Air Masses
• An air mass is a large section of the
troposphere with uniform temperature
and moisture in the horizontal.
Moisture Content
• Formed over water: Maritime
• Formed over land: Continental
Temperature
• Cold air mass: Arctic
• Cool air mass: Polar
• Warm air mass: Tropical
Maritime Arctic – mA
Maritime Polar – mP
Maritime Tropical – mT
Continental Arctic – cA
Continental Polar – cP
Continental Tropical - cT
Areas of Air Mass Formation
• Air mass type is determined by the area
over which the air has stagnated.
(Or mA)
Air Mass Movement and
Modification
• Air masses can move from their formation
zone by advection (wind) and be modified
by temperature or water content.
• The transition takes place slowly and it
can take days to weeks before the entire
air mass is transformed.
Air Masses of Canada
• Continental Arctic: cA
– not in summer; low water content; warmed from below, strong winds produce turbulence;
heap clouds and snow showers; rarely in B.C. except as a cold-air invasion
• (Continental Polar: cP) not according to Air Command Weather Manual
• Maritime Arctic: mA
– starts as cA that spends some time over the northern Pacific ocean; moist and unstable
at high altitudes; stratocumulus and cumulus; pe/sn/-shra; Summer: northern lakes affect
air mass
• Maritime Polar: mP
– more time spent over Pacific ocean; warmer in lower levels; more stable than mA;
orographic lifting makes rain west of mountains and dry east of mountains; Summer:
Tsra/Cb
• Maritime Tropical: mT
– very warm and moist; Gulf of Mexico, Caribbean & south of 30°N; Winter: rarely at
surface N of Great lakes, but present at high altitudes; unstable when Frontal lift;
sn/ra/zr/icing and turbulence; FOG (east coast); Summer: shra/tsra
FRONTS vs AIR MASSES
(ARTIC FRONT)
(Maritime Front)
(POLAR FRONT)
Winter: mT moves far south and Canada generally only
sees Continental arctic front and maritime arctic front.
Summer: cA moves far north (or disappears) and we
only see Maritime Arctic front or Maritime Polar front
cA
mA
mP
mT
Pg 6-7
POLAR
FRONTS
The
transition
zone
between
two air
masses is
called a
front.
Fronts
• Named by the movement of the cold air:
– Cold Front: that portion of the front where
the cold air is advancing
– Warm Front: that portion of the front where
the cold air is retreating
– Stationary Front: the cold air is neither
advancing nor retreating.
– Occluded Fronts and Trowals: trough of
warm air aloft.
Fronts found around a cold
airmass
Formation of Fronts
• Fronts are always located in troughs of low pressure
between two different air masses.
1.
2.
3.
Speed of Fronts
• The
speed of the front is equal to the component
of the geostrophic wind (parallel to isobars) in the
cold air mass that is perpendicular to the front.
20 kts
15 kts
25 kts
10 kts
FRONTS
Cold Fronts
• Has a relatively
steep frontal slope
• Tends to lift the
warm air up (like a
wedge)
• Clouds are of the
vertical
development
category. Cu,
TCu, Acc, Cb
• Showery
precipitation is
likely near the
front.
The Cold Front
• Factors:
• moisture of the warm air mass
• stability of the warm air mass
• speed and steepness of the frontal surface
• Wind: veers, some gusts
• Temperature: drops
• Visibility: improves after passage
• Pressure: approaching front, pressure will drop, then rise after
passage
• Turbulence: usually associated with Cb’s
• Severe Cb’s can spawn tornados and hail
• Precipitation: showery in character, usually a narrow band 50 n.m.
Cold
Warm
Warm Fronts
• Has a relatively
shallow frontal
slope.
• Warm air overruns
the cold air mass
(moves up slope)
• Clouds gradually
change from Ci, to
Cs, to As, to St and
Ns as the front
approaches.
• Continuous
precipitation is likely
within 400 miles of
the front.
Warm air mass
is stable
Embedded Thunderstorms
• If the warm air mass is moist and unstable
thunderstorms can form within the normal clouds
associated with the warm front.
Embedded Cb
Warm air mass
is unstable
• Factors:
•
•
•
•
•
•
•
•
Moisture
Degree of overrunning
Stability
Wind: Veers
Frontal Slope: 1 in 150 to 1 in 200
Temperature: gradual rise
Visibility: low ceiling and low visibility; fog
Pressure: drop, then rise
Turbulence: usually little
Precipitation: steady precipitation
CI, CS, AS, NS
The Warm Front
Warm
Cold
Winter Warm Front = Freezing Rain!
If you encounter ice
pellets, the freezing rain
is above you and ahead
of you in the direction of
the front.
KaBoom
Frontal Slopes
• The slope of a warm front is approximately 1:200
(meaning it rises 1 foot vertically for every 200
feet horizontally)
• The slope of a cold front is between 1:50 to
1:100 depening on the speed of the front.
• A very fast moving cold front may have a vertical
slope in the lower thousand feet causing severe
weather due to rapid uplifting of the warm
airmass.
Warm Front
Normal
Cold Front
Fast moving
Cold Front
WIND SHEAR @ WARM FRONT
Airplanes in flight will cross the front in advance of a warm front and behind
a cold front (when the fronts are depicted at their surface positions).
Frontogenesis and Frontolysis
Frontogenesis – The formation or strengthening of a front
Frontolysis – The dissipation or weakening of a front
Little graves!
7-16 Frontogenesis and 7-18 Frontolysis
1
2
3
4
5
6
FRONTAL WAVE
FRONTAL WAVE
Warm sector high level wind
Cold sector
upper wind
Warm sector upper wind
Cold sector
surface wind
Warm sector
surface wind
TROWALS chap.8-12
Warm
Colder
Cold
Warm
Colder
Cold
Warm
OCCLUSION or Occluded Front
Colder
Cold
Frontal Passage
• Temperature: Temperature will tend to rise after
a warm front passes and fall after a cold front
passes. (there are always exceptions to these
rules).
• Dew point: Will rise after warm front passes and
fall after cold front passes.
• Visibilities: visibility often improves with the
passage of a cold front.
Warm Front Passage
(Pressure and Winds)
Winds:
170/20
230/22
Winds: 270/25
Pressure:
1001
995
mb
Pressure: 998
1010mb
Pressure: falls as the
front approaches and
rises after it passes
Winds: always veer
after the front passes
Cold Front Passage
Pressure and Winds
Winds:
270/25
Winds: 020/25
340/25G35
Pressure:
1009
1002
mb
Pressure: 1000
995 mb
Pressure: falls as the
front approaches and
rises after it passes
Winds: always veer
after the front passes
(Gusty winds when
the front passes)
Pg 12-13
JET
STREAM
Pg 12-11
Jet Streams
• Rapidly flowing streams of air embedded
within the main airflow.
• Usually thousands of miles long, a few
hundred miles wide, and a few thousand
feet thick.
• Minimum wind speed to qualify as a Jet is
60 knots.
Jet Streams and Fronts
• Jet Streams roughly follow frontal waves.
• Temperatures to the north of a jet stream
will be colder. (and warmer to the south)
JET
STREAM
Jet Stream Turbulence (CAT)
• Looking at the jet from west to east
(looking downwind), this shows the most
turbulent areas.
Low Level Nocturnal Jet
Summer, often over the
prairies on clear night.
Sheet of wind 700 to
2000 agl begins about
dusk and peaks in
early morning. Wind
shear is major
problem. Speeds
excessive for isobar
spacing.
40-65 KNOTS
Pg 11-13