Cold Front 1-2 days in advance of the front's
arrival, there are high cirrus clouds -
indicating the lifting mechanism As the front
advances, wind shifts, temperature drops,
barometric pressure drops due to lifting Lowest
pressure occurs as the line of most intense
lifting passes, just ahead of front itself
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Cold Front Clouds build along the front into
characteristic cumulonimbus - appears as an
advancing wall of clouds rain is heavy, can
include hail, lightening, thunder After the front
passes, usually have northerly winds in N
hemisphere because the anti-cyclonic high
pressure mass moves in causes lower
temperatures, increasing air pressure (cooler,
denser air), broken cloud cover
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Cold front Fast advancing cold front causes
violent uplift and creates a zone along or
slightly ahead of front called a squall line
Wind patterns are turbulent, precipitation is
intense, may evolve tornadoes Well defined
frontal clouds rise up to 17,000m with
thunderstorms forming along the front
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On weather maps Cold front drawn as a line with
triangular spikes pointing in direction of
frontal movement In N America, the front would
be along an advancing cP or mP air mass
Usually over N America, contrast between cP and
mT air masses are best developed and can easily
access each other can create large storms and
dramatic temperature shifts on each side of the
front
Figure 8.18
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Warm Front
Leading edge of advancing warm air is unable to
displace cooler more dense passive air, so warm
air pushes cold air into characteristic wedge
shape as warmer air slides over colder air In
region of cooler air, a temperature inversion
forms, causing poor air circulation
Figure 8.13
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Warm front As warm air is lifted, stratiform
clouds develop into characteristic nimbostratus
clouds with drizzly rain Progression of clouds,
with high cirrus and cirrostratus clouds
preceding the coming front, and as front
approaches, clouds lower and thicken into
altostratus and finally into stratus clouds
within several 100 km of front
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On weather maps Warm front drawn as a line with
semicircles pointing in direction of frontal
movement
In N America, warm fronts are usually mT air
masses
Figure 8.18
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Average Annual precipitation
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Air Mass Modification Lake Effect
Figure 8.5
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Reading a weather map Called synoptic analysis
evaluation of weather data collected at a
selected time See top of legend for explanation
of symbols (next slide) Note isobars delineating
H and L pressure cells (L pressure over
Nebraska) Winds move counterclockwise around L
pressure cell Also information about wind speed,
precitation, cloud cover Cold front moving east
away from L pressure and warm front moving north
on edge of counterclockwise motion Air mass over
Nevada and Utah is cP - dry over those regions
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Weather Map
Figure 8.18
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Figure 8.17
Weather predictions Predictions are based on
computer models, called numerical models These
are complicated and often try and mimic chaotic
behavior Which means tiny variations in the
input or in the assumptions made will greatly
change the answer you get
Information needed to make a weather map and also
to input into computer models barometric
pressure pressure tendency (is it rising,
falling, steady) surface air temperature
dew-point temperature wind speed, direction
clouds - type and movement current weather sky
conditions visibility precipitation