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Air Masses and Midlatitude Cyclones. ATS 351 Lab 11 November 16, 2009. Air Masses of North America. Air mass: extremely large body of air whose properties of temperature and humidity are fairly similar in any horizontal direction at any given latitude Named for their region of origin

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Air Masses and Midlatitude Cyclones

ATS 351 Lab 11

November 16, 2009

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Air Masses of North America

Air mass: extremely large body of air whose properties of temperature and humidity are fairly similar in any horizontal direction at any given latitude

Named for their region of origin

Continental Polar

Continental Arctic

Maritime Polar

Maritime Tropical

Continental Tropical

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Cold Front

Cold, dry stable polar air is replacing warm, moist, conditionally unstable subtropical air

Steep vertical boundary due to surface friction slowing down the surface front

Has strong vertical ascent along the surface front

Strong upper level westerlies push ice crystals far ahead of the front, creating cirrus advance of the front.

  • Cold, dense air wedges under warm air, forcing the warm air upward, producing cumuliform clouds

  • Can cause strong convection, severe weather, and squall lines.

  • Air cools quickly behind the front

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Air Masses of North America

Continental Polar (cP) & Arctic (cA)‏

Cold, dry, stable air in winter

In summer, cP air mass usually brings relief from oppressive heat in central and eastern US

Maritime Polar (mP)‏

In winter, cold and dry continental air masses are carried over Pacific Ocean where moisture and warmth is added

Air mass at Pacific Coast is cool, moist, and conditionally unstable

East of Rockies - brings fair weather and cooler temperatures (moisture has been removed by mountains)‏

East coast mP air mass: originates in N. Atlantic

Storms may develop (heavy rain or snow, coastal flooding)‏

Late winter, early spring

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Air Masses of North America

Maritime Tropical (mT)‏

Subtropical east Pacific Ocean: very warm and moist by the time it reaches east coast

Heavy precipitation

Gulf of Mexico: warm, humid subtropical air

Formation of dew, fog and low clouds along Gulf coast

Could lead to record heat waves

Continental Tropical (cT)‏

Source is N Mexico and arid SW: only exist in summer

Hot, dry and conditionally unstable at low levels

Clear skies and hot weather (severe drought possible)‏

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Air Masses and Fronts

A front is a transition zone between two air masses of different densities

Fronts extend both horizontally and vertically

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Cold Front

Rising motion causes decreased surface pressure ahead of the front

On a surface pressure map, frontal location can be seen by “kinks” in the isobars, changes in wind direction from a southwesterly to a northwesterly wind, and decreases in temperature.

Pressure is lowest at the surface front.

On weather maps, cold fronts are indicated by blue lines with triangles pointing in the direction of frontal motion (towards warmer air)‏

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Cold Front






Gusty, shifting




Sudden drop

Steady drop


Steady fall

Min, then sharp rise

Steady rise


Increasing, Ci, Cs, Cb




Brief showers

Heavy rains, severe weather

Showers, then clearing


Fair to poor



Dew Point

High, remains steady

Sharp drop


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Warm Front

Occurs at the leading edge of an advancing warm, moist, subtropical air mass from the Gulf replacing a retreating cold, maritime, polar air mass from the North Atlantic

Slowly advances as cold air recedes; moves at about half the speed of an average cold front

Speed may increase due to daytime mixing

Speed may decrease due to nighttime radiational cooling

Smaller vertical slope than cold front

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Warm Front

Warmer, less-dense air rides up and over the colder, more-dense surface air


Produces clouds and precipitation well in advance of the front

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Warm Front










Slowly warming

Steady rise

Warmer, then steady



Leveling off

Slight rise, followed by fall


(in order) Ci, Cs, As, Ns, St, fog (Cb in summer)‏


Clearing with scattered Sc



Drizzle or none

Usually none





Dew Point

Steady rise


Rise, then steady

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Stationary Front

Essentially no movement

Surface winds blow parallel to front, but in opposite directions on either side of it

Separates two air masses

Seen often along mountain ranges when cold air cannot make it over the mountain ridge

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Hourly surface observations at Gage, Oklahoma showing the passage of a primary and secondary cold front (left) and at Bowling Green, Kentucky showing the passage of a warm front (right).

Source: Wallace and Hobbs, 2006.

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Occluded Fronts passage of a primary and secondary cold front (left) and at Bowling Green, Kentucky showing the passage of a warm front (right).

Cold fronts generally move faster than warm fronts

Occlusion occurs when cold front catches up to and overtakes a warm front

Occlusions can be warm or cold

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Dry Lines passage of a primary and secondary cold front (left) and at Bowling Green, Kentucky showing the passage of a warm front (right).

Think of a dry line as a moisture boundary

Separates warm, humid air from warm, dry air

Drier air behind dry lines lifts the moist air ahead of it, triggering storms along and ahead of it

Induces lifting along front

Often produces severe thunderstorms in OK & TX

Unique to southern great plains of US because of the Rocky mountains and the Gulf of Mexico

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A guide to the symbols for weather fronts that may be found on a weather map:

#1 cold front

#2 warm front

#3 stationary front

#4 occluded front

#5 surface trough

#6 squall/shear line

#7 dry line

#8 tropical wave

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Midlatitude (Extratropical) Cyclone on a weather map:

A cyclone (area of low pressure) in the middle latitudes (35°-70°)‏

Important for global heat transport

Help to redistribute energy between the tropics (equator) and the poles

Often associated with significant weather events

Described by the Polar Front Theory

Form on boundaries between warm and cold air

Cold Polar air meeting warm tropical air

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Features of a Midlatitude Cyclone on a weather map:

Deep low pressure area with attached cold and warm fronts

Often an occlusion forms, the triple point lending to the formation of severe weather

Precipitation associated with the cold and warm fronts organizes in typical “comma cloud” structure

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Polar Front Theory on a weather map:

  • Initially, there is a stationary front that acts as the boundary separating cold, continental polar air from warm, maritime tropical air

  • Winds blow parallel to this front on either side

  • Polar Fronts are discontinuous

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Cyclogenesis on a weather map:

Central Pressure

  • A wave forms on the front due to a shortwave disturbance

    • Frontal Wave

  • The front develops a "kink" where the wave is developing

  • Precipitation will begin to develop along the front

    • Overrunning and lifting

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Strengthening on a weather map:

  • The cyclonic circulation around the low becomes more defined

  • The central pressure intensifies

  • The cold front and warm front have more organized motion

  • Cyclone usually pushed east or northeast by the winds aloft

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Mature Cyclone on a weather map:

  • The cold front catches up with the warm front and an occlusion forms

  • The cyclone is at its strongest at this point

  • Severe weather often develops near the “triple point”

    - Intersection of cold, warm, and occluded fronts

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Dissipation on a weather map:

  • The occlusion grows with time

  • Eventually, the occlusion is so great that the supply of warm, moist air into the low is cut off

    • Cold air on both sides

  • When this happens, the system starts to dissipate

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Interaction with Upper Levels on a weather map:

  • Previous model for cyclone development only includes surface characteristics – but what happens higher up can determine what happens below

  • Divergence aloft can help to remove mass from a column, hence lowering the surface pressure even more

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Interaction with Upper Levels on a weather map:

  • Downstream of an upper level trough, the air tends to diverge

  • If a surface low is located slightly downstream of an upper level trough, the divergence will be located above the low and help to intensify it

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Mid-latitude Cyclones: on a weather map:The Upper Level Life Cycle

  • A 500 mb trough develops westward of a surface stationary front

  • As the 500 mb trough deepens, the associated upper level divergence strengthens, helping to intensify the surface low

  • Stronger winds aloft force the upper level trough to move eastward faster, and eventually it becomes located above the surface low

  • When the surface and upper level low are “stacked”, convergence at both levels starts to “fill” the low pressure area, weakening the cyclone

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Mid-latitude Cyclones: on a weather map:The Upper Levels

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Closed Low on a weather map:

  • A closed low is an upper level area of low pressure that is completely encircled by at least one isobar

  • This may be partially or completely detached from the main flow

  • Often results from occlusion of surface cyclone

  • These can persist for several days without moving and produce several days worth of precipitation over the area where they park

  • Called cut-off low when completely separated from main flow at upper levels