Air Masses & FrontsAir Masses & Fronts

This chapter discusses:This chapter discusses:

1.1. Classification of 4 North American air masses based Classification of 4 North American air masses based on cold or warm land mass and ocean originon cold or warm land mass and ocean origin

2.2. Air mass delineation and movement along stationary, Air mass delineation and movement along stationary, cold, warm, and occluded frontscold, warm, and occluded fronts

Regional Weather PatternsRegional Weather Patterns•Surface maps of Surface maps of US temperature, US temperature, dew point, and dew point, and pressure reflect pressure reflect synoptic (large-synoptic (large-scale) trends.scale) trends.

•In this image, In this image, nearly every nearly every station around the station around the high pressure high pressure anticyclone anticyclone reports cold, dry reports cold, dry air, suggesting the air, suggesting the air mass formed in air mass formed in a common source a common source region.region.

Air Mass Classification and Characteristics

** cA (continental Artic) - very cold, dry and very stable. Very shallow air mass.

Source Regions & ClassificationSource Regions & Classification

• Air masses of similar temperature and humidity form above flat, Air masses of similar temperature and humidity form above flat, uniform regions with light surface winds.uniform regions with light surface winds.

• Air masses are associated with surface high pressure.Air masses are associated with surface high pressure.

• Typical U.S. air masses:Typical U.S. air masses:maritime Polar (mP)maritime Polar (mP)

- moist and cold- moist and coldcontinental Polar (cP)continental Polar (cP)

- dry and cold- dry and coldmaritime Tropical (mT)maritime Tropical (mT)

- moist and warm- moist and warmcontinental Tropical (cT) (continental Tropical (cT) (warm season onlywarm season only))

- dry and hot- dry and hotcontinental Artic (cA) (continental Artic (cA) (cold season onlycold season only))

- very dry and very cold- very dry and very cold

cP Wind FlowcP Wind Flow•Western Western mountains, such mountains, such as the Rocky and as the Rocky and Sierra Nevada Sierra Nevada ranges, normally ranges, normally protect the Pacific protect the Pacific Northwest from Northwest from cP air.cP air.

•Strong Strong anticyclone highs, anticyclone highs, however, can however, can create northeast create northeast winds that cause winds that cause cold outbreaks cold outbreaks along the western along the western coast.coast.

Lake-effect snows and the associating air masses

• Lake-effect snows form as a result of colder air traversing over “relatively” warm lake water.

• 25F (air temperature) vs. 40F (lake surface temperature)

• The warmth of the lake provides an instability which promotes cloud growth.

• Lake effect snow events can last between several hours to a few days.

Artic outbreak associated Artic outbreak associated with continental artic air with continental artic air

mass.mass.

Top image: 12 UTC 24 Top image: 12 UTC 24 December 1983December 1983

Bottom image: 00 UTC Bottom image: 00 UTC February 18 2006February 18 2006

Summer & Winter cP AirSummer & Winter cP Air

•Summer cP air over Summer cP air over the US brings welcome the US brings welcome relief from heat, but relief from heat, but also triggers steeper also triggers steeper environmental lapse environmental lapse rates and cumulus rates and cumulus cloud development.cloud development.

•Cold surfaces during Cold surfaces during the winter create the winter create temperature temperature inversions.inversions.

Modification of cP AirModification of cP Air

As the cP air mass moves over the warmer Gulf of Mexico and Gulf Stream, As the cP air mass moves over the warmer Gulf of Mexico and Gulf Stream, ocean surface warmed the cooler air. The lower atmosphere becomes unstable ocean surface warmed the cooler air. The lower atmosphere becomes unstable and forms extensive rows of cumulus cloud streets.and forms extensive rows of cumulus cloud streets.

Origin of mP AirOrigin of mP Air

Cold Asian & Cold Asian & polar air passing polar air passing over the ocean over the ocean south of the south of the Aleutian low will Aleutian low will pick up warmth pick up warmth and moisture, and and moisture, and reaches the reaches the Pacific Coast as Pacific Coast as cool, moist, and cool, moist, and unstable, bringing unstable, bringing rain and snow.rain and snow.

Modification of mP AirModification of mP Air

•Orographic precipitation lowers the moisture content of mP air, Orographic precipitation lowers the moisture content of mP air, called Pacific air east of the Rockies, during its westward flow.called Pacific air east of the Rockies, during its westward flow.

•Leeward of the Rockies, the air is dry and may develop chinook Leeward of the Rockies, the air is dry and may develop chinook winds.winds.

East Coast mP AirEast Coast mP Air

•A strong anticyclone in eastern A strong anticyclone in eastern Canada creates northeasterly Canada creates northeasterly winds that may bring cold, winds that may bring cold, unstable Atlantic mP air and unstable Atlantic mP air and storms into New England and storms into New England and the middle Atlantic States.the middle Atlantic States.

•This creates a damming effect This creates a damming effect of the cold air due to the of the cold air due to the Appalachian mountain range.Appalachian mountain range.

This cold mP air mass adjacent This cold mP air mass adjacent to the warm Gulf Stream air to the warm Gulf Stream air produces a temperature produces a temperature difference zone where difference zone where developments of extratropical developments of extratropical cyclones are favorable.cyclones are favorable. •These storms are known as These storms are known as northeasters.northeasters.

Tropical Pacific mT AirTropical Pacific mT Air

•Warm and moist maritime air from the tropical Pacific may reach the West Coast as a Warm and moist maritime air from the tropical Pacific may reach the West Coast as a series of unstable and powerful thunderstorms. series of unstable and powerful thunderstorms.

•This stream of moist air is sometime referred to as the This stream of moist air is sometime referred to as the ““pineapple express.pineapple express.””

•The subtropical jet (STJ) is associated with bringing in moist, warm, mT air into southern The subtropical jet (STJ) is associated with bringing in moist, warm, mT air into southern California during the El-Nino-influenced winter causing landslides.California during the El-Nino-influenced winter causing landslides.

El Nino STJEl Nino STJ

Regular STJRegular STJ

La Nina STJLa Nina STJ

Gulf & Caribbean mT AirGulf & Caribbean mT Air

•Gulf of Mexico and Caribbean Sea warmth and moisture flows into the East Coast by a Gulf of Mexico and Caribbean Sea warmth and moisture flows into the East Coast by a strong anticyclone.strong anticyclone.

•When the moist, hot mT air rises above dense cP air, heavy and widespread precipitation When the moist, hot mT air rises above dense cP air, heavy and widespread precipitation can result.can result.

•Thunderstorms can occur in the moist mT air mass just ahead of the cold front.Thunderstorms can occur in the moist mT air mass just ahead of the cold front.

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Thunderstorms Threat

Mexican Origin of cT AirMexican Origin of cT Air

•Dry, hot air from the Dry, hot air from the Mexican desert can cause low-Mexican desert can cause low-level instability in the U.S. level instability in the U.S. interior during summer, and interior during summer, and may trigger dust devils.may trigger dust devils.

•An upper level ridge of high An upper level ridge of high pressure may add pressure may add compressionally heated air to compressionally heated air to the region, enhancing the dry, the region, enhancing the dry, hot conditions.hot conditions.

•This summer upper-level This summer upper-level high pressure is a common high pressure is a common feature in the central US feature in the central US which causes the Polar Front which causes the Polar Front Jet to migrate even further Jet to migrate even further north.north.

The Results of Different Air Masses CollisionThe Results of Different Air Masses Collision

•Two air masses entering a region, such as the U.S. middle latitudes, have a front, or Two air masses entering a region, such as the U.S. middle latitudes, have a front, or transition zone, between the strong temperature, humidity, wind direction differences.transition zone, between the strong temperature, humidity, wind direction differences.

•Fronts can also be defined as troughs of lower pressure from analyzing isobars. Fronts can also be defined as troughs of lower pressure from analyzing isobars.

•Four different fronts are used on weather maps: cold, warm, stationary, and occluded Four different fronts are used on weather maps: cold, warm, stationary, and occluded fronts.fronts.

•An extratropical cyclone (mid-latitude cyclone) is made up of 2 or more different air An extratropical cyclone (mid-latitude cyclone) is made up of 2 or more different air masses.masses.

FrontsFronts“the transition zone between two distinct air masses”

• Cold – a transition zone where a cold air mass advances and replaces a warm air mass.

• Warm – a front that moves in such a way that warm air replaces cold air.

• Occluded (occlusion-“closed off”) – a complex frontal system that ideally forms when a cold front overtakes a warm front. – When the air behind the front is colder than the air ahead

of it, the front is called a cold occlusion. Cold Cool– When the air behind the front is milder than the air ahead

of it, the front is called a warm occlusion. Cool Cold• Stationary – a front that is nearly stationary with

winds blowing parallel and from the opposite directions on each side of the front at the surface.

Front IdentificationFront Identification•Locating a front on a weather Locating a front on a weather map involves finding sharp map involves finding sharp changes in:changes in:a) temperaturea) temperatureb) dew pointb) dew pointc) wind directionc) wind directiond) pressure and d) pressure and e) cloud/precipitation patterns.e) cloud/precipitation patterns.

•Pressure tendency values are Pressure tendency values are different along a front.different along a front.

•West of the front the surface West of the front the surface stations are reporting rising stations are reporting rising pressure trend while stations pressure trend while stations ahead of the front are ahead of the front are experiencing falling pressure experiencing falling pressure tendency.tendency.

•(/) rising(/) rising•(\) falling(\) falling•((//\) rise than fall\) rise than fall•((\\/) fall than rise/) fall than rise

Cold Front PassageCold Front Passage

Click Here for Animation

Cold Front TransitionCold Front Transition

•Important cloud, wind, and temperature changes are revealed in this cross-section view of a typical Important cloud, wind, and temperature changes are revealed in this cross-section view of a typical cold front.cold front.

• The front The front rises steeplyrises steeply (1km rise across a 50km distance) and high clouds protrude ahead. (1km rise across a 50km distance) and high clouds protrude ahead.

• The slope is also dependent on the speed of the front as well, therefore the fast the front, the steeper The slope is also dependent on the speed of the front as well, therefore the fast the front, the steeper the slope which leads to greater upward vertical motion along the cold front.the slope which leads to greater upward vertical motion along the cold front.

•Slow moving cold fronts are associated with broader cloud and precipitation coverage behind the Slow moving cold fronts are associated with broader cloud and precipitation coverage behind the front.front.

• Fast moving cold fronts may send a fast moving squall line of showers ahead of the front.Fast moving cold fronts may send a fast moving squall line of showers ahead of the front.

cold, dry aircold, dry air warm, moist airwarm, moist air

Strengthening FrontStrengthening Front

•Frontogenesis is the process in which a front is strengthening or developing; Frontogenesis is the process in which a front is strengthening or developing; tightening of the thermal gradient; strong vertical motion is associated with tightening of the thermal gradient; strong vertical motion is associated with this process.this process.

•Frontolysis is the process where a front is weakening or dissipating; thermal Frontolysis is the process where a front is weakening or dissipating; thermal gradient becomes diffuse; vertical motion is weak.gradient becomes diffuse; vertical motion is weak.

•Satellite imagery shows the temporal transition between a weak front and its Satellite imagery shows the temporal transition between a weak front and its frontogenesis, or strengthening, as it moves offshore over warmer water.frontogenesis, or strengthening, as it moves offshore over warmer water.

Cold Front Weather

Most cold fronts move toward the south, southeast, or east.Most cold fronts move toward the south, southeast, or east.

Back Door Cold FrontBack Door Cold Front

•Eastern Canadian high pressure can Eastern Canadian high pressure can generate cold fronts from the generate cold fronts from the northeast, which mix with the warm, northeast, which mix with the warm, moist Gulf air over the northeastern moist Gulf air over the northeastern US.US.

•Cold air damming describes how the Cold air damming describes how the Appalachian Mountains confine the Appalachian Mountains confine the front's westward movement.front's westward movement.

•Surface winds shifts from westerly to Surface winds shifts from westerly to easterly or northeasterly, and easterly or northeasterly, and temperature falls.temperature falls.

•Denver area does experience back Denver area does experience back door cold front.door cold front.

6:00 CDT 28 Sept 2005

Frontal Weather TrendsFrontal Weather Trends

•Observed wind, temperature, Observed wind, temperature, pressure, humidity, clouds, and pressure, humidity, clouds, and rain weather experiences typical rain weather experiences typical patterns before, during, and after patterns before, during, and after a warm front passage as well.a warm front passage as well.

•Note the cyclonic rotation of Note the cyclonic rotation of winds and change in temperature winds and change in temperature along this warm front.along this warm front.

Warm Front PassageWarm Front Passage Click Here for Animation

Warm Front TransitionWarm Front Transition

•Unique clouds and precipitation patterns are associated with warm fronts, with a broader Unique clouds and precipitation patterns are associated with warm fronts, with a broader range of showers than in a cold front.range of showers than in a cold front.

•The cross-sectional view shows the gentle slope of The cross-sectional view shows the gentle slope of ““overrunningoverrunning”” warm air, a typical warm air, a typical temperature inversion, and the shifting winds.temperature inversion, and the shifting winds.

•Overrunning is where a warmer air mass is Overrunning is where a warmer air mass is ““glidinggliding”” over the top of a colder air mass. over the top of a colder air mass.

•A warm front is associated with a gentle slope (2km rise across a 600km distance).A warm front is associated with a gentle slope (2km rise across a 600km distance).

•The upward vertical motion associated with a warm front is not as intense as a cold front.The upward vertical motion associated with a warm front is not as intense as a cold front.

cold, moist air

warm, moist air

Warm Front Weather

Cold Occluded FrontsCold Occluded Fronts•Fast moving cold fronts Fast moving cold fronts may overtake the slower may overtake the slower moving warm front, moving warm front, particularly when they are particularly when they are influenced by cyclonic winds influenced by cyclonic winds eventually leading to an eventually leading to an occlusion.occlusion.

•The colder, heavier air The colder, heavier air behind a cold front is able to behind a cold front is able to lift the less cold, lighter air lift the less cold, lighter air ahead of the warm front.ahead of the warm front.

•Cold occlusion describes Cold occlusion describes where very cold air is behind where very cold air is behind the cold front and cool/cold the cold front and cool/cold air ahead of the warm front.air ahead of the warm front.

Click Here for Animation

Cold Occlusion Cross-Sectional Views

Warm Occluded FrontsWarm Occluded Fronts

•The air ahead of the warm The air ahead of the warm front is colder than the air front is colder than the air behind the cold front.behind the cold front.

•Warm occlusion describes the Warm occlusion describes the case where the cold front case where the cold front catches up to and overtakes the catches up to and overtakes the warm front but the milder, warm front but the milder, lighter air is unable to lighter air is unable to ““undercutundercut”” the colder, heavier the colder, heavier air ahead of it.air ahead of it.

•The cold air associated with The cold air associated with the cold front rides along the the cold front rides along the slope of the warm front – slope of the warm front – ““piggy-backing.piggy-backing.””

Occluded Front Weather

Mid-Latitude CycloneMid-Latitude Cyclone

•Occluded fronts are common Occluded fronts are common along mid-latitude cyclones, or along mid-latitude cyclones, or deep low pressures centers about deep low pressures centers about which the cold and warm fronts which the cold and warm fronts pivot.pivot.

•Mid-latitude cyclones are also Mid-latitude cyclones are also known as extratropical cyclone. known as extratropical cyclone.

•These storms appear frequently These storms appear frequently in the mid-latitudes.in the mid-latitudes.

•These storms are formed as a These storms are formed as a result of colliding air masses result of colliding air masses (polar front theory).(polar front theory).

Upper Air FrontUpper Air Front

A division between cold and warm air masses in the tropopause is A division between cold and warm air masses in the tropopause is described as an upper-air front, which forms when polar jet rides described as an upper-air front, which forms when polar jet rides near the tropopause through tightly packed isotherms.near the tropopause through tightly packed isotherms.

Dryline

• Not a true front but rather a moistures boundary.• Separates warm humid air to the east (Gulf of Mexico)

and hot, dry air to the west (high Mexican plateau).• Moisture and wind shift differences are used to depict

the dryline.• Acts as a focus mechanism for severe weather.• Climatologically found in the south-central US during

Spring and Fall but can be observed over the CO high plains.

• There are two types of dryline:– Synoptically-driven (active)– Quiescent (passive)

Active Synoptic Pattern Dryline: 1800 UTC 4 May 2003

Passive Synoptic Pattern Dryline: 1800 UTC 6 May

High Plains Dryline: 1800 UTC 18 Aug 2005

High Plains Dryline: 1800 UTC 18 Aug 2005