Chapter 9 Notes
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.
originate in Polar regions
originate in tropical or subtropical regions
form over oceans or maritime areas
form over land surfaces or continental surfaces
Primary Air Mass Regions:
(P)-cold or cool
(T)-hot or warm
(c)-formed over large land masses, dry
(m)-formed over oceans, moist
main air mass types:
Polar (cP)- form over interior high latitude regions, such as Canada
Tropical (cT)- form over tropical and subtropical deserts and plateaus,
Ex: SW USA
Polar (mP)- form over oceans at high latitudes, ex: North Pacific,
Tropical (mT)- form over warm oceans such as Gulf of
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.
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
masses are not confined to their source regions after they form, and
travel by upper air patterns.
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.
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
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.
Mass Modification- air masses moving from place to place change their
properties as they exchange heat and moisture with the underlying surface.
primary mechanisms that modify air masses:
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
lifting- forced by topography
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.
are classified by the temperature changes that result after an air mass
passes over a given location
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.
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
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: 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.
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
form at the boundaries between air masses of different temps and moisture
air always slopes upward over cold air
and precipitation form as a warm air mass rises over more dense colder air
front always slopes upward over the cold air
drops as a front approaches
the Northern Hemisphere, wind direction near the ground shifts clockwise as the