1 Lecture 14 Air Mass Fronts
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Lecture 14
Air MassFronts
Air Masses
Weather map 11/02/2009
Weather strongly depends on where an
air mass comes from!!!
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What is an air mass?
An air mass is a large body of air with similar temperature and humidityAn air mass is named by its birthplace (source region)
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The birthplace
Must be dominated by light wind (or non at all)Must have an extensive, uniform surface
1. Temperature
Tropical TPolar PArctic A
air mass nameBy two letters with two properties
2. MoistureContinental cMaritime m
Names
cAcPcTmPmT
air mass nameBy two letters with two properties
cAcPcTmPmT
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The birthplace
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The birthplace
Typical weather map showing eastern 2/3 of North America dominated by a cP air mass (note: anticyclone)
Why cold cP passage causes clear weather?
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The upper air flow on the map below would bring ____ air masses into the Pacific Northwest and ____ air masses into the eastern United States.
a. mT, mPb. mP, cPc. mT, cPd. mP, cT
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Air masses of North AmericamP/west coast
mP is modified by time as it reaches the interior of US, how?
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What would happen as cP passes over great lakes?
Figure 1, p. 207
Lake-effect snow is produced in the winter when cold,
arctic winds move across long warmer lake water, and is deposited
on the lee shores.The greater the contrast in temperature between water and air, the greater the potential for snow shows.Why?
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What would happen as two air masses meet?
Fronts
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Atmospheric FrontsBoundary, transition zone between two different air masses
frontal boundary can be 1-100 km wide
Namescold frontswarm fronts
occluded fronts
stationary fronts
cPm
T
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Cold FrontsCold air replaces warm Much steeper than warm
frontsAdvances faster than warm
frontMore violent weather –
cumulonimbus cloudsShort, sharp showers
cPmT
Weather Before Passing While Passing After Passing
Winds south-southwest gusty; shifting west-northwest
Temp. warm sudden drop steadily dropping
Dew Point
high; remains steady sharp drop lowering
Pressure falling steadily minimum, then sharp rise rising steadily
Precip. short period of showers heavy rains, sometimes with hail, thunder and lightning
showers then clearing
Weather associated with cold front passing
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Warm FrontsWarm air replaces coldGentle slopeCovers a wide area with its
weatherStratus clouds get lower as
front approachesDrizzle or steady rain
mT
cP
cPmT
Before Passing While Passing After PassingWinds south-southeast variable south-southwest
Temperature cool-cold, slow
warming steady rise
warmer, then steady
Dew Point steady rise steady rise, then steady
Precipitation light-to-moderate rain, snow, sleet, or drizzle
drizzle or none usually none,
sometimes light rain or showers
Weather associated with warm front passing
Why?
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Warm FrontsWarm air replaces coldGentle slopeCovers a wide area with its
weatherStratus clouds get lower as
front approachesDrizzle or steady rain
mT
cP
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Stationary Fronts no moving
weather: clear, partly cloudy, cloudy, light precipitation
if the statio
nary front
starts moving north, it
will become….?
if th
e stationary fron
t starts m
oving sou
th, it
will b
ecome…
.?
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Occluded Fronts separate two cold air massesLifts the warm air off the groundWeather is complexCold occluded front– cold front lifted warm frontWarm occluded front- cold front rises up and over warm front
cold front moves quicker catches up to the warm front
26cold front moves quicker catches up to the warm front
Cold Occluded Front
Catches up
overtakes
Touches down
27cold front moves quicker catches up to the warm front
Warm Occluded Front
Catches up
Rises up
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Atmospheric FrontsBoundary, transition zone between two different air masses
frontal boundary can be 1-100 km wide
Namescold frontswarm fronts
occluded fronts
stationary fronts
cPm
T
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The diagram below represents a side view of a ____ occluded front with the coldest air located at position ____.
a. cold type, Bb. warm type, Bc. cold type, Ad. warm type, A
Review Questions1. What is an air mass? 2. What and where are good source regions for air
masses? 3. How are air masses classified? 4. What are the characteristics of the air masses that
commonly affect weather over the U.S.? 5. How can air masses be modified? 6. What are the fundamental, physical processes involved
with generating lake effect snows? 7. What is a front? 8. What are the types of fronts? How are they analyzed
(what symbol) on a surface weather map? 9. What are the characteristics of each kind of front? 10. What is observed during frontal passage for each kind
of front? 11. Why is the weather at LA much drier than at Atlanta? 12. What are the two jet streams and how are they
created? 13. What is the Walker circulation? 14. What is El Nino? How often does it occur? 15. How do pressure, precipitation, SSTs and ocean
currents change in the equatorial pacific during an El Nino event?
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