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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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