Transcript Air Masses, Fronts, and Storms

Air Masses, Fronts and Storms
Pages 670-673
An air mass is a large "bubble" of
air that sits over an area a long
time.
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It sits there long
enough to take on
the
characteristics of
the area.
General Characteristics
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Maritime will be moist
Continental dry
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Maritime and
continental are
abbreviated with small
letters
Arctic will be cold
Polar not quite as cold
Tropical Warm
The names are a combination of
letters that tell where they formed
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1st over land or sea
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Continental c
Maritime m
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Small letters
2nd approximate
latitude
 Artic A
 Polar P
 Tropical T
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Capital letters
Continental Polar (cP)
Source
 Formed over
continent
 Near poles
Characteristics
 Cold
 Dry
Put the letters together to
form the name of an air
mass
What will the characteristics of these air masses
be?
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mA
cA
cT
cP
Maritime Tropical (mT)
Source
 Near equator and
over water
Characteristics
 Warm
 Moist
Characteristics
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cP continental polar
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cT continental tropical
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hot, dry, stable air aloft--unstable surface air
mP maritime polar
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cold, dry, stable
cool, moist, and unstable
mT maritime tropical
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warm, moist, usually unstable
OK, remember air
masses?
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Continental polar and continental
arctic produce cold dry winters
Maritime polar and maritime arctic
pick up moisture and bring cool
moist weather
Continental tropical - hot, dry
Maritime tropical - warm, humid
Fronts
Fronts form where
air masses collide
 Cold front
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Cold air invades
warm air
Warm front
Warm air invades
cold air
 We find a variety of
weather at fronts
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Front types
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When the two masses meet, the differences
in temperature, moisture and pressure can
cause one air mass to override the other.
They are named for the invader
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If cold air invades warm air--cold front
If warm air moves into cold air--warm front
If neither air mass is moving a stationary front
If a cold front over takes a warm front it is an
occluded front
Fronts are accompanied by wind, clouds,
rain, and storms.
Approaching cold front
Approaching warm front
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Less dense air gradually rises over the cold
denser air
Less obvious and more gradual than cold front
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Cirrus clouds
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Thicken into altocumulus and altostratus
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Sky turns gray
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Light to moderate rain or snow develops
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At front rain or snow turns to drizzle
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Warm front animation
source: http://www.learn-line.nrw.de/angebote/klima/medio/bilder/wfront.gif
we find
a variety of
weather
fronts
Atmospheric Lifting
For clouds to form air must lift.
 3 lifting mechanisms
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Convectional
Orographic
Frontal
I. Convectional Lifting
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Some areas of
Earth's surface pick
up heat better than
others.
As air warm it
becomes less
dense and rises. As
it rises it cools and
sinks
Circulatory motion is called
convectional lifting
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If cooling occurs
close to the air's
saturation
temperature,
condensing
moisture forms a
cumulus cloud
Form and dissipate
over the same area
II. Orographic Lifting
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An air mass is
lifted when it is
pushed
upward over
an obstacle
such as a
mountain
range
As rising air cools, if it is
humid, it forms a cloud
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If stable air
stratus
If unstable air
cumulus
As air moves down the other slope it
warms
The Name of the Wind is Chinook
The descending air is dry because the
moisture was removed on the other side.
This is called the
Rain Shadow Effect.
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Example: Eastern
Colorado and
western Kansas
See figure 27.10
III. Frontal Lifting
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When fronts collide warm less dense
air is forced up over cold dense air
Thunderstorm Formation
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As cold air pushes into
warm moist air, the
cold more dense air
stays low and the
warmer less dense air
is pushed up rapidly
This rapid upward
movement forms
thunderstorms
Fronts—dryline
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The source region for cT air is the desert Southwest, the high
plains and Mexico with relation to the United States. The air has
low dewpoints and warm to hot afternoon temperatures but with
mild nighttime temperature. Skies are generally clear in cT air.
This allows daytime heating during the day and radiational
cooling at night. The cT air mass is most prevalent in summer; in
the cool season it is not as discernible. Due to the buoyancy and
elevation of cT air across North America, this air will advect into
the mid-levels of the atmosphere once it moves out of its source
region. This creates a cap of mild dry air. If this air advects over
PBL mT air, the severe thunderstorm threat increases
significantly. The boundary of cT is most noticeable with the
creation of a dryline. A dryline separates mT air from cT air.
Depending on the strength of the dryline, convergence along the
dryline and the dynamics above the dryline, severe
thunderstorms can form near a dryline boundary.
Hail Formation
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Hail stones form by accretion
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Small hail gets caught up in the
updraft and goes to the top of the
cloud where it receives another
layer of ice
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This repeats until the hail cannot
be supported by the updraft and
it falls
Hail requires
strong updrafts to form
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For the smallest hail to form, an
updraft of around 36 to 54 km/h
(24-34 mph) is required.
golf-ball size (1 3/4 inch
diameter) -- require updrafts of
around 88 km/h (55 mph) to
form.
Softball-size hail involves
updrafts exceeding 160 km/h
(100 mph).
Record Hail
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Credit: NOAA Photo Library
Record hail fell in
Coffeyville, KS
diameter 14.4 cm
(5.67 inches)