Request:1. Post either asks a question related to the corresponding module or shares/describes something interesting (any sources are cited) and weather related. The post is thoughtful, well written, and is of substantial meaning to the discussion as a whole. The post doesn’t just regurgitate course material, but expresses your understanding and reflection of the material.
2. Reply to someone else’s post, either answering their question, or building on the information provided in the original post.
In first part, you must ask one question about the module. Once, you finish first part, and give the first part to me I will give back some’s discussion to write the second part.
Each discussion should be more than 50 words
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Understand the key properties of an Air
Identify an air mass based on its source
Explain the conditions
before/during/after a frontal passage.
Learn the steps of a Mid-Latitude
Cyclone’s life cycle.
An Air Mass is an extremely large body of
air that has a similar temperature and
They can span thousands of Kilometers.
Air masses are constantly moving: The
boundary between them is called a
The Region where an air mass comes
from will determine its characteristics.
› The region where an air mass comes from is
called a Source Region.
› A source region is a relatively flat location
with a uniform composition.
Latitudes of Source Regions:
› Tropical Air Masses: Warm air masses that
come from South of 30°N
› Polar Air Masses: Cold air masses that come
from North of 50°N
› Arctic Air Masses come from even further
Composition of Source Regions
› Maritime Air Masses are moist air masses that
come from the oceans.
› Continental Air Masses are dry air masses
come from inland locations.
So to classify an air mass, we look at these
two characteristics of the air mass.
› Tropical Air Masses (Warm):
Maritime Tropical (mT): Warm, Moist, Usually
Continental Tropical (cT): Hot, Dry, Unstable
Surface/Stable Aloft Air.
› Polar Air Masses (Cool):
Maritime Polar (mP): Cool, Moist, Unstable Air.
Continental Polar (cP): Cold, Dry, Stable Air.
› Arctic Air Masses (VERY COLD):
Continental Arctic (cA): Extremely Cold, Dry, Stable
This is a stream of Maritime, Tropical (mT) air
moving into California referred to as the
In December, 2010, a
Express set up occurred
over most of California.
Southern California got
the brunt of it (while I
was down there for
Christmas… I GOT
Dumped over 10 INCHES
of rain in Long Beach
between December 1822nd
› Average annual LB
rainfall is ~15 inches
Source: NASA Goddard Space Flight Center
Major cold snap in the Eastern US.
Not Actually a “Polar Vortex” event… The
Media has just dubbed it that.
› The worst was in January/February of 2014, but
2015 has rivaled it at times.
› In Chicago:
Low of -16°F on 1/6/2014
High of 8°F on 1/7/2015 (I was there… it was bad!)
High of 7°F on 2/19/2015
The culprit has been a huge trough, which
allowed cold cA air to reach the Midwest
A plane landing in a frigid
Chicago. Image taken by
Mid-latitude regions (like the US) are a
transition zone between air masses.
› Because of this, the US experiences a
massive variation of weather.
Much of this weather is delivered in the
transition zone between two air masses…
these are called Fronts.
The Transition zone between two air
masses of different densities… and thus
One air mass overruns (rides over) the
other air mass.
› Note: Air Masses have vertical extent too…
› The upward extension of a front is referred to
as a frontal zone or frontal surface.
Fronts have vertical extent…
Four Main Types of
› Cold Front
› Warm Front
› Stationary Front
› Occluded Front
Source: National Weather Service
A stationary front is a front
that is… well… stationary!
› What this means is that the air
masses are not moving
relative to each other.
A cold front represents a
boundary where a cold
air mass is advancing and
a warm air mass is
On a weather map, a
cold front is denoted with
a blue line with spikes
pointing in the direction
the front is traveling in.
behind a cold front.
As the cold air mass advances, it acts like a
wedge, forcing the warm, moist air ahead
of the front upwards at a very steep angle
(sometimes almost straight up).
› As the air rises, it quickly becomes buoyant,
shooting upwards, producing towering
cumulonimbus clouds ahead of the frontal
› Stormy weather is typically present ahead of,
and slightly behind a cold front.
› The weather typically improves once the front
Explain to me how the
weather changes as a
cold front passes
How is the weather
A. Ahead of the front?
B. Behind of the front?
Look at the handout
Most Cold Fronts
South/East in the
However, in some
areas (like New
England), Cold Fronts
can move in off the
cooler Atlantic basin.
These are called
“Back Door” fronts.
Along a warm front, warm
air is advancing while cold
air is retreating.
Denoted on a Weather
Map by a red line with
“humps” pointing in the
direction of travel.
Warm fronts move much
slower than cold fronts:
Air temperature increases
noticeable as a warm front
As the warm air advances, it rides up the
cold air in a process called Overrunning.
The slope of warm frontal surfaces is
significantly more gentle than cold frontal
› Precipitation, if any, is lighter/less convective.
The overrunning of warm air over cold air
can produce an inversion ahead of the
› The Inversion allows for the formation of
sleet/freezing rain ahead of the front during
Explain how the weather
changes as a warm front
Tell me the weather:
A. When the front is
approaching, but still pretty
B. How does the weather
change ahead of the front,
but as the front gets closer.
C. How does the weather
change once the front
Cold fronts and Warm fronts usually
travel together, each a part of a larger
system called a Mid-Latitude Cyclone.
In a mid-latitude cyclone, the warm front
travels ahead of the cold front.
But… the cold front travels faster than
the warm front… so what happens when
the cold front catches up to the warm
Occluded fronts form!
An occluded front forms
when the cold front
catches up to the warm
front, and cuts it off, in a
process called an
› When this occurs, the
“middle-man” warm air mass
does not reach the surface.
Warm Air Occlusion: When a cold, dry cP
air mass is replaced by a cool, moist mP
› Common along the US West Coast.
Cold Air Occlusion: When a cool, moist
mP air mass is replaced by a cold, dry cP
› Common along the US East Coast
A well-developed mid-latitude
cyclone off the US East Coast.
This is a typical nor-Easter
So how do these cyclones
Cyclogenesis is the term used to
describe the formation of cyclones
The formation and life cycle of a cyclone
is described in the Norwegian Model of
Cyclogenesis, also known as the Polar
Developed from a group of scientists in
Norway, based on Observations.
The model has six steps.
› Polar Front Stage (stationary boundary between
Tropical and Polar Air)
Frontal Wave (development of a “kink” along
the frontal boundary)
Open Wave (warm air advances ahead, cold
air advances behind)
Initial Occlusion (mature stage)
Advanced Occlusion (cold air on both sides of
cyclone… cyclone begins to die)
Cut-off low (Low pressure splits off from front).
Initially, a stationary front between warm
and cold air exists.
Something causes a “kink” in the front…
› A Low Pressure Develops.
› Cold air to the west begins to move south.
› Warm air to the east begins to move north.
The Low Pressure begins to Intensify.
Precipitation develops ahead of the
The Cold Front catches
up to the Warm Front…
The storm is most intense
at this point.
The Warm Sector begins
to shrink as the cold
front “zips-up” with the
The North-West of the
cyclone is the most
The Occluded Front grows as the Warm
Front meets the Cold Front.
The Cyclone begins to weaken.
Without any warm,
rising air, the
occluded front dies
off… the cyclone
breaks off from the
The “kink” at the surface begins due to
› Recall the two column model… divergence
aloft fuels convergence below.
› This causes cyclonic motion, advecting
warm air ahead of the cyclone, and cold air
Increased Divergence aloft fuels
convergence at the surface.
› This happens on the right side of upper-level
› Convergence happens on the left side of
the trough… Surface High Develops.
As air travels over
mountains, it becomes
Once the air makes it over
the lee side of the
mountain, it expands,
fueling divergence aloft and
promoting convergence at
Some of the strongest storms
in the US form East of the
Typical areas of formation and
movement for Mid-Latitude
Which of these lows typically
gives us our rain? Which of
these lows creates
Typical areas of formation and
movement for anticyclones.
During the winter time, what
kind of weather can you expect
Air Masses are large bodies of air with
similar temperature and humidity.
Fronts are boundaries between two
different air masses.
Occluded fronts occur because cold
fronts catch up to warm fronts.
Cyclones form when a “kink” develops
along the polar front, caused by
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