Chapter 9 Notes


  • Air Mass- extremely large body of air whose properties of temperature and moisture content are similar to any horizontal direction.  Formed when air stagnates for long periods over a uniform surface, thus their characteristics are determined by surface over which they are formed.
  • Air Mass Types:
    • Cold- originate in Polar regions
    • Warm- originate in tropical or subtropical regions
    • Moist- form over oceans or maritime areas
    • Dry- form over land surfaces or continental surfaces
  • 5 Primary Air Mass Regions:


1)      Polar (P)-cold or cool

2)      Tropical (T)-hot or warm

3)      Arctic (A)-very cold

4)      Continental (c)-formed over large land masses, dry

5)      Maritime (m)-formed over oceans, moist


  • Four main air mass types:
    • Continental Polar (cP)- form over interior high latitude regions, such as Canada
    • Continental Tropical (cT)- form over tropical and subtropical deserts and plateaus, Ex: SW USA
    • Maritime Polar (mP)- form over oceans at high latitudes, ex: North Pacific, Northeaster (Atlantic)
    • Maritime Tropical (mT)- form over warm oceans such as Gulf of Mexico
  • Arctic air masses are much colder than polar air masses and form in winter over snow-covered surfaces in Siberia, the Arctic Basin, North America, or Greenland.  They form over frozen Arctic.
  • Air mass source regions are the “birthplaces” of air masses and must have light or no winds, so the air has time to acquire the temperature and moisture properties of the region’s surface.  They must also have an extensive and homogeneous surface.
  • Air masses are not confined to their source regions after they form, and travel by upper air patterns.
  • The Static Stability of the atmosphere is found by comparing the temperature of a rising parcel with the temperature of the atmosphere at the same altitude as the parcel.
    • If parcel is colder than its environment, it is denser and will descend, which is stable.
    • If a parcel is warmer than the surrounding air, it will rise and this is unstable.
    • If warm air is over cold air, this is stable.  If cold air is over warm air this is unstable.  For this reason, tropical air masses are usually less stable than polar air masses.
  • Air Mass Modification- air masses moving from place to place change their properties as they exchange heat and moisture with the underlying surface.
  • Two primary mechanisms that modify air masses:
    • Heat exchanges with the surface- primarily affect lowest regions of the air mass, the greater the temp difference between 2 air masses, then the greater heat exchange.  When a cold air mass moves over a warm one, it increases the instability of the air mass, and vice versa.                            Ex: Steam Fog, Lake Effect Snow
    • Mechanical lifting- forced by topography




  • Front- transition zone between two different air masses.  Can be hundreds of miles long and exist as long as the air masses they separate remain distinct.  Normally can be detected through decreasing pressure and shifts in winds and temperatures.
  • Frontal Zone- sloping surface that separates two air masses.
  • Front are classified by the temperature changes that result after an air mass passes over a given location
    • Cold Front: Cold air replaces a warm air mass; colder air will follow front’s passage.  They are often associated with a narrow band of clouds and intense precipitation as front passes.  Characterized by a steep slope of shifting temperatures.
    • Warm Front: Warm air replace a cold air mass, warm air will follow front’s passage.  They are often detectable through their approach with a large deck of steadily lowering and thickening clouds and moderate precipitation occurring as the front nears, although it can be far in advance of the front.  Slope of front is less steep than a cold front.
    • Stationary Front: Occurs when neither a cold or warm air mass is advancing.  Although the front can appear stationary at the surface, the air above can be moving, causing overrunning.  This warm air overrunning causes clouds and precipitation associated with a stationary front.
    • Occluded Front: formed when a cold front catches up with and overtakes a warm front.  Marks the surface boundary between two polar air masses, with a warm air mass aloft being lifting by its interaction with the polar air masses.  There are two types of occluded front, Cold-Type Occlusion and Warm -Type Occlusion.
  • Drylines- frontal zone defined by moisture and wind rather than temperature contrasts.  They are moisture fronts that help trigger thunderstorms over the southwest USA.


General Conclusions about Fronts


1)      Fronts form at the boundaries between air masses of different temps and moisture levels

2)      Warmer air always slopes upward over cold air

3)      Clouds and precipitation form as a warm air mass rises over more dense colder air

4)      The front always slopes upward over the cold air

5)      Pressure drops as a front approaches

6)      In the Northern Hemisphere, wind direction near the ground shifts clockwise as the front passes