1. AIR MASSES Large bodies of air
SOURCE REGIONS – areas where air masses originate
Uniform in composition
Light surface winds
Dominated by high surface pressure
The longer the air mass remains over a region, the more likely it will acquire properties of the region
Temperature and moisture

2. AIR MASSES Similar horizontal properties
Temperature
Moisture content
Lapse rates
These characteristics are carried with it as air mass moves to other areas
Challenge of weather forecasting
To predict behavior of air masses
Where it will go and how it will change

3. AIR MASS CLASSIFICATION
Bergeron classification – 1920s
Consists of 2 letters ab
Source region (a)
Thermal property (b)
Air mass properties gradually change as it travels
Acquire characteristics of invading regions
Thermal properties
Stability
Whether it is heated or cooled from below
Nomenclature only applies to recent history of air mass

4. AIR MASS CLASSIFICATION
SOURCE REGIONS
Maritime (m) – originates over oceans or large bodies of water
Continental (c) – originates over land
m or c indicate influence of surface on air mass characteristics (water and land)
THERMAL TYPES
Tropical (T) – from low latitudes
Polar (P) – from mid-high latitudes
Arctic (A) – from high latitudes (> 65°N)
P and T suggest importance of latitude of source regions

6. AIR MASS PROPERTIES OVER NORTH AMERICA
WINTERTIME cP
Source – Central Canada and Siberia
Frozen surface – ice and snow
Intense radiation cooling, lack of insolation heating
Extremely cold, stable, and dry
Clouds are non-existent

7. AIR MASS PROPERTIES OVER NORTH AMERICA
SUMMERTIME cP
Source – central Canada
Ample warming of surface through insolation
Melts snow and permafrost
Cool, dry and sometimes unstable due to insolation heating of lower layers

8. AIR MASS PROPERTIES OVER NORTH AMERICA
SUMMERTIME cT
Source – Northern Mexico and extreme SW deserts of US
Hot, dry and unstable

9. AIR MASS PROPERTIES OVER NORTH AMERICA
WINTERTIME mP
Source – open oceans of high latitudes – Gulf of Alaska and North Atlantic
cP air mass from Syberia becomes mP air mass as it moves over ocean
Lower layers modified by warmer water surface
Cool, moist, unstable in surface layers
Cool, dry aloft

10. AIR MASS PROPERTIES OVER NORTH AMERICA
SUMMERTIME mP
Source – open oceans in high latitudes – Gulf of Alaska and North Atlantic
Cool and moist in lower layers and cool and dry aloft
Overall temperature higher than in winter
Instability in lower layers

11. AIR MASS PROPERTIES OVER NORTH AMERICA
WINTERTIME mT
Source – over open ocean near 30N
Great semi permanent subtropical high pressure centers and in SW Caribbean
Warm, moist and very unstable

12. AIR MASS PROPERTIES OVER NORTH AMERICA
SUMMERTIME mT
Source – semi permanent high centers near 15N
Very warm, moist and unstable

13. AIR MASS MODIFICATION
As air masses migrate, changes in their properties exist – AIR MASS MODIFICIATION
As air masses move from source regions they carry with them physical characteristics of the region – INITIAL CONDITIONS
Most variations within air masses are found in vertical distribution
Air masses are modified by the climatic characteristics of the regions over which they move
Types of modification:
Lapse rate modifications
subsidence, uplift,
Moisture modifications
Thermal modifications

14. AIR MASS MODIFICATION cP MODIFICATIONS
WINTER – moving from land to water (lakes or ocean)
Increased instability
Surfaces in their paths are warmer than those of their source regions
Warming from below increases instability

15. AIR MASS MODIFICATION
cP MODIFICATIONS
LAKE EFFECT SNOW – as cold dry air mass moves over warmer water (winter), instability is enhanced
Produces snow belts on eastern shores of Great lakes

16. AIR MASS MODIFICATION cP MODIFICATIONS SUMMER
cP air moves from land to water, becomes more stable
Decrease of lapse rate
Haze, fog and low stratus clouds appear

17. AIR MASS MODIFICATION mP MODIFICATIONS WINTER
Becomes more unstable in lower layers as it reaches coast
Showers, squalls occur as air ascends over coastal mountains
Becomes dry and stable as it moves inland due to subsidence
SUMMER – increases instability

18. AIR MASS MODIFICATION mT MODIFICATIONS WINTER Moisture rich
Releases in precipitation as arrives at coast
Stability increases inland
SUMMER
From Gulf of Mexico
Penetrates as far as Canada flowing along the east side of Rocky Mountains
Instability increases as it moves inland

19. FRONTS Transition zone between two air masses of different densities
Temperature
Humidity
FRONTAL SURFACE (ZONE) – upward extension of a front
Types of fronts
Stationary
Cold
Warm
occluded

20. Frontolysis – the weakening or dissipation of a front
Decreased temperature contrast between two air masses
Frontogenesis – a formation, strengthening or generation of a front
Increased contrast of temperature conditions between two air masses

21. CRITERIA OF FRONT LOCATION
Contrasting conditions in the following exist on either side of a frontal system
Surface temperature
Air moisture content
Winds
Speed and direction
Cloud types & precipitation
Sea level pressure and its tendencies

22. STATIONARY FRONTS Fronts that have no movement
Designation – alternating red half-circles and blue triangles
Obstacles prevent front from progressing
Mountain range
Drawn along a line that separates two air masses

23. STATIONARY FRONTS Example:
Cold cP air from Canada against Rocky Mountains in US
cP air to the north
mP air to the west of mountains
Cold air unable to cross barrier
No westward movement

24. STATIONARY FRONTS Weather
Winds blow parallel to front but in opposite direction on either side
Clear to partly cloudy
Cold to the east
Warm to the west
Little or no precipitation
Both air masses are dry
Stationary front can become warm front or cold front

25. COLD FRONTS
Transition zone where a cold air mass advances and replaces a warm air mass
Leading edge of cold air
cP replacing mT
cP due south, mT due north
Designation – solid blue triangles oriented towards the direction in which front is moving
Rapid movement – up to 50 km/hr

26. COLD FRONTS Cold dense air wedges under warm air forcing it upwards
Sharp slope at front’s edge
Steepness due to friction which slows the airflow near the ground
Slope = 1:50 (vertical to horizontal distance from front’s edge to middle of frontal system)

27. COLD FRONTS The faster the front, the steeper the slope
Movement of the front causes most of the weather associated with its passage
Cold fronts are associated with dramatic shifts in weather

28. COLD FRONTS
Affected local meteorological conditions as front passes over:
Temperature
Seal level pressure tendency
Wind direction
Cloud cover
Dew point
visibility

29. COLD FRONTS
Position of the leading edge of advancing portion of cold air mass
FACTORS THAT DETERMINE COLD FRONTAL WEATHER
There is no average cold front
Weather associated with cold fronts may vary from minor wind shifts to severe thunderstorm activity
Weather is determined by
The nature of warm air that is being lifted
Moist versus dry – precipitation production
Stability of air mass - uplift
Degree of lift to which the warm air is subjected by the advancing cold air wedge
Speed of the cold front and the steepness of frontal surface
Mechanical uplift possible, precipitation

30. COLD FRONTS
with slow-moving cold front, clouds and precipitation usually cover a broad area behind the front
e.g. if rising air is dry and stable scattered clouds are all that form – no precipitation
e.g. during the winter, a series of cold polar outbreaks may travel across the US so quickly that warm air is unable to develop ahead of the front.
frigid arctic air usually replaces cold polar air, and a drop in temperature is the only indication that a cold front has moved through the area

31. COLD FRONTS
SURFACE WEATHER CHANGES ASSOCIATED WITH COLD FRONTS (cP replacing mT)
Surface winds
Change direction when cold front passes over
Cold air moves from W-NW and is undercutting a flow of warm air from S-SW
Winds S-SW ahead of cold front and W-NW behind cold front
Temperature
Arrival of cold front will result in a decrease in temperature (warm to cold)
After passage, temperature keeps decreasing as the cold air begins to modify surface conditions

32. COLD FRONTS
SURFACE WEATHER CHANGES ASSOCIATED WITH COLD FRONTS (cP replacing mT)
Moisture content
Onset of cold air results in drop in dew point
Cloud and precipitation
Ahead – cirrus and cirro-stratus clouds due to stronger upper level winds that push uplifted air at the front’s edge ahead
While passing – towering cumulus clouds (lots of moisture in warm air and lots of precipitation, hail and thunder)
After passing – as warm air is drying out, showers decrease in intensity and skies clear

33. COLD FRONTS
SURFACE WEATHER CHANGES ASSOCIATED WITH COLD FRONTS (cP replacing mT)
Visibility
Usually improves after a cold frontal passage
Air behind front is unstable as a result of passing over a warm surface
Vertical motion will carry pollution aloft
Tropical air accumulates considerable smoke from industrial areas during its movement north
Polar and arctic air masses are relatively free of pollution
Pressure
Because front lies in a trough, approach will be accompanied by decrease in pressure
A marked rise will be observed as trough passes

34. WARM FRONTS
Transition zone between a retreating cold air mass and advancing warm air mass
Designation – red semi-circles pointing in the direction where the warm air is advancing
Warm and moist mT replacing dry cold mP

35. WARM FRONTS
Frontal changes are less abrupt than cold air frontal passages
Long spells of cold weather do not come to a rapid end
Weather at the warm front portion of a frontal system is more extensive than at the cold front portion
Cloud system & precipitation cover extensive areas

36. WARM FRONTS Advancing warm air overruns the retreating wedge
Cold air lies as a wedge under warm air
As warm front approaches depth of cold air decreases
Advancing warm air overruns the retreating wedge
Forces air to rise – creates frontal inversion
Air expands and cools
Extensive cloud on top of cold air

37. WARM FRONTS Average slope of 1:300 Warm fronts are slow
About half that of average cold front
Average slope of 1:300
More gentle than cold front

38. WARM FRONTS Weather associated with warm Fronts
Weather patterns associated with warm fronts depend on:
Moisture content of warm air mass
precipitation
Stability of the warm air mass
uplift
Degree of overrunning
Uplift and precipitation

39. WARM FRONTS Weather associated with warm fronts
If overrunning air is dry and stable, only high and middle clouds will form
no precipitation
If overrunning air is moist and unstable, heavy showers can develop as thunderstorms become embedded in the cloud mass
Arrival of warm front produces wind shifts, warmer temperatures and overall improvement of weather conditions

40. OCCLUDED FRONTS
OCCLUSION – frontal system that forms when a cold front overtakes a warm front
Designation – alternating purple triangles and half-circles that point towards the direction to which the front progresses
Development – rapidly approaching cold front catches up to slow-moving warm front and overtakes it

41. OCCLUDED FRONTS WARM OCCLUSION
When the air behind the occluded front is warmer than the air ahead
cP air overrun by mP from Pacific ocean in NW USA
As cold front overtakes warm front, the milder and lighter air behind the cold front is unable to lift the colder and heavier air off the ground
Surface weather associated with this occlusion is similar to that of warm front

42. OCCLUDED FRONTS COLD OCCLUSION
When the air behind the occluded front is cooler than the air ahead
Most common to Pacific coast states
cP overrun by cA

43. OCCLUDED FRONTS COLD OCCLUSION WEATHER
Cold front rapidly approaches warm front
Warm air rides up the cool air in front of the warm front
As cold air overtakes warm front, the warm front and the warm air mass are lifted off the ground
As front approaches weather is similar to warm fronts
Frontal passage brings weather similar to that of cold fronts