Download 12. Weather Patterns

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12. Weather Patterns
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12. Weather Patterns
Reading Assignment:
• A&B:
Ch. 9, 10
1. Air Masses
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Extremely large body of air whose T and moisture (q) are
fairly similar in any horizontal direction at any given
altitude
Very large > 1600 km across
Extend across a range of latitudes
Small differences in comparison to the rapid rates f
experienced across boundaries (fronts)
Source regions
• region where air masses originate
• determine the characteristics of the air mass
• associated with anticyclones (H)
o very weak winds - new air sink
o tend to stay in region so pick up the characteristic T
and q
Moisture status
m Maritime
c continental
Temperature status
A Arctic
T Tropical
P Polar
E Equatorial
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2 letter code to describe the moisture and Temperature status
code
Definition
mP Maritime Polar
cP Continental
(cA) Polar (Arctic)
mT Maritime
Tropical
Conditions
moist – cool air
dry – cold
(extremly cold)
moist - warm
Source region
North Pacific
North Canada &
Siberia
Atlantic, S. Pacific,
Gulf of Mexico
Air Mass Modification
• Pressure differences cause the air to move out of the
source region
• As air moves into a new region it is modified
• Air masses
• Modify the weather of the area it traverses
• Are modified by the surface
• Example: If a cP or cA air moves over the ocean in winter
what will happen to the:
• Moisture at the surface: -- ↑
• Temperature at the surface: -- ↑
• What will happen to the stability?
---Unstable as airmass is heated from below--• Given that stability: what will happen to the T and
Moisture distribution
------increased mixing--moist and warmer (cool)-------• What will happen to the air mass class?
------changed from c to m--------
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Two general classes:
a) Air moves over a colder surface – air warmer ⇒ increases
stability
b) Air moves a warmer surface ⇒ instability
If becomes more stable: characteristics will remain the same
for a longer period of time
If becomes more unstable: characteristics through the air
mass change rapidly because of the mixing
2. FRONTS
Transition zone between the air masses.
• Air masses have different T ⇒ different densities
• Fronts – between contrasting T and often different
humidity
• Manually use the station model to see the locations where
there is a contrast. Front located between these places
cA--F---cP---F--------warmer air mass
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Passage of a Polar front (continental Polar air mass)
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• Air masses have horizontal & vertical
extent – frontal surface between them
• 4 Types of fronts
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a) WARM Front (boundary of advancing warm air mass)
Surface position of a front moves so that warm air occupies
area that was formerly occupied by cooler air
• Average speed: ½ that of cold front (~5 m s-1 = 18 km h-1)
• due to smaller pressure gradient
• Warmer less dense air rides up and over the colder more
dense surface air
⇒ overrunning
• Average slope: 1:200 (lifting over extensive horizontal
distances)
• Warm air ascend the retreating wedge of cold air (similar to
orographic lift)
• Cools adiabatically
• Produces clouds and light continuous precipitation
• Sequence of clouds: cirrus ⇒ stratiform
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Cirrus:
• 1st sign of warm front
• high clouds
• form far in advance of the surface expression of the
front
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• overrunning warm air ascended high up the wedge of
cold air
Aircraft contrails
• Another indicator of warm front approaching
• On a clear day they can persist for several hours
• Relative warm moist air ascending overhead
• Slow rate of advance and very gradual slope
• Light to moderate precipitation over a large area for an
extended period
• If the overrunning air is inherently unstable and front is
rather sharp ⇒ cumulonimbus clouds & thunderstorms
Passage of Warm front
• Gradual increase in T
• most apparent when there is a large T contrast exist
between air masses
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b) COLD Front (advancing wedge of cold air)
• Cold air actively advances into a region occupied by warmer
air
• Friction tends to slow surface position compared to the air
aloft
• Cold front steepens as it moves
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Normally 2 x steeper than warm front (1:100)
• Average speed 35 km h-1 (0-50 km h-1)
• More violent weather
• Stronger uplift
• Displacement of air along a Cold front – often rapid enough
to permit the release of latent heat
• Significant Increase air buoyancy
• Sudden down pours
• Vigorous gusts (mature cumulonimbus)
• Clouds frequently develop
• Produces the same amount of lifting as a Warm front but
over a shorted distance
• Greater Intensity of precipitation
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• Shorter duration of precipitation
• Sometimes preceded by altocumulus clouds
• Towering clouds can be seen in the distance
Near the front – dark band of clouds
Significant T drop
Wind shift
Pressure decrease at vicinity of front
• Goes up afterwards
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c) STATIONARY Front (nonmoving boundaries)
• Occasionally flow on both sides of a front are almost parallel
to the position of the front
• Surface position of the front does not move
• May get overrunning (inclined surface over cold air)
d) OCCLUDED front (cutting off warm air mass from the
surface..merging of two cold air masses )
• An active cold front overtakes a warm front
• Advancing cold air wedges the warm upwards – new front
emerges– small temp. difference between air masses
• Precipitation – warm air forced aloft
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• Two types
• Cold type of occluded front eg east of the Rockies
• Air behind the Cold front is colder than the air ahead of
the warm front
• Warm type of occluded front e.g. along Pacific coast
milder mP invades cP
• Air behind the Cold front (mP) advancing from over the
ocean on to land is milder than the very cold cP air
mass it meets up with at the occluded front.
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3. Wave Cyclones or Mid-latitude cyclones
Large Weather systems
• Travel great distances
• Affect wide areas (week or more)
• Precipitation----sometimes severe weather
• Forms and moves along a front
• Circulation of winds about the cyclone tend to produce a
wavelike deformation on the front
• Develop along a polar front
• Polar front - semi-continuous global boundary separating
cold polar air from warm subtropical air
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• Cyclogenesis (formation mid latitude cyclone)--Stages
Stationary front
Air flow parallel to front
in opposite directions
wave like kink →frontal wave develops along boundary
cold air pushes south and
displaces warm air north
(ccw rotation in NH)
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Cyclogenesis Intensifies
Low pressure deepens
and distinct cold and
warm front emerge
-Uplift
-Cloud formation
-Distinct frontal
characteristics
Steered by winds
aloft move E or NE
Occlusion occurs
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Common, but not exclusive
-Interruption in the isobar pattern
-Abrupt transition of wind direction
-SE → SW across warm front
-SW →NW across cold front
Warm Front always located ahead
of cold front