Weather Systems of Middle Latitudes

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Weather Studies Introduction to Atmospheric
ScienceAmerican Meteorological Society

• Chapter 10
• Weather Systems of Middle Latitudes

Credit This presentation was prepared for AMS by
Michael Leach, Professor of Geography at New
Mexico State University - Grants
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Case-In-Point

• Extra-tropical cyclones are major weather makers
  in middle and high latitudes
• In 1703, Daniel Defoe was the first to propose
  that storms generally track from west to east in
  middle latitudes
• In 1743, Benjamin Franklin was the first American
  to discover that storms usually move in an
  easterly or northeasterly direction
• Based on observations, he concluded that wind
  direction in a storm was not an indication of the
  storms direction of movement

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Driving Question

• What systems shape the weather of the middle
  latitudes?
• Middle latitudes extend from Tropics of Cancer
  and Capricorn, poleward to the Arctic/Antarctic
  Circles
• Weather is particularly dynamic because of the
  migration of cyclones and anticyclones embedded
  in the prevailing westerlies
• This chapter examines
• Air masses, fronts, cyclones, and anticyclones
• Local and regional circulation systems

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Air Masses

• An air mass is a huge expanse of air covering
  thousands of square kilometers, and is relatively
  uniform horizontally in temperature and water
  vapor concentration (humidity)
• Abbreviations for air mass types
• Cold (polar or P) or warm (tropical or T)
• Dry (continental or c) or humid (maritime or m)
• Arctic (A) air
• Air mass source regions have nearly homogeneous
  surface characteristics over a broad area with
  little topographic relief
• The air mass stays over the source region for an
  extended period, and takes on the characteristics
  of the source region

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North American Types and Source Regions
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North American Air Masses
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Air Masses

• Modification of Air Masses
• Air masses eventually move out of their source
  region
• As they move, their properties are modified by
  the surface they pass over
• Air mass modification occurs from
• Exchange of heat or moisture, or both, with the
  surface over which the air mass travels
• Radiational heating or cooling
• Adiabatic heating or cooling associated with
  large-scale vertical motion

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Air Masses

• Modification of Air Masses, continued
• In winter, as a cP air mass travels southeastward
  from Canada into the lower 48-states, its
  temperature usually modifies rapidly
• By the time it arrives in the southern states,
  temperatures will not usually drop much below
  freezing
• The sun warms snow-free ground, and the warmer
  ground heats the bottom of the air mass. Heat is
  then distributed vertically.
• A similar process of heating and destabilization
  occurs when a cP air mass crosses the East Coast
  and moves over the western Atlantic. Evaporation
  from the sea surface leads to extensive areas of
  low clouds and fog
• cP traveling over snow-covered ground experiences
  less modification
• Much of the incoming solar radiation is reflected
  rather than being absorbed

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Air Masses

• Modification of Air Masses, continued
• Tropical air masses modify less than polar masses
• They are often warmer than the ground they travel
  over
• The bottom of the air mass cools, and stabilizes
• Convective currents are suppressed
• If a tropical air mass moves over a warmer
  surface, the air mass can become even warmer
• Air masses undergo significant modification
  through orographic uplifting (e.g., mP air mass
  sweeping inland from the Pacific Ocean)
• Rising air cools adiabatically,
  condensation/deposition occur, and precipitation
  is triggered on the windward slopes
• Descent on the leeward side leads to adiabatic
  warming and cloud dissipation
• Air mass emerges considerably milder and drier
  (e.g., modified Pacific air)

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Frontal Weather

• A front is a narrow zone of transition between
  air masses that differ in density
• Density differences are usually due to
  temperature contrasts, hence the names cold
  fronts and warm fronts
• Density differences may also be caused by
  humidity contrasts
• A fronts transition zone may be 100 km and a
  line representing a front on a weather map is
  drawn along the warm edge of the zone
• A front is also associated with a trough in the
  sea-level pressure pattern, a corresponding wind
  shift, and converging winds
• When air masses meet at fronts, the colder,
  denser air forces the warmer, less dense air to
  rise
• This induces adiabatic cooling and often
  cloud/precipitation development
• The slope of the front influences the types of
  clouds that form
• Cold and warm fronts have different slopes
  associated with them

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Frontal Weather

• Stationary Front
• A front that exhibits essentially no lateral
  motion
• This often happens along the Front Range of the
  Rocky Mountains when a shallow pool of polar air
  surges south over the plains and the leading edge
  is too shallow to cross the mountains. Milder
  air remains in the Great Basin to the west of the
  Rockies.
• May also develop when a preexisting front becomes
  parallel to the upper-level flow pattern or along
  a boundary in the surface temperature pattern
• Typical front
• Slopes from Earths surface towards denser air
• Lies in a trough in the pressure pattern
• Wind changes direction rather abruptly across the
  front
• May have broad region of clouds and precipitation
  (e.g., overrunning)

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Stationary Front
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Frontal Weather

• Warm Front
• Warm air advances while cold air retreats
• Overall characteristics very similar to a
  stationary front
• As a warm front approaches
• Clouds thicken and become lower in altitude
• Sequence is cirrus, cirrostratus, altostratus,
  nimbostratus, and stratus when the advancing warm
  air is relatively stable
• Initial cirrus appearance may be more than 1000
  km (620 mi) ahead of the front
• Just ahead of the front, steady precipitation
  usually gives way to drizzle and sometimes
  frontal fog
• If advancing warm air is unstable, more vigorous
  uplift can occur with thunderstorms embedded in
  the overrunning zone

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Warm Front
Cirrus clouds
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Frontal Weather

• Cold Front
• Colder air displaces warmer air
• In North America in winter, the temperature
  contrast along a cold front is usually greater
  than across a warm or stationary front
• In summer, temperatures on either side of the
  front may be essentially the same
• Density contrasts arise because of humidity
  differences
• The slope on a cold front is much steeper than
  the slope on a warm front
• Uplift is confined to a narrow area at or near
  the cold fronts leading edge
• If the warm air is unstable, thunderstorms may
  form and a squall line can develop
• If the warm air is relatively stable,
  nimbostratus and altostratus may form

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Cold Front
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Advancing Back-Door Cold Front
A cold front generally trails south or
southwestward from the center of an
extra-tropical cyclone. Back-door cold fronts
move south along the eastern side of the
Appalachian Mountains.
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Frontal Weather

• Occluded Fronts
• Typically form late in a cyclones life cycle as
  it moves into colder air
• Faster moving cold front catches up with the warm
  front
• There are 3 types of occlusions, distinguished by
  the temperature contrast between the air behind
  the cold front and ahead of the warm front
• Cold occlusion
• Air behind cold front colder than cool air ahead
  of warm front
• Like a cold front at the surface but, with less
  air mass contrast
• Warm occlusion
• Air behind cold front is not as cold as the air
  ahead of the warm front
• Like a warm front at the surface
• Neutral occlusion
• Little difference between air masses
• Marked by a trough, wind shift line, band of
  cloudiness precipitation

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Cold-Type Occlusion

• Air behind advancing cold front colder than cool
  air ahead of warm front
• More common in eastern North America, where the
  colder air follows behind the front on northwest
  winds

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Warm-Type Occlusion

• Air behind the advancing cold front is not as
  cold as the air ahead of the warm front
• Occurs in northerly portions of western coasts,
  such as in Europe or the Pacific Northwest, where
  mP air is behind the cold front

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Air Masses

• Summary
• Fronts are characterized based on the movement of
  the cold air mass
• Clouds and precipitation may develop along fronts
  when there is a significant density contrast
  between air masses and there is an adequate
  supply of water vapor
• Properties that define a front are differences in
  temperature and humidity, wind shift,
  convergence, and a pressure trough
• Frontogensis front forms or grows stronger
• Frontolysis front weakens

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Extra-tropical Cyclones
The extra-tropical cyclone (also called a
low-pressure system or low), is a major weather
maker of middle and high latitudes. Surface
winds blow counterclockwise and inward. Surface
winds converge, air rises, expands, and cools,
resulting in clouds and precipitation.
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Extra-tropical Cyclones
The comma-shaped cloud pattern is characteristic
of a well-developed extra-tropical cyclone.
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Extra-tropicalCyclones

• Life Cycle
• Norwegian cyclone model conceptual model
  originally developed around WWI still closely
  approximates our current understanding
• (A) Incipient cyclone Cyclogenesis (birth of a
  cyclone) usually takes place along the polar
  front directly under an area of strong horizontal
  divergence in the upper troposphere
• Air pressure at the bottom of the air column
  falls, a horizontal air pressure gradient
  develops, and cyclonic circulation begins
• Westerlies aloft steer and support the cyclone as
  it progresses through its life cycle
• West of the low center, the polar front pushes
  southeast as a cold front. East of the low, the
  polar front advances north as a warm front.

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Extra-tropicalCyclones
warm sector

• Life Cycle
• (B) Wave cyclone In this stage, the central
  pressure continues to drop and winds strengthen
  due to an increased pressure gradient. The
  upper-level trough deepens while remaining west
  of the surface low center.
• Warm sector becomes better defined
• Fronts form a pronounced wave pattern and comma
  cloud is seen in satellite images
• Extensive stratiform cloudiness appears north of
  the warm front
• Cyclone moves eastward or northeastward at 40-55
  km per hr (25-35 mph)

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Extra-tropicalCyclones

• Life Cycle
• (C) Beginning of occlusion
• Faster moving cold front advances on the warm
  front
• Warm sector area diminishes and occluded front
  begins to form
• Upper level pattern shows closed circulation and
  is directly over the surface low (vertically
  stacked)
• Dry slot separates the cold front cloud band from
  the comma cloud
• Cyclone moves slower at approximately 30 km per
  hr (20 mph)

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Extra-tropicalCyclones
triple point

• Life Cycle
• (D) Bent-back occlusion
• Surface low may become detached from the westerly
  steering flow and the occluded front is drawn
  around the low center
• Warm sector is detached from cyclone center
• Triple point favors development of a secondary
  cyclone
• Eventually the cyclone weakens (cyclolysis)

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Extra-tropical Cyclones

• Entire cycle can occur over several days, or a
  much shorter period
• Speed of development depends on upper air support
• Weak divergence aloft will cause poorly defined
  systems
• Sometimes cloudiness and precipitation occur with
  an upper-level or surface trough, which is not
  associated with a closed surface cyclonic
  circulation
• When upper-level conditions are ideal, the entire
  life cycle can occur in less than 36 hours

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Extra-tropical Cyclones

• Cyclone Bomb
• This is the term applied to a rapidly
  intensifying cyclone, and is defined as a central
  pressure drop of at least 24 mb in 24 hours
• Few cyclones meet this criteria, and most that do
  occur in winter over a warm ocean surface current
  (e.g., Gulf Stream)
• Conveyor Belt Model
• This is an alternate, 3-D model to the steps
  discussed previously (Norwegian cyclone model)
• Combines horizontal and vertical air motions
• Depicts the circulation in a mature cyclone in
  terms of three broad interacting systems called
  conveyor belts, which transport air with certain
  properties from one location to another
• Belts are (1) warm and humid, (2) cold, and (3)
  dry

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Extra-tropical Cyclones

• Conveyor Belt Model, continued
• (1) Warm and humid conveyor belt originates in
  the cyclones warm sector
• Ascends slightly as it flows northward in the
  warm sector at low levels and then ascends more
  rapidly over the warm front
• Helps explain the broad region of
  clouds/precipitation north of the warm front
• (2) Cold conveyor belt originates north of the
  warm front
• Ascends as it progresses toward the west
• Forms the comma cloud and produces precipitation
• Turns clockwise at upper levels and follows
  westerly flow aloft
• (3) Dry conveyor belt
• This air originates high in the troposphere and
  low stratosphere upstream of the upper-level
  trough
• One branch descends southward behind the cold
  front the other forms the dry slot that
  separates the head tail of the comma cloud

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Conveyor Belt Model
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Extra-tropical CyclonesCyclone Weather

• Figure below represents an intensifying cyclone
  in the Upper Midwest
• Four sectors about the low center
• Strong cold air advection, stratiform clouds, and
  non-convective precipitation northwest of the low
• Cold front south of low is accompanied by
  convective precipitation. Sinking air and mostly
  clear skies characterize the southwest sector
  behind the cold front.
• The mildest air is in the southeast (warm) sector
  of the cyclone
• An extensive overrunning zone is found to the
  northeast of the low center

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Principal Cyclone Tracks

• As a general rule, the cyclone center moves
  forward in the same direction and at about
  one-half the speed of the 500-mb winds
• Principal storm tracks tend to converge toward
  the northeast
• Storm tracks appear to originate just east of the
  Rocky Mountains, but actually form over the
  Pacific Ocean near Alaska
• As a cyclone travels over the mountains, it often
  loses its identity, but reforms over the Great
  Plains
• Noreasters often intensify off the North
  Carolina coast and track toward the northeast
  along the East Coast
• 2 motions exist
• Movement of the cyclone along the coast
• Counterclockwise flow of winds around the storm
  center winds in northeast sector of the cyclone
  blow from the northeast (gives the name
  nor-easter)
• Some may become powerful systems drawing copious
  amounts of water vapor from the ocean and
  producing large amounts of precipitation over a
  broad area
• Generally, cyclones that form in the south yield
  more precipitation because they have access to
  greater amounts of mT air
• Cyclogenesis is more frequent in the winter when
  the mean position of the polar front and jet
  stream shift southward

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Principal Cyclone Tracks
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Extra-tropical Cyclones

• Cold Side/Warm Side
• Storm track determines weather at points on the
  ground
• Track A puts Chicago on the warm side with
  passage of the warm and cold fronts
• Track B puts Chicago on the cold side with no
  frontal passage
• Table summarizes the general sequence of weather
  conditions at Chicago

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Extra-tropical Cyclones

• Winter Storms
• An extra-tropical cyclone that produces any
  combination of frozen or freezing precipitation
• An associated hazard is a cold wave, which often
  follows a winter storm
• Necessary ingredients include cold air (typically
  brought in by a sprawling cold high to the
  north), a moisture supply, and uplift mechanisms
• A major storm requires warm and humid air brought
  northward
• A storm moving to the northeast produces heaviest
  snow to the north and west of the low center
• Blizzard a severe storm characterized by high
  winds and reduced visibility due to falling or
  blowing snow

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Colorado-track Winter Storm System
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Extra-tropical CyclonesCold and Warm Core Systems

• An occluded cyclone is a cold-core system
• Lowest temperatures occur in a column just above
  the surface low
• Depth of low increases with altitude
• Cyclonic circulation prevails throughout the
  troposphere and is most intense at high altitudes
• The requirement that thickness (mean temperature)
  be lowest at the low center produces the classic
  isobar pattern

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Extra-tropical CyclonesCold- and Warm-Core
Systems

• A non-occluded cyclone is a warm-core system
• Lowest temperatures are northwest of the
  cyclones center, and highest temperatures are to
  the southeast
• Low aloft is displaced to cold side of the storm
• The system tilts with altitude
• Upper-level low lags behind surface low

Vertical cross-section of a low from northwest
(cold) to southeast (warm)
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Extra-tropical CyclonesCold and Warm Core Systems

• Warm-core cyclone (thermal low)
• Stationary, have no fronts, and are generally
  associated with fair weather
• From over a broad expanse of arid/semiarid land
  in response to intense solar heating of the
  ground
• Hot surface heats the overlying air and lowers
  the density of the air column enough for a low to
  form
• Usually very shallow
• Anticyclone aloft overlies low

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Anticyclones

• In anticyclones, subsiding air and diverging
  surface winds favor formation of a uniform air
  mass, no fronts, and generally fair skies
• Arctic and Polar Highs (cold-core anticylone)
• Labeled either a polar high (cP air) or arctic
  high (A air) and are products of extreme
  radiational cooling, often over snow-covered land
• Clockwise circulation weakens with altitude, and
  may reverse
• Usually has a cold trough overlying it
• These exert the highest pressure in winter
• They are extremely stable, with an inversion in
  the lower km or so
• Interact with the circulation of an
  extra-tropical cyclone by helping to maintain and
  strengthen the temperature contrast along the
  cyclones cold front

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Anticyclones

• Warm High (warm-core anticyclone)
• Forms south of the polar front and consists of
  extensive areas of subsiding warm and dry air
• These strengthen with altitude
• Examples are Bermuda-Azores high and systems that
  may develop over the interior of North America,
  especially in summer
• The greater mass of air over the anticyclone
  center (related to a higher tropopause) is
  responsible for the high surface pressure
• A cold-core anticyclone can become warm-core as
  it moves south and modifies

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Anticyclones

• Anticyclone Weather
• Fair weather system because surface winds blowing
  in a clockwise and outward direction (Northern
  Hemisphere) induce subsidence over a broad area
• Arctic highs produce the lowest temperatures of
  winter
• A stalling warm anticyclone can lead to drought
  and excessive summer heat
• A weak horizontal air pressure gradient near the
  center leads to intense nighttime radiational
  cooling
• Ahead of an anticyclone, there may be strong
  northwest winds ushering in polar or arctic air
• May bring heavy lake-effect snows to the lee side
  of the lakes
• In the summer, the most noticeable effect is not
  a lowering of temperatures, but a lowering of
  humidity
• Highs may become entrenched east of the Rockies
  in summer, and form blocking highs
• High temperatures and eventually drought result

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Local and Regional Circulation SystemsLand and
Sea (or Lake) Breezes

• Sea Breeze
• Under exposure to the same intensity of solar
  radiation, the land surface warms more than the
  water surface
• Highest pressure over water, and cool breeze
  sweeps inland
• Shallow circulation has maximum strength in
  mid-afternoon
• Uplift may lead to thunderstorms
• Land Breeze
• By late evening, winds blow offshore due to a
  reversal in the heat differential between land
  and water
• Obtains maximum strength around sunrise but is
  weaker than a sea breeze

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Local and Regional Circulation SystemsChinook
Winds

• A relatively warm and dry wind that develops when
  air descending the leeward slopes of a mountain
  range is adiabatically compressed
• Strong winds cause stable air in the lower
  troposphere to ascend on the windward side
• On the leeward side, stable air descends to the
  original altitude, and the larger scale of
  circulation causes further descent
• Called Santa Ana winds in southern California
• The figure is a schematic representation of the
  surface weather pattern that favors development

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Local and Regional Circulation SystemsChinook
Winds

• Boulder, CO, situated in the foothills of the
  Rocky Mountains, experiences particularly strong
  and destructive downslope winds, sometimes
  gusting to 160 km per hr (100 mph)
• On average, the community sustains about 1
  million in property damage each year due to these
  winds

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Local and Regional Circulation SystemsDesert
Winds

• Hot surfaces (i.e., deserts) may develop
  superadiabatic lapse rates in the lowest levels
  of the atmosphere
• These are highly unstable, and generate vigorous
  upwelling and gusty surface winds, but very few
  clouds form
• A dust devil is a whirling mass of dust-laden air
  formed by localized hot spot
• Air is heated, and rises rapidly
• Cooler surface winds converge on the hot spot
• Horizontal wind shear causes the column of rising
  hot air to spin about a nearly vertical axis
• Dust and debris are picked up, making these
  visible to altitudes topping 900 m (3000 ft)
• May cause damage, as some have winds as higher
  than 75 km per hr (45 mph)
• Strong thunderstorm downdrafts may generate dust
  storms known as a haboob

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Local and Regional Circulation SystemsMountain
or Valley Breezes

• Valley Breeze
• Bare valley walls absorb solar radiation and heat
  the surrounding air
• Cooler, denser air over the valley sinks and air
  adjacent to the valley walls blows upslope
• Cumulus clouds may form near summit
• Best developed between late morning and sunset
• Mountain Breeze
• Bare valley walls are chilled by radiational
  cooling and cool the surrounding air
• Colder, denser air near the valley walls sinks
  and gusty breeze blows downslope
• Fog or low stratus clouds may form in the valley