Transcript Geography 210:

Unit 2: Synoptic Scale
(Regional) Weather & Climate
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Synoptic scale:
• Length: ~1000km (~600miles) to ~6000km
(~3500miles)
• ~Length of Alabama to the length of the U.S.
• Time: Hours to Days (up to 1 week)
So far we’ve been talking about more global
scale weather & climate
Synoptic scale includes air masses, fronts, midlatitude cyclones, large hurricanes, large scale
precipitation & temperature patterns
Airmasses and fronts
Air masses
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An airmass is a large (usually thousands of km across) volume of air that
has horizontally uniform properties of temperature and moisture.
Air masses acquire their properties from spending days to weeks over the
same part of the Earth.
“Polar” air masses are colder than “tropical” air masses
“Maritime” air masses are wetter than "continental" air mass
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Other specific air mass types include "arctic", "equatorial", and “monsoon”
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Bergeron classification of air masses
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3 letters: e.g. mTk, cPw
1st letter for moisture properties: c - continental, m - maritime
2nd letter for thermal characteristics: T - tropical, P -polar, A Artitic/Antarctic, M - monsoon, E - equatorial, S -superior air(dry air
formed by significant downward motion in the atmosphere)
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3rd letter for stability: k/w - air colder/warmer than ground
Source regions
The areas where air
masses form are called
source regions.
Air Masses on Weather Maps
Surface weather analysis
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is a special type of weather map summarizing the information
from all weather stations.
Wind speed code
5
10
50
Present weather
Type
Intensity
Fronts
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A weather front is a boundary separating two air masses
Types: cold front, warm front, occluded front, stationary
front, dry line, squall line
What is an atmospheric front?
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A front is a transition zone between two air masses of different
densities.
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The density contrast results from:
• Difference in temperature;
• Difference in humidity.
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The frontal zone (surface) is the
upward extension of the front.
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Sometimes the frontal zones can be very sharp.
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The intensity of the weather along the front depends on the contrast
of the air mass properties.
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The type of front depends on both the direction in which the air mass
is moving and the characteristics of the air mass.
Types of Fronts
Cold front: cold, dry stable air is
replacing warm, moist unstable air.
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Warm front: warm, moist unstable
air is replacing cold dry stable air.
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Stationary front: boundary between
the two air masses is not moving.
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Occluded front: when a cold front
catches up with a warm front
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The symbols on a map are in the
direction of the air mass motion.
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Weather Map
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Shown: surface-pressure systems, air masses, fronts,
isobars, winds and air flow (large arrows)
Green-shaded area: precipitation
What are the Signs
of a Passing Front?
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Signs
• Sharp temperature changes over a relatively
short distance.
• Changes in the air’s moisture content (as
indicated by changes in the dew point).
• Shifts in wind direction
• Pressure and pressure changes.
• Clouds and precipitation patterns.
The location of the front is not always very
obvious! Even meteorologists sometimes
disagree…
Cold Fronts
• A cold front is a mass of cold air advancing towards warm air.
• Typically associated with heavy precipitation, rain or snow, combined
with rapid temperature drops.
• Since friction decreases with height, winds move faster at higher
altitude. Then the surface of cold front becomes more steeper
through time, leading to a narrow belt of precipitation.
• Moving speed 0-30mph
Cold Fronts
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Cold front- a front in which cold air is
replacing warm air at the surface.
Notice the difference in
• Temperature
• Dew point
• Wind direction
• Pressure
Associated with low pressure centers (low
pressure troughs): follow the dashed line
The pressure is minimum as the front
passes (first decreases as the front
approaches and then increases behind the
front)
Satellite and radar images of cold fronts
(narrow belt of clouds/precipitation)
Cold Front: cloud and precipitation patterns
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The warm, moist air ahead of
the front is forced upward and
condenses
• Cirrus clouds well ahead of
the front
• Strong thunderstorms with
heavy showers and gusty
winds along and ahead of
the front: squall lines
• Broad area of cumulus
clouds immediately behind
the front (although fast
moving fronts may be mostly
clear behind the front).
Warm Fronts
• Warm fronts are warm air moving towards cold air.
• This overrunning process produces large amounts of warm, moist air
over cooler, drier air.
• Shallow stratus clouds dominate and bring light precipitation to
affected regions. Stable regions above the warmer air create
vertically limited clouds and light precipitation. Frontal fogs may occur
as rain evaporates in the colder air near the surface.
• Moving speed about 12 mph
Warm Fronts
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Warm front - in which warm air
replaces cooler air at the
surface
Notice the difference in
• Temperature
• Dew point
• Wind direction
• Pressure
Notice the presence of
precipitation well ahead of the
front
Slope of warm Fronts
• Friction decreases with
height, so winds move
faster at higher altitude
• This causes the surface of
the front to become less
steep through time. Then
clouds will be spread to a
wider region.
Warm Fronts: cloud and precipitation patterns
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Although they can trigger thunderstorms, warm fronts are more
likely to be associated with large regions of stratus clouds and
light to moderate continuous rain.
Warm fronts are usually preceded by cirrus first, then
altostratus or altocumulus, then stratus and possibly fog.
At the warm front, gradual transition.
Behind the warm front, skies are relatively clear.
Satellite and radar images of warm fronts
(wide region of clouds/precipitation)
Stationary Fronts
• Stationary fronts do not move. They do not advance.
They are two unlike air masses side by side.
• They may slowly migrate and warmer air is displaced
above colder.
From Environment
Canada
Stationary Front
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Stationary front- a front which does not move or barely moves.
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Stationary fronts behave like warm fronts, but are more quiescent.
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Many times the winds on both sides of a stationary front are parallel
to the front and have opposite direction.
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Typically stationary fronts form when polar air masses are modified
significantly so as to lose their character (e.g., cold fronts which
stall).
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Typically there is no strong precipitation associated with stationary
fronts (why? – no big contrast in the air mass properties, no air
uplifting and condensation).
Occluded Fronts
• Occluded fronts occur when two fronts meet, the warm air
mass between them is displaced aloft.
• This typically occurs when a cold front meets a warm front as
it circulates the low pressure center of a mid-latitude cyclone.
• The cold and warm fronts curve naturally poleward into the
point of occlusion, which is also known as the triple point.
Formation of
Occluded
Fronts
Occluded fronts.
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Cold fronts move faster than warm fronts.
They can catch up and overtake their
related warm front. When they do, an
occluded front is formed.
Cold occlusion: very cold air behind, not
so cold air ahead of, the warm front
The upper warm front follows the surface
occluded front
Cold occlusion
Warm Occlusion
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Very cold air ahead of, not so cold air
behind, the warm front
The cooler air from the cold front cannot
lift the very cold air ahead, rides
“piggyback”
The warm front aloft precedes the
surface occluded front
Different types of occluded fronts
• A cold-type occlusion usually
occurs in the eastern half of the
continent where a cold front
associated with continental/Polar
air meets a warm front with
maritime/Polar air ahead.
• A warm-type occlusion is typical
of the western edges of continents
where the cold front, associated
with maritime/Polar air, migrates to
an area that is occupied by
continental/Polar air.
Drylines
• Drylines are boundaries between lighter humid air and denser
dry air.
• Air masses with similar temperatures but strong humidity
gradients will act as fronts.
• They frequently occur throughout the Great Plains, and are a
favored location for thunderstorm development.
Weakening/Strengthening of the Front
• Frontolysis:
♦ The front weakens and dissipates
♦ Why?-the air masses start losing
their identities.
♦ The temperature (humidity)
contrast across the front is
decreasing.
♦ Typical for slow moving fronts
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Frontogenesis:
• The front intensifies.
• Why? – The temperature
(humidity) contrast across the
front is increasing.
• Example: cP air mass moves
over warm ocean water.
Summary
1. Definition of airmasses. Bergeron classification of air
masses (3 letters)
2. Surface weather analysis: Station model, wind speed
code, present weather
3. Fronts: 6 types.
4. What is a cold front? Steep, narrow, fast
5. What is a warm front? Less steep, wide, slow
6. What is an occluded front? Two types