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
Fronts
- 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