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- Air masses
- Contain uniform temperature
and
- humidity characteristics.
- Fronts
- Boundaries between unlike air
- masses.
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- Air masses have fairly uniform
temperature and moisture content in horizontal direction (but not
uniform in vertical).
- Air masses are characterized by
their temperature and humidity properties.
- The properties of air masses are
determined by the underlying surface properties where they originate.
- Once formed, air masses migrate within the general circulation.
- Upon movement, air masses displace residual air over locations thus
changing temperature and humidity characteristics.
- Further, the air masses themselves moderate from surface influences.
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- The areas of the globe where air masses from are called source regions.
- A source region must have certain
temperature and humidity properties that can remain fixed for a
substantial length of time to affect air masses above it.
- Air mass source regions occur
only in the high or low latitudes; middle latitudes are too variable.
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- Air masses are classified
according to the temperature and moisture characteristics of their
source regions.
- Bases on moisture content:
continental (dry) and maritime (moist)
- Based on temperature: tropical
(warm), polar (cold), arctic (extremely cold).
- Naming convention for air masses:
A small letter (c, m) indicates the moist content followed by a capital
letter (T, P, A) to represent temperature.
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- Theoretically, there should be 6
types of air masses (2 moisture types x 3 temperature types).
- But mA-type (maritime Arctic)
does not exist.
- cA: continental Arctic
- cP: continental Polar
- cT: continental Tropical
- mP: maritime Polar
- mT: maritime Tropical
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- Continental Polar air masses form
over large, high-latitude land masses, such as northern Canada or
Siberia.
- cP air masses are cold and
extremely dry.
- Wintertime cooling over these
land areas cause the atmosphere to become very stable (even inversion).
- The combination of dry and stable
conditions ensure that few if any clouds form over a cP source region.
- Summer cP air masses are similar
to winter cP, but much less extreme and remain at higher latitudes.
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- Migrations of cP air induce colder, drier conditions over affected
areas.
- As cP air migrates toward lower latitudes, it warms from beneath.
- As it warms, moisture capacity increases while stability decreases.
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- Continental Arctic (cA) air
represents extremely cold and dry conditions as, due to its temperature,
it contains very little water vapor.
- The boundary between cA and cP
air is the shallow (~1-2 km) arctic front.
- cA air masses can extend as far southward as the Canadian-United State.
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- Mainly a summertime phenomenon exclusive to the desert southwest of the
U.S. and northern Mexico.
- Characteristically hot and very dry.
- Very unstable, yet clear conditions predominate due to a lack of water
vapor.
- Thunderstorms may occur when
moisture advection occurs or when air is forced orographically.
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- Maritime polar air masses form
over upper latitude oceanic regions and are cool and moist.
- mP air masses form over high-latitude ocean as cP air masses move out
from the interior of continents. (i.e., cP à mP).
- Oceans add heat and moisture into the dry and cold cP air masses.
- Along the west coast of the U.S., mP air affects regions during winter
and may be present before mid-latitude cyclones advect over the
continent.
- Along the east coast, mP air typically affects regions after cyclone
passage as the mP air wraps around the area of low pressure.
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- Form over low latitude oceans and as such are very warm, humid, and
unstable.
- mT air masses from Atlantic and Gulf of Mexico is the primary source
region for the eastern U.S.
- As air advects over the warm continent in summer the high humidity and
high heat occasionally combine to dangerous levels.
- mT air masses have an enormous influence on the southwestern U.S,
particularly in summer.
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- Meteorologists classify fronts based on the thermal and moisture
characteristics of the air masses, the direction of the movement of the
air masses, and whether the boundary between the airmasses is in contact
with the ground (a surface front), or can be found aloft (an upper level
front).
- There are four general types of fronts associated with mid-latitude
cyclones with the name reflective of the advancing air mass.
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- Cold fronts form when cold air displaces warm air.
- Indicative of heavy precipitation events, rainfall or snow, combined
with rapid temperature drops.
- Steep front slope, typically 1:100.
- Moving faster, up to 50 km/hr (30 mph).
- Northwesterly winds behind a cold front, and southwesterly in ahead of
the front.
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- The type of precipitation that will occur along a cold front depends on
the characteristics of the warm air ahead of the front.
- If the warm air is moist and conditionally unstable, thunderstorms can
be triggered ahead of the cold front; the thunderstorms will form in a
line called squall line; if supercell thunderstorms form, tornadoes,
hail, damaging wind can occur.
- If the warm air is stable, the clouds that form by lifting only produce
light rain.
- If the warm air is dry and stable, no clouds will form at all.
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- Created when warm air displaces colder air.
- Shallow horizontal stratus clouds and light precipitation.
- Frontal fogs may occur as falling raindrops evaporate in the colder air
near the surface. Sleet and freezing rain may also formed.
- Half the slope of cold fronts, typically (1:200).
- Moving slower, about 20 km/hr (12 mph).
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- Clouds are deepest just north of the warm front boundary and
progressively becomes thinner and higher toward the north.
- The intensity of the precipitation depends on the stability of the warm
air.
- If the warm air is conditionally unstable, thunderstorms may develop
over the warm front.
- If the warm air is stable, the clouds will be layered.
- If the temperature in the cold air is below freezing, snow, ice pellets,
or freezing rain may occur.
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- When two unlike air masses remain side by side, with neither encroaching
upon the other, a stationary front exists.
- The cold air on the north side of the front is moving parallel to the
front, while the warm air to the south moving toward the front and get
lifted along the frontal boundary.
- If the warm air is conditionally unstable, a line of showers and
thunderstorms may develop in the warm side of the front.
- If the warm air is stable, widespread layered clouds may form over the
front, with rain falling on the cold side of the front.
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- Occlusion: the warm air is cut off from the surface by the meeting of
two fronts.
- Usually, a fast-moving cold front catches a slow-moving warm front.
- A cold-type occlusion: eastern half of the continent where a cold front
associated with cP air meets a warm front with mP air ahead.
- A warm-type occlusion: western edges of continents where the cold front,
associated with mP air, invades an area in which colder cP air is
entrenched.
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25
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26
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Notes