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2. WEATHER Specialty Course AUXWEA
Chapter 4 – Air Masses, Fronts and Cyclones

3. Air Masses and Pressure Systems
Section 1
Air Masses and Pressure Systems

4. Overview Air masses. High and low pressure systems. Types.
Source regions.
Modification.
High and low pressure systems.
Characteristics.
How they move.

5. Air Masses
Air masses are high pressure areas with fairly uniform characteristics.
Temperature.
Humidity.
They are labeled by source region:
Continental (c) or maritime (m).
Tropical (T), Polar (P) or Arctic (A).
Types are:
cP (dry and cold), cT (dry and warm).
mP (moist and cold), mT (moist and warm).
cA—continental Arctic (dry and very cold).
Classification
Polar (cold)
Tropical (warm)
Continental (dry) cP cT
Maritime (moist) mP mT

6. Sources of Air Masses
Source regions impart characteristics to the air mass within a few days.
As they move, they are modified by the surface over which they pass.
Temperature increased or decreased.
Moisture added or removed.
Fronts are where they meet.
Courtesy of NOAA

7. High and Low Pressure Systems
High pressure systems (highs) are characterized by:
Stable, sinking air with surface divergence.
Light winds that increase with distance from the center.
Weather is clear to partly cloudy with no precipitation.
Clear skies at night can produce dew, frost or fog.
Low pressure systems (lows) are characterized by:
Unstable, rising air, with surface convergence.
Strong, gusty winds that increase closer to the center.
Unsettled, cloudy weather, often with precipitation.
Can have steady rain, showers, or thunderstorms.

8. Movement of Highs and Lows
Highs and lows are steered by the winds in the upper atmosphere, at and above about 18,000 feet (500 mb).
Divergence or Convergence aloft affects highs and lows.
Divergence aloft (spreading isoheights or winds speeding up) causes rising air, forming or deepening lows.
Convergence aloft (tightening isoheights or winds slowing down) causes falling air, forming or strengthening highs.
Temperature advection aloft also affects highs and lows.
Positive temperature advection lowers surface pressure.
Negative temperature advection raises surface pressure.
As the wind pattern aloft moves, the highs and lows move with it.
Positive temperature advection means that the winds blow higher temperatures into a area. Negative temperature advection blows in colder temperatures.
The reason this changes temperature has to do with the weight of the air above. Warmer air is less dense, and therefore lighter. Colder air is heavier.

9. Movement of Highs and Lows
In this diagram, convergence is caused by winds slowing down and divergence by winds speeding up. The cyclone to the right is represented by the fronts and the cyclonic circulation.
©2008, US Power Squadrons. Reprinted with permission

10. Fronts and Storm Systems
Section 2
Fronts and Storm Systems

11. Overview Fronts. Storm systems (extra-tropical cyclones) Types.
Symbols.
Characteristics.
Storm systems (extra-tropical cyclones)
Structure.
Cloud types.
Formation.
Movement.

12. Fronts Fronts are the boundaries between different air masses.
There are four different types (one with subtypes).
Types are based on which air mass is advancing.
Cold fronts are where cold air is pushing under warm air.
Warm fronts are where warm air is riding over cool air.
Stationary fronts have very little movement.
Occluded fronts are where a fast-moving cold front overtakes a warm front. It involves three air masses: cold, cool, and warm.
In cold occlusions, the cold air pushes under the cool air.
In warm occlusions, the cool air overrides the cold air.
In both cases, the warm air is raised off the ground.
The next few slides expand on these definitions.

13. Fronts on Surface Charts
The four types of fronts are shown how they appear on surface charts. Also included is a trough (or trof). Fronts move in the direction of the pips.
Cold Warm Stationary Occluded Trof
Occluded fronts can be colored either purple, or alternating red and blue.

14. Cold Fronts
Cold fronts are fast moving and aggressively push under warm air, making it unstable and producing convective activity. They have strong winds and showery precipitation.
©2008, US Power Squadrons. Reprinted with permission

15. Warm Fronts
Warm fronts are gentler and have steady precipitation and stratiform clouds. Depending on the temperature gradient, it can be rain, snow, sleet, or freezing rain.
Cool
©2008, US Power Squadrons. Reprinted with permission

16. Comparison of Cold and Warm Fronts
This figure compares cold and warm fronts—their clouds, slopes, and extent.
©2008, US Power Squadrons. Reprinted with permission

17. Stationary Fronts
Stationary fronts remain in nearly the same place, sometimes for days, wandering back an forth. Over time, the weather comes to resemble that of a warm front.
©2008, US Power Squadrons. Reprinted with permission

18. Occluded Fronts Occluded fronts involve three air masses.
Whether the occlusion is warm or cold depends on the relative arrangement of the cold, cool, and warm air masses.
Top figure is a warm occlusion.
Bottom figure is a cold occlusion.
Warm occlusions are more common on the west coast, primarily the Pacific NW. Warm occlusions are more common on the east coast.
©2008, US Power Squadrons. Reprinted with permission

19. Cyclones Cyclone means the system has cyclonic circulation.
In the mid-latitudes, they are more properly called extra-tropical cyclones and almost always have fronts .
This is to distinguish them from tropical cyclones, which are related to hurricanes.
The graphic is illustrative only. Details will be covered later in this chapter. Most cyclones have fronts associated with them. The primary exception is “thermal” lows, such as in the SW (AZ and NM).
Note the area of precipitation, the wind shift across the fronts, and the kinks in the isobars (normally more pronounced at the cold front).
©2008, US Power Squadrons. Reprinted with permission

20. How is the Cyclone Moving?
In this picture, note how the low moves relative to an observer.
If the low is passing to the south of the observer, the winds will back.
If the low is passing to the north of the observer, the winds will veer.
Remember Buys-Ballot.
©2008, US Power Squadrons. Reprinted with permission

21. The Formation of Lows Cyclogenisis is the formation of cyclones.
There are two theories of how cyclones form:
The classic theory involves waves forming on the polar front.
The polar front resides between the polar easterlies and the prevailing westerlies.
Disturbances on the polar front develop into cyclones because the winds are flowing in opposite directions.
The modern theory is based on the upper air convergence and divergence discussed earlier. The polar jet stream is over the polar front and divergence aloft forms lows.
The next slide shows a series of images depicting the classic theory.

22. Classic Cyclogenisis Theory
©2008, US Power Squadrons. Reprinted with permission

23. Modern Cyclogenisis Theory
As explained earlier, divergence aloft causes rising air, forming low pressure areas (cyclones).
©2008, US Power Squadrons. Reprinted with permission

24. Storm Tracks There are three basic source regions for cyclones.
The Pacific Ocean.
The Midwest.
The Gulf.
Storms tend to follow the tracks shown here.
mP comes from the Pacific.
cP comes from the Midwest.
mT comes from the Gulf.
©1997, USA Today. Reprinted with permission

25. Summary (1 of 2)
Air masses are pools of air with similar characteristics.
Temperature—tropical, polar or arctic.
Moisture—continental or maritime.
Source regions determine type: cP, cT, mP, mT, cA.
They are modified as they move over new areas.
Addition or subtraction of moisture.
Heating or cooling.
A front is where two or more air masses meet.
Cold front—cold air pushes aggressively under warm air.
Warm front—warm air rises over cool air.
Stationary front—little movement.
Occluded—cold front overtaking warm front.

26. Summary (2 of 2)
High pressure areas are air masses with anti-cyclonic rotation.
Low pressure systems are extra-tropical cyclones.
They are where fronts meet.
There are other lows that are due to convergence or heating.
Pressure systems are steered by upper level winds and modified by upper level convergence or divergence.
If a low passes to the south, the wind backs.
If a low passes to the north, the wind veers.
Lows form by cyclogenisis.
Divergence aloft lowers surface pressure.
Circulation develops, forming a wave on the polar front.

27. Chapter 4 Questions QUESTION ANSWER
In high pressure systems, the air moves in what manner?
In low pressure systems, the air moves in what manner?
Precipitation ahead of and on a cold front is typically:
When a cold front passes, the wind does what?
Cyclones often form on the polar front because:
Spirals clockwise out of the center.
Spirals counter-clockwise into the center.
Short in duration and showery.
Slide 27: Some Chapter Questions (1 of 2)
ANIMATIONS
Five questions are displayed.
[CLICK] to display each answer in sequence. Font change used to simulate human handwriting.
After last answer displayed, one more [CLICK] to next screen (allows instructor pause for last answer).
* * * * * * * * * * (end comments).
Veers sharply.
The prevailing westerlies and polar easterlies oppose each other.

28. Chapter 4 Questions QUESTION ANSWER
Clouds ahead of a warm front are typically of what type?
Clouds ahead of a cold front are typically of what type?
When the wind speeds up with distance, it is said to be:
Divergence aloft encourages cyclone development because it:
The symbol for a cold front is:
Stratiform.
Cumuliform.
Diverging.
Slide 28: Some Chapter Questions (1 of 2)
ANIMATIONS
Five questions are displayed.
[CLICK] to display each answer in sequence. Font change used to simulate human handwriting.
After last answer displayed, one more [CLICK] to next screen (allows instructor pause for last answer).
* * * * * * * * * * (end comments).
Supports convection and surface convergence.
A line with triangular pips.

29. Chapter 4 Questions QUESTION ANSWER The symbol for a warm front is:
A line with triangular and semi-circular pips pointing in different directions marks:
A line with triangular and semi-circular pips pointing in the same direction marks:
A cP air mass from Canada, moving through the central United States will be modified in what way?
The paths of cyclones through the U.S. is determined by:
A line with semi-circular pips.
A stationary front.
An occluded front.
Slide 29: Some Chapter Questions (1 of 2)
ANIMATIONS
Five questions are displayed.
[CLICK] to display each answer in sequence. Font change used to simulate human handwriting.
After last answer displayed, one more [CLICK] to next screen (allows instructor pause for last answer).
* * * * * * * * * * (end comments).
Warmed from the surface upward.
Winds at and above 18,000 feet.

30. End of Chapter 4 Are there any questions?
Chapter 5 covers Clouds, Precipitation, and Optical Effects.