Understanding Weather and Climate 3rd Edition Edward Aguado and James E. Burt

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Understanding Weather and Climate 3rd
EditionEdward Aguado and James E. Burt

• Anthony J. Vega

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Part 3. Distribution and Movement of Air

• Chapter 9
• Air Masses and Fronts

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Introduction

• Air masses contain uniform temperature and
  humidity characteristics
• They affect vast areas
• Fronts are boundaries between unlike air masses
• Fronts are spatially limited
• They are inherently linked to mid-latitude
  cyclones

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• Formation of Air Masses
• Surface energy and moisture exchanges cause
  initial temperature, pressure, and moisture
  characteristics in overlying air
• Such exchanges are limited in spatial scope,
  leading to variation in these parameters from
  place to place
• Source regions
• Sites of air mass genesis
• Form when air stagnates over particularly large
  surface regions, typically those which are
  topographically uniform
• Overlying air gains temperature and humidity
  characteristics of the surface
• Air masses are characterized based on these
  properties
• Moisture characteristics are expressed first and
  in lower case in the classification system
• Temperature is expressed next in upper case

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• The wettest air is called maritime air, while the
  driest is called continental
• Air deemed, from warmest to coolest, tropical,
  polar, or arctic
• 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

North American air masses and air mass source
regions
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• Continental Polar (cP) and Continental Arctic
  (cA) Air Masses
• Wintertime source regions for continental polar
  (cP) air include northern Canada and Asia
• cP air takes on cold, dry characteristics and is
  inherently stable
• During summer, cP air is warmer and more humid by
  comparison but still cool and dry as compared to
  other air masses
• 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 arctic
  front

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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. This
sequence depicts the movement of cP air
equatorward.
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• Maritime Polar (mP) Air Masses
• Maritime polar air masses form over upper
  latitude oceanic regions and are cool and moist
• 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
• Referred to as a Noreaster for the dominant
  northeasterly winds
• Continental Tropical (cT) Air Masses
• 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 Tropical (mT) Air Masses
• Form over low latitude oceans and as such are
  very warm and humid
• mT air is inherently unstable due to high
  temperature and humidity
• The 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
• Advection of mT air also promotes the so-called
  Arizona monsoon
• Fronts
• Fronts separate air masses and bring about
  changes in temperature and humidity as one air
  mass is replaced by another
• 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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The four fronts
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• Cold Fronts
• Cold fronts form when cold air displaces warm air
• Indicative of heavy precipitation events,
  rainfall or snow, combined with rapid temperature
  drops
• Extreme precipitation stems from rapid vertical
  lifting associated with the steep cold front
  boundary profile
• Because cold air is dense, it spills across the
  surface producing a steeply inclined leading edge
  
• Warm moist air ahead of the front is forced aloft
  with great vertical displacement
• This accounts for large vertical cumulonimbus
  clouds and heavy precipitation
• Such sharp transitions between the colder, drier
  air behind the front and the warmer, moisture air
  ahead of the front, can be easily detected on
  satellite images and radar composites

Frontal development about a low pressure system
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Above The vertical displacement of air along a
cold front boundary Right The sharp cold front
boundary is evident on both satellite pictures
and radar composites
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• Warm Fronts
• Created when warm air displaces colder air
• Even though the warmer air advances, it is
  displaced aloft
• This overrunning process places large amounts of
  warm, moist air over cooler, drier air along
  extensive spatial areas
• Shallow horizontal stratus clouds dominate and
  bring light precipitation to affected regions
• Stable regions above the warmer air aloft help
  propagate vertically limited clouds and light
  precipitation
• Frontal fogs may occur as falling raindrops
  evaporate in the colder air near the surface
• In a similar, but more extreme situation, sleet
  and freezing rain may result
• Stationary Fronts
• When two unlike air masses remain side by side,
  with neither encroaching upon the other, a
  stationary front exists
• Fronts may slowly migrate and warmer air is
  displaced above colder
• Fronts are zones of transition rather than sharp
  boundaries

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Profile of a warm front
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• Occluded Fronts
• When two fronts meet, the warm air mass between
  them is displaced aloft resulting in an occluded
  front
• This typically occurs when a cold front closes
  on, and meets a warm front as it circulates about
  the low pressure center of a mid-latitude cyclone
• Cold air now occupies the surface completely
  around the low while warmer air is displaced
  aloft
• A cold-type occlusion usually occurs in the
  eastern half of the continent where a cold front
  associated with cP air meets a warm front with mP
  air ahead
• This situation resembles a cold front in profile
• A warm-type occlusion is typical of the western
  edges of continents where the cold front,
  associated with mP air, invades an area in which
  colder cP air is entrenched
• This results in a vertical profile which
  resembles a warm front

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Occlusion sequence
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Some occlusions form when the surface low
elongates and moves away from the junction of
the cold and warm fronts
Some occlusions occur when the intersection of
the cold and warm fronts slides along the warm
front
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• Drylines
• Because humidity is an important determinant of
  air density, air masses with similar temperatures
  but strong humidity gradients will act as fronts
• Boundaries between dry and moister air are called
  drylines
• They frequently occur throughout the Great Plains
  and are an important contributor to storm
  development

A dryline over Texas
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End of Chapter 9 Understanding Weather and
Climate 3rd EditionEdward Aguado and James E.
Burt