Air Masses & FrontsAir Masses & Fronts

• Learning ObjectivesLearning Objectives– Define air mass, source region, warm Define air mass, source region, warm

and cold air massand cold air mass– Characteristics of air massesCharacteristics of air masses– Define fronts & frontal surfaceDefine fronts & frontal surface– Weather at cold and warm frontsWeather at cold and warm fronts– Occlusion, Occluded & Stationary frontsOcclusion, Occluded & Stationary fronts

Air Masses & FrontsAir Masses & Fronts

• Why need to study about air masses Why need to study about air masses & fronts ?& fronts ?

1.1. Weather forecast – as it involves Weather forecast – as it involves identification of air masses, their identification of air masses, their properties, movement & properties, movement & changes/modificationschanges/modifications

2.2. Safety of life - severe weather Safety of life - severe weather conditions might be associated with conditions might be associated with frontsfronts

Air Masses - CharacteristicsAir Masses - Characteristics

• Air mass – large body of air whose Air mass – large body of air whose temperature and relative humidity are temperature and relative humidity are same in the horizontal directionsame in the horizontal direction

Q.1Q.1 What could be the size/dimension What could be the size/dimension of of an air mass ?an air mass ?

Q.2Q.2 What about temperature & What about temperature & relative relative humidity in vertical humidity in vertical direction ?direction ?

Air Masses – Source regionAir Masses – Source region

• Birth place, where the air stays for a long Birth place, where the air stays for a long timetime

• What could be the Characteristics of a What could be the Characteristics of a source region ? source region ?

1.1. Light winds or no windsLight winds or no winds

2.2. An extensive & homogeneous surfaceAn extensive & homogeneous surface

(large ocean areas, flat uniform (large ocean areas, flat uniform continental lands etc)continental lands etc)

Q.Q. Can coastal regions be a good source Can coastal regions be a good source region for an air mass ?region for an air mass ?

Air Masses – Source regionAir Masses – Source region

Air Masses – Source regionAir Masses – Source region

Air Masses - PropertiesAir Masses - Properties

Depends on following factors:Depends on following factors:

• Source region (eg. Polar region – cold air Source region (eg. Polar region – cold air mass)mass)

• Age (no of days spent in source region)Age (no of days spent in source region)

• Route or track over earth’s surface (over Route or track over earth’s surface (over ocean – absorb moisture)ocean – absorb moisture)

• Rate of travel (faster – uninfluenced)Rate of travel (faster – uninfluenced)

Modification

Air Masses - transportationAir Masses - transportation

• How air is transported ?How air is transported ?– Vertically – Convection & TurbulenceVertically – Convection & Turbulence– Horizontally – Heat advectionHorizontally – Heat advection

• Convergence & DivergenceConvergence & Divergence– If air current flows towards each other – If air current flows towards each other –

Convergence, if flows away from each other - Convergence, if flows away from each other - DivergenceDivergenceBOM_AirBOM_Air pressure pressure

• Which of the above process creates an air Which of the above process creates an air mass ??mass ??

Air Masses - ClassificationAir Masses - Classification

Dual classificationDual classification– Relative – depends on the underlying Relative – depends on the underlying

surface over which it moves – if the surface over which it moves – if the surface below is cold – the air mass is surface below is cold – the air mass is warm, and vice – versawarm, and vice – versa

– Absolute – depends on the source region Absolute – depends on the source region & properties of air mass& properties of air mass

Cold & Warm Air Masses Cold & Warm Air Masses

• Relative Classification – warm & cold Relative Classification – warm & cold air mass……air mass……– If the temp of an air mass is 15If the temp of an air mass is 15°C & the °C & the

temp of temp of the underlying surface is 20the underlying surface is 20°C, °C, the air mass is a warm air mass.the air mass is a warm air mass.

True or False ?? True or False ??

Wx at Warm & Cold air Wx at Warm & Cold air massmass

– Cold air massCold air mass absorbs heat from absorbs heat from underlying surface & becomes unstable.underlying surface & becomes unstable.

– Instability causes – Cumuliform cloud, Instability causes – Cumuliform cloud, gusty wind, showers & thunderstormgusty wind, showers & thunderstorm

– Warm air massWarm air mass is cooled by the is cooled by the underlying surface & becomes stable.underlying surface & becomes stable.

– Stability causes – Low Stratiform cloud, Stability causes – Low Stratiform cloud, steady wind, drizzle & fogsteady wind, drizzle & fog

Wx at Warm & Cold air Wx at Warm & Cold air massmass

Observed Observed WxWx

Stable air Stable air massmass

Unstable Unstable air massair mass

VisibilityVisibility PoorPoor GoodGood

WindWind SteadySteady GustyGusty

CloudCloud StratiformStratiform CumuloforCumuloformm

PrecipitatioPrecipitationn

DrizzleDrizzle showeryshowery

Air Masses - ClassificationAir Masses - Classification

• Absolute classificationAbsolute classificationAir massAir mass AbbrvAbbrv SourceSource TemTem

ppHumHum

Tropical Tropical maritimemaritime

TmTm Sub tropical Sub tropical oceanocean

HighHigh V V highhigh

Tropical Tropical ContinentContinent

alal

TcTc Sub tropical Sub tropical desertsdeserts

V V highhigh

V lowV low

Polar Polar ContinentContinent

alal

PcPc Temperate Temperate continentscontinents

VarieVariess

LowLow

Polar Polar MaritimeMaritime

PmPm Ocean Ocean areas lat > areas lat >

505000

LowLow HighHigh

ArcticArctic AA Arctic/Arctic/

AntarcticAntarcticV lowV low lowlow

Air MassesAir Masses

• Arctic – Arctic/Antarctic Arctic – Arctic/Antarctic

• Polar – Pole ward up to abt 60Polar – Pole ward up to abt 60° N/S° N/S

• Tropical – Within 30° of EquatorTropical – Within 30° of Equator

• Over sea – Moist (Maritime)

• Over land – Dry (Continental)

FrontsFronts

Fronts - CharacteristicsFronts - Characteristics

• On weather maps, boundary zones between two On weather maps, boundary zones between two different air masses are drawn as lines, called fronts or different air masses are drawn as lines, called fronts or weather fronts.weather fronts.

– Surface of separation between 2 different air mass is Surface of separation between 2 different air mass is the frontal surface. And, front is the line or zone of the frontal surface. And, front is the line or zone of intersection between the frontal surface & a intersection between the frontal surface & a horizontal plane.horizontal plane.

– Front is a narrow layer of width between 5-60 mls.Front is a narrow layer of width between 5-60 mls.

Fronts - CharacteristicsFronts - Characteristics

• Frontal surface does not have perfect discontinuities. Frontal surface does not have perfect discontinuities. • Some mixing takes place along the boundary which Some mixing takes place along the boundary which

is a very narrow zone, known as is a very narrow zone, known as mixing zonemixing zone..• Frontal surface is slightly inclined with the Frontal surface is slightly inclined with the

horizontal , which is known as horizontal , which is known as Slope or GradientSlope or Gradient..• Gradient - Ratio of vertical rise to horizontal Gradient - Ratio of vertical rise to horizontal

distance.distance.

Q.1Q.1 Why the surface of separation between 2 different Why the surface of separation between 2 different air air masses is not horizontal ?masses is not horizontal ?

Q.2Q.2 Between Convergence & Divergence, which Between Convergence & Divergence, which process process creates a weather front ??creates a weather front ??

Fronts - CharacteristicsFronts - Characteristics

• Isobars at front must Isobars at front must be refracted so that be refracted so that “kink” in the isobar “kink” in the isobar points away from the points away from the LP & points towards LP & points towards the HP.the HP.

• More intense fronts & More intense fronts & fronts with greater fronts with greater speed have sharper speed have sharper kinks.kinks.

Fronts - CharacteristicsFronts - Characteristics

• Front is a wind-shift line.Front is a wind-shift line.

• If you face the wind in If you face the wind in advance of a front, the wind advance of a front, the wind will shift to starboard as the will shift to starboard as the front passes.front passes.

• Wind speed across the front Wind speed across the front depends on Pressure depends on Pressure gradient.gradient.

• If isobars are close wind If isobars are close wind would be stronger & vice-would be stronger & vice-versa.versa.

Fronts - TypesFronts - Types

•Cold front Cold front

•Warm frontWarm front

•Stationary frontStationary front

•Occluded frontOccluded front

Cold Fronts Cold Fronts

• Cold front – a transition zone on the earth’s surface Cold front – a transition zone on the earth’s surface where cold air replaces warm air for a stationary where cold air replaces warm air for a stationary observer.observer.

• After a front passes over a place, if the After a front passes over a place, if the temperature is lower than it was before, the front is temperature is lower than it was before, the front is called cold front.called cold front.

On wx map solid blue line with triangles shows direction of cold front

Cold Fronts - CharacteristicsCold Fronts - Characteristics

• Cold air replacing warm air.Cold air replacing warm air.

• Generally moves from NW to SE.Generally moves from NW to SE.

• Behind cold front air is colder & drier.Behind cold front air is colder & drier.

• When a cold front passes through, temp can When a cold front passes through, temp can drop even by >15drop even by >15°C.°C.

• Slope / gradient of the Cold front is 1:50.Slope / gradient of the Cold front is 1:50.

• Slow moving cold fronts have gentle Slow moving cold fronts have gentle gradient.gradient.

• Avg speed of cold front is abt 25 kts.Avg speed of cold front is abt 25 kts.

COLDCOLD Front Animation Front Animation

3D View of a cold front3D View of a cold front

Cold Fronts - CharacteristicsCold Fronts - Characteristics

• Cold air wedges under warm air & forces Cold air wedges under warm air & forces warm air to rise upward.warm air to rise upward.

• As moist air rises upward, forms cloud.As moist air rises upward, forms cloud.• Cumuliform cloud appear far in advance. Cumuliform cloud appear far in advance. • At front - narrow layer of Cb produces heavy At front - narrow layer of Cb produces heavy

shower with gusty wind.shower with gusty wind.• Behind front – sky is clear or have fair wx Behind front – sky is clear or have fair wx

Cu.Cu.• Behind front air cools more quickly & have Behind front air cools more quickly & have

more clouds & causes heavy precipitation.more clouds & causes heavy precipitation.• If rising air is dry, scattered cloud & no If rising air is dry, scattered cloud & no

precipitation.precipitation.

Cold Fronts – Vertical Cold Fronts – Vertical C/sectionC/section

Cold Fronts - CharacteristicsCold Fronts - Characteristics

• http://www.classzone.com/books/earth_science/tehttp://www.classzone.com/books/earth_science/terc/content/visualizations/es2002/es2002page01.cfrc/content/visualizations/es2002/es2002page01.cfm?chapter_no=visualizationm?chapter_no=visualization

http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/af/frnts/cfrnt/prcp.rxml

CloudsClouds

Clouds - CbClouds - Cb

ElementElement In advanceIn advance

(before (before passing)passing)

At frontAt front

(while (while passing)passing)

RearRear

(after (after passing)passing)

PressurePressure Little changeLittle change Rapid Rapid increaseincrease

IncreasesIncreases

WindWind SteadySteady Veers, squallyVeers, squally Decreases, Decreases, may veermay veer

TempTemp SteadySteady DecreasesDecreases Steady, may Steady, may decreasedecrease

CloudCloud StSt Cu, CbCu, Cb Cu, CbCu, Cb

WeatherWeather Cloudy, drizzleCloudy, drizzle ThunderstormThunderstormss

ShowersShowers

VisibilityVisibility Poor, Poor, advection fogadvection fog

PoorPoor Good except Good except in rainin rain

Cold fronts – Weather Cold fronts – Weather patternpattern

Cold fronts – Weather Cold fronts – Weather ObservationObservation

http://www.victoriaweather.ca/events/2002-04-14/

Cold Fronts - IndicationsCold Fronts - Indications

– Abrupt change in temperature over short Abrupt change in temperature over short distancedistance

(ahead temp is much higher than behind)(ahead temp is much higher than behind)

– Sudden shift of wind directionSudden shift of wind direction

(eg. ahead S - SW, behind W – NW)(eg. ahead S - SW, behind W – NW)

– Change in the air’s relative humidityChange in the air’s relative humidity

(Before front passes air is more humid)(Before front passes air is more humid)

Cold & Warm FrontsCold & Warm Fronts

Cold Front & PrecipitationCold Front & Precipitation

Warm FrontsWarm Fronts

• Warm front - a transition zone on the earth’s Warm front - a transition zone on the earth’s surface where warm air replaces cold air for a surface where warm air replaces cold air for a stationary observerstationary observer

• After a front passes over a place, if the After a front passes over a place, if the temperature is higher than it was before, the front temperature is higher than it was before, the front is called warm front.is called warm front.

Solid red line with semi circles pointing towards the colder air & in the direction of movement

COLDCOLD Front Graphic Front Graphic

WARM Front GraphicWARM Front Graphic

Warm Fronts - Warm Fronts - CharacteristicsCharacteristics• Warm air replacing cold airWarm air replacing cold air

• Generally moves from SW to NEGenerally moves from SW to NE

• Behind warm front air is warmer & more Behind warm front air is warmer & more moist than ahead.moist than ahead.

• When a warm front passes through, air When a warm front passes through, air becomes noticeably warmer & more humid.becomes noticeably warmer & more humid.

• Slope / gradient of the warm front is abt Slope / gradient of the warm front is abt 1:2001:200

• Avg speed of warm front is 10 kts.Avg speed of warm front is 10 kts.

WARM Front AnimationWARM Front Animation

Warm Fronts - Warm Fronts - CharacteristicsCharacteristics• Warm air slides upward over the cold air.Warm air slides upward over the cold air.• As moist air rises upward it forms cloud.As moist air rises upward it forms cloud.• Cloud appear far in advance of a approaching Cloud appear far in advance of a approaching

front, in order of Ci, Cs, As, Ns & St.front, in order of Ci, Cs, As, Ns & St.• At front, cloud is mainly of Ns type.At front, cloud is mainly of Ns type.• Behind the front, clear sky with scattered St.Behind the front, clear sky with scattered St.• Before passing of w/front, - lt to moderate Before passing of w/front, - lt to moderate

rain, At the front - drizzle or none; After it rain, At the front - drizzle or none; After it passes – cloudy, lt rain or drizzlepasses – cloudy, lt rain or drizzle

• If overrunning warm air is dry, scattered If overrunning warm air is dry, scattered cloud & no precipitationcloud & no precipitation

Warm Fronts – C/sectionWarm Fronts – C/section

Warm Fronts - Warm Fronts - CharacteristicsCharacteristics

http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/af/frnts/wfrnt/prcp.rxml

http://www.classzone.com/books/earth_science/terc/content/visualizations/es2002/es2002page01.cfm?chapter_no=visualization

Warm front – Weather Warm front – Weather patternpattern

ElementElement In advanceIn advance

(before (before passing)passing)

At frontAt front

(while (while passing)passing)

RearRear

(after (after passing)passing)

PressurePressure DecreasesDecreases Stops Stops decreasingdecreasing

Little changeLittle change

(Slt +/-)(Slt +/-)

WindWind Increases, Increases, may backmay back

Veers, may Veers, may decreasedecrease

SteadySteady

TempTemp Slow riseSlow rise Quick riseQuick rise SteadySteady

CloudCloud Ci, Cs, As, NsCi, Cs, As, Ns NsNs StSt

WeatherWeather Fine, then Fine, then rainrain

Rain stopsRain stops Cloudy, Cloudy, drizzledrizzle

VisibilityVisibility Fine (except Fine (except in showers), in showers), then worsethen worse

Poor, frontal Poor, frontal fogfog

Poor, Poor, advection advection

fogfog

Warm Fronts - IndicationsWarm Fronts - Indications

– Great extent of w/front cloud & their changing Great extent of w/front cloud & their changing pattern is the 1pattern is the 1stst clue of the approaching warm clue of the approaching warm front.front.

– Noticeable change in temperature over short Noticeable change in temperature over short distance. (ahead temp is much lower than behind)distance. (ahead temp is much lower than behind)

– Sudden shift of wind direction.Sudden shift of wind direction.(eg. ahead S or SE, behind S or SW)(eg. ahead S or SE, behind S or SW)

– Change in the air’s relative humidity.Change in the air’s relative humidity.(After the front passes air is more humid)(After the front passes air is more humid)

Precipitation along warm Precipitation along warm frontfront

Warm front & Cold front Warm front & Cold front

• Compare and contrast warm and coldCompare and contrast warm and cold fronts. fronts.

Warm fronts – Weather Warm fronts – Weather ObservationObservation

Weather RhymeWeather Rhyme

• Red sky at morningRed sky at morning

Sailors take warning,Sailors take warning,

Red sky at nightRed sky at night

Sailors Delight.Sailors Delight.

Thought for the day ??Thought for the day ??

1.1. What would be the type of wind shift at the What would be the type of wind shift at the front ?front ?

2.2. What is the difference in cloud formation What is the difference in cloud formation between the cold & warm front ?between the cold & warm front ?

3.3. What is the difference in precipitation between What is the difference in precipitation between the cold & warm front ?the cold & warm front ?

4.4. Fog is usually common with _______.Fog is usually common with _______.5.5. Thunderstorm & squally weather are associated Thunderstorm & squally weather are associated

with __________.with __________.6.6. What are the indications of an approaching warm What are the indications of an approaching warm

front ?front ?

Recent heat waves ??Recent heat waves ??

• http://www.washingtonpost.com/wp-http://www.washingtonpost.com/wp-dyn/content/article/2006/07/28/AR20dyn/content/article/2006/07/28/AR2006072801648.html06072801648.html

Stationary frontStationary front

• A stationary front has A stationary front has no movement.no movement.

• On a weather map, it is On a weather map, it is drawn as an alternating drawn as an alternating red & blue lines.red & blue lines.

• Red semicircles facing Red semicircles facing colder air & blue colder air & blue triangles pointing warm triangles pointing warm air.air.

Stationary frontStationary front

• If warm air from W begins to move & replace cold If warm air from W begins to move & replace cold air on E, it will no longer be stationary & becomes air on E, it will no longer be stationary & becomes a warm front.a warm front.

• If cold air from E replaces the warmer air, the If cold air from E replaces the warmer air, the front becomes a cold front.front becomes a cold front.

• Weather along a stationary front is clear to partly Weather along a stationary front is clear to partly cloudy.cloudy.

• Both air masses are relatively dry & cause no Both air masses are relatively dry & cause no precipitation.precipitation.

• Surface wind tends to blow parallel to the front Surface wind tends to blow parallel to the front but in opposite directions on either side of it.but in opposite directions on either side of it.

Stationary front animationStationary front animation

Occlusion – occluded frontOcclusion – occluded front

• Occlusion means “closing off”.Occlusion means “closing off”.• Occluded fronts are formed by Occluded fronts are formed by

merging of cold & warm fronts.merging of cold & warm fronts.• If a cold front catches up & overtakes If a cold front catches up & overtakes

a warm front, the frontal boundary a warm front, the frontal boundary created between the two air mass is created between the two air mass is called an occluded front.called an occluded front.

• On a surface weather map, it is On a surface weather map, it is represented by a purple line with represented by a purple line with alternating triangles & semicircles.alternating triangles & semicircles.

Occlusion – occluded frontOcclusion – occluded front

• Developing cyclone has a preceding warm Developing cyclone has a preceding warm front & a faster moving cold frontfront & a faster moving cold front

• As the storm intensifies, the cold front rotates As the storm intensifies, the cold front rotates around the storm & catches up with the warm around the storm & catches up with the warm front, & forms occluded frontfront, & forms occluded front

Occluded front animationOccluded front animation

Fronts - animationFronts - animation

http://http://www.metoffice.com/weather/charts/awww.metoffice.com/weather/charts/animation.htmlnimation.html

QuestionsQuestions

• Define – air mass, source region, Define – air mass, source region, warm front, cold front.warm front, cold front.

• List the properties and classification of List the properties and classification of the various air masses using a tablethe various air masses using a table

• Describe using a sketch the weather Describe using a sketch the weather at a warm and cold front for a at a warm and cold front for a stationary observerstationary observer

• Describe an occlusion with a sketchDescribe an occlusion with a sketch