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Air
masses |
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Contain uniform temperature and |
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humidity characteristics. |
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Fronts |
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Boundaries between unlike air |
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masses. |
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Air
masses have fairly uniform temperature and moisture content in horizontal
direction (but not uniform in vertical). |
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Air
masses are characterized by their temperature and humidity properties. |
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The
properties of air masses are determined by the the underlying surface
properties where they originate. |
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Once formed, air masses migrate within the
general circulation. |
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Upon movement, air masses displace residual air
over locations thus changing temperature and humidity characteristics. |
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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. |
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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. |
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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. |
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Bases on
moisture content: continental (dry) and maritime (moist) |
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Based on
temperature: tropical (warm), polar (cold), arctic (extremely cold). |
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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). |
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But
mA-type (maritime Arctic) does not exist. |
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cA: continental Arctic |
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cP:
continental Polar |
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cT:
continental Tropical |
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mP:
maritime Polar |
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mT:
maritime Tropical |
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Continental Polar air masses form over large, high-latitude
land masses, such as northern Canada or Siberia. |
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cP air
masses are cold and extremely dry. |
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Wintertime cooling over these land areas cause the atmosphere
to become very stable (even inversion). |
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The
combination of dry and stable conditions ensure that few if any clouds form
over a cP source region. |
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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. |
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As cP air migrates toward lower latitudes, it
warms from beneath. |
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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. |
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The
boundary between cA and cP air is the shallow (~1-2 km) arctic front. |
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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. |
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Characteristically hot and very dry. |
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Very unstable, yet clear conditions predominate
due to a lack of water vapor. |
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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. |
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mP air masses form over high-latitude ocean as
cP air masses move out from the interior of continents. (I.e., cP à mP). |
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Oceans add heat and moisture into the dry and
cold cP air masses. |
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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. |
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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. |
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mT air masses from Atlantic and Gulf of Mexico
is the primary source region for the eastern U.S. |
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As air advects over the warm continent in summer
the high humidity and high heat occasionally combine to dangerous levels. |
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mT air masses have an enormous influence on the
southwestern U.S, particularly in summer. |
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Fronts separate air masses and bring about
changes in temperature and humidity as one air mass is replaced by another. |
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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. |
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Indicative of heavy precipitation events,
rainfall or snow, combined with rapid temperature drops. |
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Steep front slope, typically 1:100. |
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Moving faster, up to 50 km/hr (30 mph). |
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Northwesterly winds behind a cold front, and
southwesterly in ahead of the front. |
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Created
when warm air displaces colder air. |
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Shallow horizontal stratus clouds and light
precipitation. |
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Frontal fogs, sleet, freezing rain may occur as
falling raindrops evaporate in the colder air near the surface. |
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Ha;f the slope of cold fronts, typically
(1:200). |
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Moving slower, about 20 km/hr (12 mph). |
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When two unlike air masses remain side by side,
with neither encroaching upon the other, a stationary front exists. |
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Fronts may slowly migrate and warmer air is
displaced above colder. |
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Fronts sloping over the cold air. |
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Occlusion: the warm air is cut off from the
surface by the meeting of two fronts. |
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Usually, a fast-moving cold front catches a
slow-moving warm front. |
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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. |
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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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Because
humidity is an important determinant of air density, air masses with
similar temperatures but strong humidity gradients will act as fronts. |
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Boundaries between dry and moister air are
called drylines. |
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They frequently occur throughout the Great
Plains and are an important contributor to storm development. |
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