Edit Web Module Lecture Slides - Air Masses and Fronts

8/10/2019 Edit Web Module Lecture Slides - Air Masses and Fronts

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ENVIR SC 1A03 -Climate and Water

Ai r Masses and Fronts

Unit 1

Source Regions and Fronts

Development of Air Masses - an air mass is an extremely large body of air whose temp &

humidity are uniform horizontally & vertically; they affect vast

areas

- the areas of the globe where air masses form are called source

regions

- large bodies of air must remain over source region for

substantial length of time

- for air mass to form, air mass will reflect air temp & humidity

characteristics of the source region


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Source Regions

Source Regions

Winter Air Masses in North America

- air masses are classified by their origin in terms of latitude &

surface type

- these characteristics are categorized using lower case & upper

case letter scheming

- labels indicated origins of the air masses

- latitude indicates the temp whereas surface type indicates

moisture content

- a source region is a region in which an air mass originates

usually w/ a flat & uniform composition & w/ light surface winds

- as air mass travels over source region for any substantial

amount of time, its properties can change

- see here the air mass source regions & their paths during

winter in North America

- continental polar air masses form over large high latitude land

masses such as Northern Canada & Siberia

- in winter these locations have short days, low sun angle, & are

usually snow covered reflecting more energy

- cold, dry air associated w/ this air mass is typically stable

- continental arctic air masses contain extremely dry & coldconditions as it contains very little water vapour- maritime polar air masses form over the north pacific & aremore moderate than continental polar air masses- maritime polar air masses migrate to the east, crossingthe Gulf of Alaska & the west coast of North America

- impact North America primarily in winter- maritime polar air masses affect the east coast ofNorth America in the form of northeasters bringingcold winds& heavy snowfall


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Summer Air Masses in North America

Seasonal Changes

Fronts

- see here the air mass source regions & their paths during

summer in North America

- continental tropical air masses form during the summer over

hot , low latitude areas such as southwestern US & Mexico

- continental tropical air masses are hot & dry; these air masses

are very unstable but due to the low availability of water vapour,

do not usually produce precipitation- when water vapour is present, thunderstorms form within theseair masses- maritime tropical air masses form over warm, tropical waters

- maritime tropical air masses form over the Atlantic & the Gulf oMexico & migrate towards North America

- high moisture content & instability cause thunderstorms& heavy rain- these air masses are ideal for precipitation & clouds

- the vertical profile of air masses can change over seasons

- this graph illustrates that the typical vertical temp profile over

land for summer continental polar & winter continental polar air

mass

- b/c of low heat capacity of land surfaces, the temp of the winter

air mass changes rapidly near the surface

- fronts are boundaries b/w diff air masses

- there are surfaces of contact b/w 2 distinct air masses

- fronts are spatially limited & usually linked to mid-latitude

cyclones

- features of fronts - sharp change in temp, sharp change in dew

point, shift in wind direction, sharp change in pressure, clouds &

precipitation

- there are 4 general types of fronts associated w/ mid-latitude

cyclones; cold fronts, warm fronts, stationary fronts, & occluded

fronts


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Frontogenesis & Frontolysis

Cold Front

Weakening of a Cold Front

- here is visible satellite image showing the modification of cold,

continental polar air as it moves over the warmer Gulf of Mexico

& the Atlantic ocean

- the concepts of frontogenesis & frontolysis are related to the

contrast in temp b/w 2 air masses

- frontogenesis is the generation or strengthening of a front & is

associated to increased temp contrast

- frontolysis is the weakening & dissipation of a front that is

associated to a reduced temp contrast

- a cold front is a mass of cold air advancing towards warm air

- it is typically associated w/ heavy precipitation, rain or snow, &

combined w/ rapid temp drops

- the infrared satellite image shows the weakening of a cold

front over land


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ENVIR SC 1A03 -Climate and Water

Ai r Masses and Fronts

Unit 2

Occluded and StationaryFronts

Occluded Fronts- occluded fronts occur when 2 fronts meet

- the warm air mass b/w them is displaced aloft

- this typically occurs when a cold front meets a warm front as it

oscillates around a low pressure centre of a mid-latitude cyclone


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Cold Occluded Front

Cold Occluded Front

Cold Occluded Front

- this slide & the next 2 show occlusion process w/ cold front

overtaking warm front

- cold type occlusion usually occurs in eastern half of continent

where cold front associated w/ continental polar air mass meets

warm front w/ maritime polar air mass ahead

- panel on the left shows the placement of the fronts relative to

the surface along w/ the location of the sample cross-section

- panel on the right shows the profiles of fronts across the

transects shown on the corresponding left panel

- here occlusion begins, warm air does not disappear but instead

gets lifted upward away from the surface

- eventually the cold front completely overtakes the warm front &

the entire system is occluded


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Formation of an Occluded Front

Storms & Weather Fronts

Warm Occluded Front

- some occluded fronts form when the surface elongates away

from the junction at the cold & warm fronts

- some occlusions also occur when the intersection at the cold &

warm front slides along the warm front

- here is visible satellite image showing mid-latitude cyclonic

storm w/ its weather fronts over the Atlantic ocean

- superimposed on the photo is the position of the surface cold

front, warm front & occluded front

- precipitation symbols indicate where precipitation is reaching

the surface

- a warm type occlusion is typical of the western edges of

continents where the cold front associated w/ maritime polar air

migrates to an area that is occupied by continental polar air

- here a faster moving cold front overtakes the slower moving

warm front in its path


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Warm Occluded Front

Warm Occluded Front

Stationary Front

- the lighter air behind the cold front rises up & over denser air

ahead of the warm front

- this diagram shows the surface map of the situation

- away from the occlusion, the faster moving cold front overtakes

the slower moving warm front

- within the occlusion, the lighter air behind the cold front rises

up & over the denser air ahead of the warm front

- stationary fronts do not move, they do not advance

- they are 2 unlike air masses side by side

- they may slowly migrate & warmer air is displaced above

colder


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This podcast was made available to you exclusively for your use, as a

student registered in ENVIR SC 1A03, at McMaster University.

You are not authorized to re distribute it, modify it or edit it

without the explicit consent of the course instructor.


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Strengthening of a Cold Front

Warm Front

This podcast was made available to you exclusively for your use, as a

student registered in ENVIR SC 1A03, at McMaster University.

You are not authorized to re distribute it, modify it or edit it

without the explicit consent of the course instructor.

- the infrared satellite image shows the same front now

intensifying as a vigorous front over the warm Gulf stream

waters

- 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 & bring light precipitation to

affected regions

- stable regions above the warmer air create vertically limited

clouds & light precipitation

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Edit Web Module Lecture Slides - Air Masses and Fronts