Download Air Mass

Air Masses and Fronts
Air Mass
• A large body of air in which there are
similar horizontal temperature and
moisture properties.
• Properties are largely acquired from the
underlying surface
Air Mass
• Air mass over cold ground
Cold and dry
• Air mass over water
More moist
How does water temp affect moisture?
Air Mass Classification
• Air masses are classified according to
their temperature and moisture
characteristics
– “continental” = dry (c)
– “maritime” = wet (m)
– “polar” = cold (p)
– “tropical” = warm (t)
– “arctic” = frigid (a)
• These are combined to create categories
Air mass classification
• mT = maritime tropical
– warm/moist; originate over tropical oceans
• cT = continental tropical
– warm/dry; originate over areas like SW U.S.
• mP = maritime polar
– cold/moist; originate over polar oceans
• cP = continental polar
– cold/dry; originate over interior continents in winter
• cA = continental arctic
– frigid/dry; form at very high latitudes
Source Regions
Figure from apollo.lsc.vsc.edu/classes/met130
Fronts
• “Boundary between different air masses”
• Types of fronts
– Cold
– Warm
– Stationary
– Occluded
Maritime Polar (mP)
• Forms over the oceans at
high latitudes
• Moist
• Cold
• Can contribute to
significant snowfall
events in mid-Atlantic
•
Figure from apollo.lsc.vsc.edu/classes/met130
Continental Polar (cP)
• Forms over the
northern continental
interior (e.g., Canada,
Alaska)
• Long, clear nights
allows for substantial
radiational cooling
(stability?)
• Assisted by snowpack
• Dry
• Cold
Arctic (A,cA)
• Similar to cP, but
forms over very high
latitudes (arctic circle)
• Dry
• Extremely cold
•
Figure from apollo.lsc.vsc.edu/classes/met130
Continental Tropical (cT)
• Forms over southwest
U.S. & Northern Mexico
• Source region includes
west Texas
• Dry
• Warm
• Limited water bodies and
vegetation limits effect of
evaporation and
transpiration
•
Figure from apollo.lsc.vsc.edu/classes/met130
Maritime Tropical (mT)
• Forms over Gulf of
Mexico as well as
subtropical Atlantic
and Pacific Oceans
• Moist
• Warm
•
Figure from apollo.lsc.vsc.edu/classes/met130
Air Mass Modification
• Air masses can be modified once they
leave their source region.
• Temperature & moisture content can
increase or decrease
• So how are air masses modified?
Air Mass Modification
Figure from ww2010.atmos.uiuc.edu
• 1. Move over warmer or colder ground
Air Mass Modification
Figure from ww2010.atmos.uiuc.edu
• 2. Move over a large body of water
Fig. 9-12, p. 264
Example: Lake Effect Snow
Box 9-2, p. 263
Air Mass Modification
Figure from www.usatoday.com/weather/wdnslope.htm
• 3. Move over a mountain range
Air Mass Modification
• Stability of the air mass can also modified
Fronts
• Air masses move from source region through
advection
• Air masses do not readily mix together
• Front – A boundary between two different air
masses
• Can be hundreds of miles long
Types of Fronts
• Cold Front
• Warm Front
• Stationary Front
• Occluded Front
Cold Front
• Cold air advances,
replaces warm air at
the surface
• Change in wind
direction/speed
• Minimum in
atmospheric pressure
Fig. 9-14, p. 266
Cold Front Cross Section
•
•
•
•
Fig. 9-15, p. 266
A front is a 3-D boundary
Front slopes back over the cold air mass
Warm, less dense air is lifted
Clouds/precipitation associated with a front depend
on stability and moisture
• Sharp vertical motion at cold front can force
thunderstorm activity
Fig. 9-16, p. 267
Slope of a Front
• Depends on temperature and wind
differences between the two air masses
• Shallow vs. steep slope
Warm Front
• Warm air advances
• Replaces the cold air
at the surface
• Change in wind
direction/speed
Fig. 9-17, p. 268
Warm Front Cross Section
Fig. 9-18, p. 269
• Front slopes back over the cold air mass
• Slope is more gentle than with a cold front (less thunderstorm
activity)
• Warm, less dense air lifted over the cold air (called
overrunning)
• Clouds/precipitation depend on moisture and stability, usually
follow a set progression with an increase in altitude
• Responsible for a lot of hazardous winter weather
Fig. 9-19, p. 270
Stationary Front
• Air masses at surface do
not move, so the front is
stationary
• Overrunning still
occurring, so we often still
see cloudiness
•
Figure from ww2010.atmos.uiuc.edu
Occluded Front
• Separates cool air from
relatively colder air at the
surface
• Sometimes thought of as
the “cold front catching up
to warm front”
• The warm air mass is
found above the ground
• Two types:
– Cold-type occluded front
– Warm-type occluded front
•
Figure from ww2010.atmos.uiuc.edu
Development of Occluded Front
Figures from ww2010.atmos.uiuc.edu
Cross Section of Occluded Front
Fig. 9-20, p. 271
Occluded Front
Dryline
• Dry air (lower dewpoint temperatures) found to west,
moist air (higher dewpoint temperatures) found to east
• Temperature change is rather limited across the
boundary
• Common in the southern plains during the spring
• It is a convergence line for wind at the surface, and is
therefore responsible for initiating many of our tornadic
thunderstorms in the south Plains
• Motion is tied strongly to insolation, and typically exhibits
a diurnal “sloshing” motion (moving eastward during the
day, westward at night)
Fig. 9-21, p. 272
Air Masses with the Dryline
www.geog.umn.edu/faculty/klink/geog1425/images/front/dryline_airmass.jpg
Surface Dew Points
Animation
• Satellite
• WTM
Fronts
• “Boundary between different air masses”
• Types of fronts
– Cold
– Warm
– Stationary
– Occluded
Identification of Fronts on a
Weather Map
Look for sharp
changes in:
a) temperature
b) dew point
c) wind direction
d) pressure and e)
cloud/precipitation
patterns.
Cold Fronts
• Divides cold/dry air (usually a cP air mass) from
a warm/moist air mass (mT)
• Cold air is advancing on the warm air
• Cold air is denser, pushes warm air up and over
• May result in heavy localized precipitation ahead
of the front
• Usually trails down and to the south of a midlatitude cyclone
Cold Front Transition
Most precipitation and deep
clouds form ahead of front
Cirriform clouds spread
ahead of front
Warm air rises in a steep
fashion over intruding cold air
cP Air mass
mT airmass
Where’s the Cold Front?
Warm Fronts
• Boundary between warm/moist air (mT)
and cool/moist or dry air (cP or mP)
• Warm air is advancing on the cold air
• Since warm air is less dense, it overruns
the cold air and rises in a more gentle
slope
• Usually results in more widespread and
light precipitation
Note the progression of cloud types
one would observe as the warm front
approaches
Stationary Fronts
• A front that is not moving
• Boundary between two air masses, usually
warm and cold
• May not be associated with a mid-latitude
cyclone
• However, mid-latitude cyclones may
develop along stationary fronts
Occluded Fronts
• Formed when a faster moving cold front
“catches up” to the slower moving warm front
• The warm mT air mass is then pinched up away
from the surface
• Thus, occluded fronts usually divide cold air
(behind the cold front) from cool air (ahead of
the warm front)
• Usually occur in the dying stages of mid-latitude
cyclones
Drylines
• Not a traditional front, but are important
• Divide moist air mass from dry air mass
– both have similar temperatures
• dry air is more dense than moist air, tends
to create steep rising motion along dryline
– results in thunderstorms, sometimes severe