Air MassesAir 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 – mAMaritime Polar – mPMaritime Tropical – mT

Continental Arctic – cAContinental Polar – cPContinental Tropical - cT

Areas of Air Mass FormationAreas of Air Mass Formation• Air mass type is determined by the area

over which the air has stagnated.

(Or mA)

Air Mass Movement and Air Mass Movement and ModificationModification

• 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 CanadaAir 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 MASSESFRONTS 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

Pg 6-7

cA mA mP mT

TROPOPAUSE

POLARPOLAR FRONTSFRONTS

The transition zone between two air masses is called a front.

FrontsFronts• 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 Fronts found around a cold airmassairmass

Formation of FrontsFormation of Fronts

• Fronts are always located in troughs of low pressure between two different air masses.

1.

2. 3.

Speed of FrontsSpeed 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.

25 kts

15 kts

20 kts

10 kts

FRONTSFRONTS

Cold FrontsCold 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 FrontThe 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 FrontsWarm 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 massis stable

Embedded ThunderstormsEmbedded Thunderstorms• If the warm air mass is moist and unstable

thunderstorms can form within the normal clouds associated with the warm front.

Warm air massis unstable

Embedded Cb

The Warm FrontThe Warm Front

• 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

ColdWarm

Winter Warm Front = Freezing Rain!Winter Warm Front = Freezing Rain!

KaBoom

If you encounter ice pellets, the freezing rain is above you and ahead of you in the direction of the front.

Frontal SlopesFrontal 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 FrontNormalCold Front

Fast movingCold Front

WIND SHEAR @ WARM FRONTWIND 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 FrontolysisFrontogenesis 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 Frontolysis7-16 Frontogenesis and 7-18 Frontolysis

1 2

3 4

5 6

FRONTAL WAVEFRONTAL WAVE

Cold sector upper wind

FRONTAL WAVEFRONTAL WAVE

Warm sector surface wind

Cold sector surface wind

Warm sector upper wind

Warm sector high level windTROW

AL /

(OCCLUSION)

TROWALS chap.8-12TROWALS chap.8-12

Colder

Colder

Colder

Cold

Cold

Cold

Warm

Warm

Warm

OCCLUSION or Occluded Front

Frontal PassageFrontal 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 PassageWarm Front Passage(Pressure and Winds)

1008

1004

1000

996

992

Winds: 170/20Pressure: 1010 mbWinds: 170/20Pressure: 1001 mbWinds: 230/22Pressure: 995 mbWinds: 270/25Pressure: 998 mb

Pressure: falls as the front approaches and rises after it passes

Winds: always veer after the front passes

Cold Front PassageCold Front PassagePressure and Winds

1008

1004

1000

996

994

Winds: 270/25Pressure: 1009 mbWinds: 270/25Pressure: 1002 mbWinds: 340/25G35Pressure: 995 mb

Winds: 020/25Pressure: 1000 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)

JET JET STREAMSTREAM

Pg 12-11

Pg 12-13

Jet StreamsJet 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 FrontsJet 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)

JETSTREAM

Jet Stream Turbulence (CAT)Jet Stream Turbulence (CAT)

• Looking at the jet from west to east (looking downwind), this shows the most turbulent areas.

Low Level Nocturnal JetLow Level Nocturnal Jet

40-65 KNOTS

Pg 11-13

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.