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Midlatitude Cyclones
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Fronts
Fronts are boundary surfaces
that
separate air masses of
different densities,
one usually warmer and
more humid than the other.
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Fronts
As one air mass moves into another,
the warmer, less dense air mass
is forced aloft in a process called
overrunning.
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Fronts
The five types of fronts are
warm front,
cold front,
stationary front,
occluded front,
and dryline.
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Warm Front
Gradual slope
Slow advancing
Light to moderate precipitation
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Cold Front
Steep slope
Advance about 80 km/hr
Violent weather
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Stationary Front
Occasionally, airflow on both
sides of a front is neither
toward the cold air mass
nor toward the warm air mass.
Rather, it is almost parallel
to the line of the front.
Consequently, the surface position of
the front
does not move, or it moves
sluggishly.
This condition is called a
stationary front.
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Occluded Front
When a rapidly moving cold front
overtakes a warm front,
the cold air wedges
the warm front upward.
A new front forms
between the advancing
cold air and the air over
which the warm front is gliding,
the cool air
a process known as
occlusion .
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The Dry Line
Classifying fronts based
solely on the temperature
differences across the frontal
boundary can be misleading.
Humidity also influences
the density of air.
All other factors being equal,
Fronts are boundary
surfaces that
separate air masses of
different densities,
humid air is LESS DENSE
than dry air.
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The Dry Line
In the summer it is not unusual
for a southeastward-moving
air mass that originated
over the northern Great Plains
to displace warm, humid air
over the lower Mississippi Valley.
A dryline is easily identified by
comparing the dew-point
temperatures of the cT air
west of the boundary
ELP 57 0F
DFW 80 0F
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with the dew points of the
mT air mass to the east.
The mid-latitude Cyclone
The primary weather producer
in the middle latitudes is the
middle-latitude, cyclone.
Midlatitude cyclones are
large low- pressure systems
with diameters often exceeding
1000 kilometers (600 miles).
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The mid-latitude Cyclone
They generally travel from
west to east.
They last a few days to
more than a week,
have a counterclockwise
circulation pattern with
a flow inward toward their centers,
and have a cold front and
a warm front
extending from the
central area of low pressure.
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Concept Check 9.1
1. Compare the weather of a typical warm front with that
of a typical cold front.
The weather associated with warm fronts is generally much milder
than that associated with cold fronts.
Warm fronts usually produce light to moderate precipitation over a
large area and for an extended period.
After a warm front passes, temperatures gradually rise.
Cold front weather is usually more violent than warm
front weather with more intense precipitation over a smaller area.
Cold fronts often produce severe weather including thunderstorms
and tornadoes.
A marked temperature drop and wind shift also usually accompany
the passage of cold fronts.
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Frontal Weather
Cold Front
Warm Front
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Question 1 Which one of these five clouds (a–e) tends to be
generated along a cold front?
c) cumulonimbus cloud.
Question 2 Assuming that a warm front is approaching your
location, list the names of the other four clouds, in the order in
which they would pass overhead
d) Cirrus, e) cirrostratus, a) altostratus, b) nimbostratus
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Life Cycle of a Midlatitude Cyclone
According to the
polar front theory,
midlatitude cyclones form
along fronts and proceed
through a generally
predictable life cycle.
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Cyclogenesis
Along the polar front,
two air masses of
different densities are
moving parallel to the front
and in opposite directions.
Cyclogenesis (cyclone formation) occurs,
and the frontal surface takes on
a wave shape that is usually
several hundred kilometers long.
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Cyclogenesis
Once a wave forms,
warm air advances poleward,
invading the area formerly
occupied by colder air.
This change in the direction
of the surface flow causes a
readjustment in the pressure pattern
that results in somewhat
circular isobars,
with the low pressure centered
at the apex of the wave.
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Cyclogenesis
Usually, the cold front
advances faster
than the warm front
and gradually closes the
warm sector and lifts
the warm front.
This process is known as occlusion.
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Cyclogenesis
Eventually, all of
the warm sector
is forced aloft,
and cold air surrounds
the cyclone at low levels.
At this point, the cyclone
has exhausted its source of energy
and the once highly organized
counterclockwise flow ceases to exist.
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Concept Check 9.3
2. Describe the surface circulation of a midlatitude cyclone.
The surface circulation
is inward directed and
counterclockwise,
with cold polar air
pushing south and
warm tropical air
flowing north.
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Divergence and Convergence Aloft
Guided by the westerlies aloft, cyclones generally move eastward across
the United States.
Airflow aloft (divergence and convergence) plays an important role in
maintaining cyclonic and anticyclonic circulation.
In cyclones, divergence aloft supports the inward flow at the surface.
In anticyclones convergence aloft supports the outward flow at the surface
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Surface Lows and the Jet Stream
Relationship between the meandering flow in the jet stream aloft and
cyclone development at the surface.
Midlatitude cyclones tend to form downstream of an upper-level low.
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Concept Check 9.5
1 Briefly explain how the flow aloft maintains cyclones at the surface.
In order for cyclones to be
maintained at the surface,
surface convergence must be
offset by outflow aloft.
As long as divergence aloft
is equal to or
greater than the surface inflow,
the cyclone can be sustained.
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Steering of Mid-latitudes Cyclones
Notice that the cyclone (low)
moved almost in a straight in a
southeastward direction.
March 21 and March 22.
On the morning of March 23,
it abruptly turned northward.
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Steering of Mid-latitudes Cyclones
This change in direction
corresponded to the change
from rather straight contours
on the upper-air chart
for March 21 to
curved contours
on the chart for
March 23
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The 500 mb Chart
The height contours on the map
are actually the height of the
500 mb pressure surface in
meters above sea level.
The average air pressure near
the ground is about 1000 mb,
and since air pressure decreases
as one moves upward,
at some altitude the air pressure
will fall to 500 mb.
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The 500 mb Chart
The height above sea level
of the 500 mb pressure surface
is measured at many locations
around the globe.
The data from around the world
is collected and maps of the
current 500 mb height are generated.
Computer weather forecast models
predict the future pattern of
500 mb heights.
The actual pattern of the 500 mb
heights changes (evolves) daily.
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Concept Check 9.6
1 List four locations where midlatitude cyclones that affect
North America tend to form.
Midlatitude cyclones affecting
North American
tend to form along the eastern
slopes of the Rockies,
off the coast of the
Pacific Northwest,
in the Gulf of Mexico, and
off the coast of North Carolina.
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Blocking High
A cold anticyclone associated with an outbreak of frigid arctic
air impacts the eastern two-thirds of North America.
Temperatures are shown in degrees Fahrenheit.
Outbreak of arctic air invades New England,
bringing subzero temperatures and mostly clear skies.
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Cut-off Low
Cut-off low pressure systems are literally cut off from
the west-to-east flow in the jet stream.
As a result, these systems can spin for days over the same area and
are capable of producing very large quantities of precipitation.
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The Conveyor Belt Model
Schematic drawing of the circulation of a mature midlatitude cyclone,
showing the warm conveyor belt (red), cold conveyor belt (blue), and
dry conveyor belt (yellow). The inset shows the cloud cover produced
by the warm and cold conveyor belts and the dry slot produced by the
dry conveyor belt.
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Chapter 9 Problems
3. (page 269)
If you were located 400 kilometers ahead of the surface
position of a typical warm front (with slope 1:200), how high
would the frontal surface be above you?
Given:
Warm front slope ratio is 1/200
(For each altitude increase of 1km the frontal boundary
is 200 km upstream.)
Your position is 400 km in front of the surface position
of the warm front.
Find: Altitude of frontal boundary above your position
Type Problem: Frontal slope ratio
Equation: set up ratio by using division
1.
(My position)
----------------- = height
(frontal slope)
2. Substituting numbers
(My position)
(400 km)
----------------- = height = ---------------- = 2 km
(frontal slope)
(200km/1 km)
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The End
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