Transcript ATMOSPHERIC PRESSURE

AIR MASSES
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Large bodies of air
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SOURCE REGIONS –
areas where air masses
originate
– Uniform in composition
– Light surface winds
– Dominated by high surface
pressure
– The longer the air mass
remains over a region, the
more likely it will acquire
properties of the region
• Temperature and moisture
AIR MASSES
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Similar horizontal properties
– Temperature
– Moisture content
– Lapse rates
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These characteristics are carried with it as air mass moves to other
areas
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Challenge of weather forecasting
– To predict behavior of air masses
– Where it will go and how it will change
AIR MASS CLASSIFICATION
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Bergeron classification – 1920s
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Consists of 2 letters ab
– Source region (a)
– Thermal property (b)
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Air mass properties gradually change as it travels
Acquire characteristics of invading regions
– Thermal properties
– Stability
• Whether it is heated or cooled from below
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Nomenclature only applies to recent history of air mass
AIR MASS CLASSIFICATION
SOURCE REGIONS
• Maritime (m) – originates over oceans or large bodies of water
• Continental (c) – originates over land
• m or c indicate influence of surface on air mass characteristics (water
and land)
THERMAL TYPES
• Tropical (T) – from low latitudes
• Polar (P) – from mid-high latitudes
• Arctic (A) – from high latitudes (> 65°N)
• P and T suggest importance of latitude of source regions
AIR MASS PROPERTIES OVER NORTH AMERICA
WINTERTIME cP
• Source – Central Canada
and Siberia
• Frozen surface – ice and
snow
• Intense radiation cooling,
lack of insolation heating
• Extremely cold, stable,
and dry
• Clouds are non-existent
AIR MASS PROPERTIES OVER NORTH AMERICA
SUMMERTIME cP
• Source – central Canada
• Ample warming of surface
through insolation
• Melts snow and
permafrost
• Cool, dry and sometimes
unstable due to insolation
heating of lower layers
AIR MASS PROPERTIES OVER NORTH AMERICA
SUMMERTIME cT
• Source – Northern Mexico
and extreme SW deserts
of US
• Hot, dry and unstable
AIR MASS PROPERTIES OVER NORTH AMERICA
WINTERTIME mP
• Source – open oceans of
high latitudes – Gulf of
Alaska and North Atlantic
• cP air mass from Syberia
becomes mP air mass as
it moves over ocean
– Lower layers modified by
warmer water surface
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Cool, moist, unstable in
surface layers
Cool, dry aloft
AIR MASS PROPERTIES OVER NORTH AMERICA
SUMMERTIME mP
• Source – open oceans in
high latitudes – Gulf of
Alaska and North Atlantic
• Cool and moist in lower
layers and cool and dry
aloft
• Overall temperature
higher than in winter
• Instability in lower layers
AIR MASS PROPERTIES OVER NORTH AMERICA
WINTERTIME mT
• Source – over open
ocean near 30N
• Great semi permanent
subtropical high pressure
centers and in SW
Caribbean
• Warm, moist and very
unstable
AIR MASS PROPERTIES OVER NORTH AMERICA
SUMMERTIME mT
• Source – semi permanent
high centers near 15N
• Very warm, moist and
unstable
AIR MASS MODIFICATION
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As air masses migrate, changes in their properties exist – AIR MASS
MODIFICIATION
As air masses move from source regions they carry with them physical
characteristics of the region – INITIAL CONDITIONS
Most variations within air masses are found in vertical distribution
Air masses are modified by the climatic characteristics of the regions
over which they move
Types of modification:
– Lapse rate modifications
• subsidence, uplift,
– Moisture modifications
– Thermal modifications
AIR MASS MODIFICATION
cP MODIFICATIONS
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WINTER – moving from land to
water (lakes or ocean)
Increased instability
Surfaces in their paths are
warmer than those of their source
regions
Warming from below increases
instability
AIR MASS MODIFICATION
cP MODIFICATIONS
• LAKE EFFECT SNOW – as cold dry
air mass moves over warmer water
(winter), instability is enhanced
• Produces snow belts on eastern
shores of Great lakes
AIR MASS MODIFICATION
cP MODIFICATIONS
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SUMMER
cP air moves from land to
water, becomes more stable
Decrease of lapse rate
Haze, fog and low stratus
clouds appear
AIR MASS MODIFICATION
mP MODIFICATIONS
WINTER
• Becomes more unstable in lower layers as it reaches coast
• Showers, squalls occur as air ascends over coastal mountains
• Becomes dry and stable as it moves inland due to subsidence
SUMMER – increases instability
AIR MASS MODIFICATION
mT MODIFICATIONS
WINTER
• Moisture rich
• Releases in precipitation
as arrives at coast
• Stability increases inland
SUMMER
• From Gulf of Mexico
• Penetrates as far as
Canada flowing along the
east side of Rocky
Mountains
• Instability increases as it
moves inland
FRONTS
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Transition zone between two
air masses of different
densities
– Temperature
– Humidity
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FRONTAL SURFACE
(ZONE) – upward extension
of a front
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Types of fronts
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Stationary
Cold
Warm
occluded
• Frontolysis – the weakening or dissipation of a front
– Decreased temperature contrast between two air masses
• Frontogenesis – a formation, strengthening or generation of a front
– Increased contrast of temperature conditions between two air masses
CRITERIA OF FRONT LOCATION
• Contrasting conditions in the
following exist on either side of
a frontal system
• Surface temperature
• Air moisture content
• Winds
– Speed and direction
• Cloud types & precipitation
• Sea level pressure and its
tendencies
STATIONARY FRONTS
• Fronts that have no movement
• Designation – alternating red
half-circles and blue triangles
• Obstacles prevent front from
progressing
– Mountain range
• Drawn along a line that
separates two air masses
STATIONARY FRONTS
• Example:
– Cold cP air from Canada
against Rocky Mountains in
US
– cP air to the north
– mP air to the west of
mountains
– Cold air unable to cross
barrier
– No westward movement
STATIONARY FRONTS
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Weather
– Winds blow parallel to front
but in opposite direction on
either side
– Clear to partly cloudy
– Cold to the east
– Warm to the west
– Little or no precipitation
• Both air masses are dry
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Stationary front can become warm
front or cold front
COLD FRONTS
• Transition zone where a cold
air mass advances and
replaces a warm air mass
• Leading edge of cold air
– cP replacing mT
– cP due south, mT due north
• Designation – solid blue
triangles oriented towards the
direction in which front is
moving
• Rapid movement – up to 50
km/hr
COLD FRONTS
• Cold dense air wedges under warm air forcing it upwards
– Sharp slope at front’s edge
– Steepness due to friction which slows the airflow near the ground
– Slope = 1:50 (vertical to horizontal distance from front’s edge to middle of frontal
system)
COLD FRONTS
• The faster the front, the steeper the slope
• Movement of the front causes most of the weather associated with its
passage
• Cold fronts are associated with dramatic shifts in weather
COLD FRONTS
• Affected local meteorological conditions as front passes over:
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Temperature
Seal level pressure tendency
Wind direction
Cloud cover
Dew point
visibility
COLD FRONTS
• Position of the leading edge of advancing portion of cold air mass
FACTORS THAT DETERMINE COLD FRONTAL WEATHER
• There is no average cold front
• Weather associated with cold fronts may vary from minor wind
shifts to severe thunderstorm activity
• Weather is determined by
– The nature of warm air that is being lifted
• Moist versus dry – precipitation production
• Stability of air mass - uplift
– Degree of lift to which the warm air is subjected by the advancing cold
air wedge
• Speed of the cold front and the steepness of frontal surface
• Mechanical uplift possible, precipitation
COLD FRONTS
• with slow-moving cold front, clouds and precipitation usually
cover a broad area behind the front
e.g. if rising air is dry and stable scattered clouds are all that form –
no precipitation
e.g. during the winter, a series of cold polar outbreaks may travel
across the US so quickly that warm air is unable to develop
ahead of the front.
frigid arctic air usually replaces cold polar air, and a drop in
temperature is the only indication that a cold front has moved
through the area
COLD FRONTS
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SURFACE WEATHER CHANGES ASSOCIATED WITH COLD
FRONTS (cP replacing mT)
1.
Surface winds
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2.
Change direction when cold front passes over
Cold air moves from W-NW and is undercutting a flow of warm air
from S-SW
Winds S-SW ahead of cold front and W-NW behind cold front
Temperature
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Arrival of cold front will result in a decrease in temperature (warm to
cold)
After passage, temperature keeps decreasing as the cold air begins
to modify surface conditions
COLD FRONTS
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SURFACE WEATHER CHANGES ASSOCIATED
WITH COLD FRONTS (cP replacing mT)
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Moisture content
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Onset of cold air results in drop in dew point
Cloud and precipitation
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Ahead – cirrus and cirro-stratus clouds due to stronger upper
level winds that push uplifted air at the front’s edge ahead
While passing – towering cumulus clouds (lots of moisture in
warm air and lots of precipitation, hail and thunder)
After passing – as warm air is drying out, showers decrease
in intensity and skies clear
COLD FRONTS
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5.
SURFACE WEATHER CHANGES ASSOCIATED WITH COLD FRONTS
(cP replacing mT)
Visibility
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Usually improves after a cold frontal passage
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Air behind front is unstable as a result of passing over a warm
surface
– Vertical motion will carry pollution aloft
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Tropical air accumulates considerable smoke from industrial
areas during its movement north
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Polar and arctic air masses are relatively free of pollution
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Pressure
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Because front lies in a trough, approach will be accompanied by
decrease in pressure
A marked rise will be observed as trough passes
WARM FRONTS
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Transition zone between a
retreating cold air mass and
advancing warm air mass
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Designation – red semi-circles
pointing in the direction where
the warm air is advancing
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Warm and moist mT replacing
dry cold mP
WARM FRONTS
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Frontal changes are less abrupt than cold air frontal passages
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Long spells of cold weather do not come to a rapid end
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Weather at the warm front portion of a frontal system is more extensive
than at the cold front portion
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Cloud system & precipitation cover extensive areas
WARM FRONTS
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Cold air lies as a wedge under warm air
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As warm front approaches depth of cold air decreases
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Advancing warm air overruns the retreating wedge
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Forces air to rise – creates frontal inversion
Air expands and cools
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Extensive cloud on top of cold air
WARM FRONTS
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Warm fronts are slow
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About half that of average cold front
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Average slope of 1:300
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More gentle than cold front
WARM FRONTS
Weather associated with warm
Fronts
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Weather patterns associated
with warm fronts depend on:
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Moisture content of warm air
mass
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precipitation
Stability of the warm air
mass
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uplift
Degree of overrunning
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Uplift and precipitation
WARM FRONTS
Weather associated with warm fronts
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If overrunning air is dry and
stable, only high and middle
clouds will form
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no precipitation
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If overrunning air is moist and
unstable, heavy showers can
develop as thunderstorms
become embedded in the cloud
mass
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Arrival of warm front produces
wind shifts, warmer temperatures
and overall improvement of
weather conditions
OCCLUDED FRONTS
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OCCLUSION – frontal system
that forms when a cold front
overtakes a warm front
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Designation – alternating purple
triangles and half-circles that point
towards the direction to which the
front progresses
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Development – rapidly
approaching cold front catches up
to slow-moving warm front and
overtakes it
OCCLUDED FRONTS
WARM OCCLUSION
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When the air behind the occluded front
is warmer than the air ahead
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cP air overrun by mP from Pacific ocean
in NW USA
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As cold front overtakes warm front, the
milder and lighter air behind the cold
front is unable to lift the colder and
heavier air off the ground
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Surface weather associated with this
occlusion is similar to that of warm
front
OCCLUDED FRONTS
COLD OCCLUSION
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When the air behind the occluded front is cooler than the air ahead
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Most common to Pacific coast states
cP overrun by cA
OCCLUDED FRONTS
COLD OCCLUSION WEATHER
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Cold front rapidly approaches warm front
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Warm air rides up the cool air in front of the warm front
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As cold air overtakes warm front, the warm front and the warm air mass are
lifted off the ground
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As front approaches weather is similar to warm fronts
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Frontal passage brings weather similar to that of cold fronts