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METEOROLOGY
GEL-1370
Chapter Eight
Air Masses, Fronts, and MiddleLatitude Cyclones
Goal for this Chapter
We are going to learn answers to the following questions:
• What are the different types of air masses?
• How fronts are formed?
• How can warm fronts cause freezing rain & sleet during
winter
• What are Lake-effect snow? How does it affect certain
regions in Michigan?
• What are characteristics of warm front, cold front and
occluded front?
• What is a polar front theory?
Air Masses
• Air mass: A large body of air that has similar
horizontal temperature & moisture characteristics
• Weather forecasting: Determination of air mass
characteristics, predicting how and why they change, and
in what direction the systems will move
• Source Region: Regions where air masses originate are
known as source regions
• For a huge mass of air to develop uniform
characteristics, its source region should be generally flat
and of uniform composition, with light winds – ideal
regions are ice- and snow-covered arctic plains in winter
and subtropical oceans and desert regions in summer
Cold winter air mass over US – upper: air temp
& lower: dew point (both in °F)
Air mass Classification & Characteristics
• Table 1
Source Region
Polar (P)
Land
Continental
cP
cT
Cold, dry, stable Hot, dry, stable,
air aloft;
Unstable suface
air
mP
mT
Cool, moist
Warm, moist
unstable
Usually unstable
Water
Martime
(m)
Tropical (T)
Air Mass Classification
• Air masses are grouped into four categories based on
their source region
–
–
–
–
Polar (P): Originate in polar latitudes
Tropical (T): Originate in warm tropical regions
Continental (c): Source region is land, air mass is dry
Water (m): source region is water
Air Masses of North Americal
Continental Polar (cP) & continental Arctic (cA):
Bitterly cold weather enters US in the winter;
originate over ice- and snow-covered regions of
northern Canada and Alaska; the air in contact with
the surface becomes very cold and stable with little
moisture; portion of this air breaks away and moves
southward as a shallow high pressure area
Air mass source regions and their paths
Average upper-level wind flow (heavy arrows) and surface
position of anticyclones during two winter season
(minimum temperature)
Air masses in North America – contd.
• Cities located in the east of the Applachian mountains
usually do not experience temperatures as low as those
on the west side (WHY??)- compressional heating
increase air temp downwind side
• cP air in summer has properties much different than
from its winter counterpart; air is moderately cool and
brings relief from the oppressive heat at the central and
eastern US
• Cold, dry cP air moving over the Gulf of Mexico warms
rapidly and gains moisture; its original characteristics
are no longer discernable
Visible satellite image showing the modification of cP air as
it moves over the warmer Gulf of Mexico and the Atlantic
Ocean
mP (Maritime polar) Air masses
• During winter, cP air originating over Asia and frozen polar
regions is carried eastward and southward over the Pacific Ocean
by the circulation around the Aleutian low; the ocean water
modifies the cP air by adding warmth and moisture to it,
gradually changing into a maritime polar air mass
• When the mP air moves inland, it looses much of its moisture
and eventually become a drier, stable air mass called Pacific air
• Along the East Coast, mP air originates in the North Atlantic as
cP air moves southward some distance off the Atlantic coast;
North Atlantic is very cold and air mass travels only a short
distance over water, wintertime Atlantic mP air masses are much
colder their Pacific counterparts
• Atlantic mP air masses are much less common
A winter upper-air pattern that brings mP air into the west
coast of North America; large arrow: the upper-level flow;
small arrows: trajectory of the mP air at the surface
Cool moist mP air from off the Pacific Ocean after
crossing several mountain ranges, at the eastern
side of the rockies
mT (Maritime tropical) Air masses
• The wintertime source region for the Pacific maritime
tropical air masses is the subtropical east Pacific Ocean;
• Air masses are warm and moist by the time they arrive
along the West Coast because air travels over many kms
of water before it reaches
• The mT air that influences much of the weather east of
the Rockies originates over the Gulf of Mexico and
Caribbean Sea; in winter, cold polar air tends to
dominate the continental weather scene, so mT air is
usually confined to the Gulf and extreme southern
states.
Maritime tropical air (heavy red arrows) moving into
northern California on January 1, 1997 – IR satellite image
Weather condition during a hot spell on 17 April, 1976; Red:
maximum temp.; blue: minimum reached; heavy arrow:
average upper-level flow; L & H: average position of the
upper-level trough & ridge
mT –contd.
• In the previous figure, upper-level flow is directing cP
southward and mT air northward
• Continental Tropical (cT) Air Masses: Source region for
hot, dry cT in North America is found during the summer
in northern Mexico & the adjacent and southwestern US
• Summary: Characteristics of each air mass depend upon
the air mass source region & the type of surface over
which the air mass moves. The winds aloft determine the
trajectories of these air masses
cT air covered a large area of the central and western US
during 29,30th June, 1990; #: maximum temp (°F); H shows
with the isobar shows the upper level position of the
subtropical high; winds aloft are weak
Lake-Effect Snow
• Lake-effect Snow: Snowstorms that form on the
downwind side of a lake
• Snow storms are highly localized, extending from a few
km to >50 km inland; one side of a city may get more
than the other side due to this effect
• Most numerous from November – January; greater the
contrast in temperature, greater the potential for snow
showers
• Longer the stretch of water over which the air mass
travels, greater the amount of warmth & moisture
derived from the lake & greater the potential for heavy
snow showers
Lake-Effect Snow – contd.
• In Late winter, the frequency and intensity of lakeeffect snows taper off as the temp contrast between air
and water diminishes and larger portions of the lakes
freeze (shaded purple: experience heavy L-E effect)
Formation of a Lake-Effect Snow; cold, dry air crossing the
lake gains moisture & warmth from the water; buoyant air
rises and cloud forms leading to snowfall
Fronts
• Front: Transition zone between two air masses of
different densities; density differences are caused by
temperature differences, fronts separate air masses with
contrasting temperatures
• Frontal surface or frontal zone: Upward extension of
a front
• Stationary Fronts: Any front that stops moving is
called stationary front; drawn as an alternating red and
blue line; semicircle face toward colder air & triangles
point toward warmer air
• Surface winds tend to blow parallel to the front, but in
opposite directions on either side of it; upper level
winds blow parallel to a stationary front
A simplified weather map showing surface pressure systems,
air masses, and front (Green-shaded: precipitation)
FRONT – CONTD.
• Cold Front: Between points B & C on the surface
weather map where cold, dry, stable polar air is
replacing warm, moist unstable subtropical air
• Triangles along the front showing its direction of
movement
• Criteria to locate a front on a surface weather map:
–
–
–
–
–
Sharp temperature change over a relatively short distance
Changes in the moisture content (dew point temp)
Shifts in wind direction
Clouds and Precipitation patterns
Pressure and pressure changes
Surface weather associated with a cold front in the southeastern
US; temperature (top), dew point (bottom), present weather,
cloud cover, sea level pressure (/:rising;\), wind speed & direction
Cold Front – contd.
• Large contrast in temp & dew point on either side
• Winds shift from southwesterly ahead of the front to
northwesterly behind
• Each isobar kinks as crosses the front, forming an
elongated area of low pressure – a trough – which
accounts for the wind shift
• At the front, a relatively narrow band of thunderstorms
produces heavy showers with gusty winds; behind the
front, the air cools quickly & precipitation ends
• Slope of the front – Horizontal distance/vertical distance
= 50 (typical for fast-moving front, with >25 knots)
• There are exceptions to ‘typical’ cold fronts
A vertical view of the weather across the cold front
along the line X-X’
Tropical weather conditions associated with a cold front
Weather Elements Before Passing
While Passing
After Passing
Winds
South or southwest
Gusty, shifting
West or northwest
Temperature
Warm
Sudden drop
Steadily dropping
Pressure
Falling steadily
Minimum, then
sharp rise
Rising steadily
Clouds
Increasing Cirrus,
Cirrostratus then
either Tcu or Cb
Towering cumulus
Stratocumulus (Sc)
(Tcu) or
when ground is
cumulonimbus (Cb) warm
Precipitation
Short period of
showers
Heavy showers of
rain or snow
Decreasing rain &
clearing
Visibility
Fair to poor in haze
Poor, followed by
improving
Good except in
showers
Dew Point
High, remains
steady
Sharp drop
Lowering
• .
Warm Fronts
• Warm front is drawn along the solid red line running
from points C to D
• Average speed of warm front ~ 10 knots (~1/2 of cold
front)
• Horizontal/ vertical ~ 150-200
• Start from P’ --- ~1,200 km away ahead of the surface front -- surface winds are light & variable --- front is moving
towards P’ --- cirrus clouds gradually thicken; --- clouds
become altocumulus (Ac) and altostratus (As); closer to the
fronts, snowflakes start falling; sheet like covering of
nimbostratus (Ns) clouds --- as we approach, snow changes
to sleet, then become freezing rain and finally rain & drizzle
as the temp climbs above freezing
Surface weather associated with a typical warm
front (Green shaded area: precipitation)
Vertical view of clouds, precipitation, & Winds
across the warm front along a line P-P’ – horizontal /
vertical ~150-200
Tropical weather conditions associated with a warm front
Weather Elements Before Passing
While Passing
After Passing
Winds
South or southeast
Variable
South or southwest
Temperature
Cool to cold, slow
warming
Steady rise
Warmer, then steady
Pressure
Usually falling
Leveling off
Slight rise, followed
by fall
Clouds
Ci, Cs, As, Ns, St,
and fog;
occasionally Cb in
summer
Stratus-type
Clearing with
scattered Sc in
summer
Precipitation
Light-to-moderate
rain, snow, sleet or
drizzle;showers in
summer
Drizzle or none
Usually none
Visibility
Poor
Poor, but improving Fair in haze
Dew Point
Steady rise
Steady
• .
• .
.
Rise, then steady
Occluded Front
• Occluded Front (occlusion): The frontal boundary formed when
a cold front overtakes a warm front; in surface weather map,
represented as purple line with alternating cold-front triangles
&warm-front half circles; both symbols point in the direction
toward which the front is moving
• Cold occlusion: Air behind the front is colder than the air ahead
of it; upper warm front follows the surface occluded front
• Warm occlusion: Air behind the front is milder than the air ahead
of it; in a warm occlusion, the upper-level cold front precedes the
surface occluded front
• Most violent weather usually occurs where the cold front is just
overtaking the warm front, where the greatest contrast in temp
occurs
Cold-occluded front; faster moving cold front (a) catches up the
slower-moving warm front (b) and forces it to rise off the ground (c)
Warm-occluded front; faster moving cold front in (a) overtakes the
slower-moving warm front in (b); surface map of the situation (c)
Polar Front Theory
• Polar Front Theory: A model to explain the life cycle
of an extratropical storm; stages of birth, growth, &
decay of mid-latitude cyclones; how weather develops
along the polar front
• Useful to describe the structure & weather associated
with a migratory storm system
• Consider a segment of the polar front as a stationary
front --- cold air to the north and warm air to the south
flow parallel to the front, but in opposite direction
• Flow in opposite direction sets-up a shear; under a set of
conditions, wavelike kink forms on the front (b); the
wave forms is known as Frontal Wave
Idealized life cycle of a wave cyclone in the Northern
Hemisphere based on Polar Front Theory; Arrow
next L shows direction of storm movement
Polar Front Theory – development of wave cyclone
• In b) we see the newly formed wave with a cold front
pushing southward and warm front moving northward
• In c): steered by wind aloft, the system moves east or
northeast and gradually becomes a fully developed open
wave in 12-24 hours; central pressure is lower and more
closely-spaced isobars develop
• In d): as the waves moves eastward, central pressures
continue to decrease, and the winds blow more
vigorously; cold front moves faster and moves closer to
warm front; eventually cold front overtakes the warm
front & the system becomes occluded
• In e): storm becomes intense, with clouds &
precipitation covering a larger area
Polar Front Theory – contd. & others
• In f): the intense storm gradually dissipates, because
cold air lies on both side of the occluded front
• Entire life cycle of a wave cyclone can last from a few
days to over a week
• Family of cyclones: Succession of storms that are
various stages of development along the polar front in
winter
• Cyclogenesis: Development or strengthening of a midlatitude cyclone
• Regions of cyclogenesis in US: Eastern slopes of the
Rockies (leeward), Gulf of Mexico, Atlantic ocean east
of the Carolinas, and Great Basin
A family of cyclones forming along the Polar front
Mid-latitude cyclones & anticyclones
• Some waves develop into huge storms whereas others
simply dissipate in a day or so – Answer lies in the
upper-wind flow, in the region of the high-level
westerlies
• Convergence: Piling up air; causes air density to
increase directly above the surface low
• Divergence: Will remove air from the column directly
above the high; surface pressure falls and the system
weakens
Typical paths of winter mid-latitude cyclones
Typical paths of winter anticyclones
Mid-latitude cyclones & anticyclones
• If upper-level (or low-level) pressure systems were
always located directly above those at the surface would
die out soon after they form
Mid-latitude cyclones & anticyclones – contd.
• In two regions, we have H & L in the surface air; in the
upper air we have convergence & divergence; surface
winds are converging about the center of the low while
aloft, the winds are diverging; for the surface low to
develop into a major storm system, upper-level
divergence must be greater than the surface convergence
of air; more air must be removed above the storm than is
brought in at the surface --- storm system is intensifying
or deepening;
• If the reverse of above takes place, the storm system will
dissipate in a process called ‘filling’
Convergence, divergence, and vertical motions associated with surface
pressure systems; for the surface storm to intensify, upper trough of low
pressure must be located to the left (or west) of the surface low
Mid-latitude cyclones & anticyclones – contd
• When the surface winds are diverging about the center of
the high, directly above the anticyclone, they are
converging; in order for the surface high to strengthen,
upper-level convergence of air must exceed low-level
divergence of air; when this occurs, surface pressure
increases and the high pressure is ‘building’
• For a surface storm to intensify, the upper-level trough of
low pressure must be located behind (to the west) of the
surface low; when the upper-level trough is in this
position, the atmosphere is able to redistribute its mass,
as regions of low-level convergence are compensated for
by regions of upper-level divergence, and vice versa
Jet Streams & mid-latitude cyclones
• Jet Streak: A region of high wind speed that moves
through the axis of a jet stream
• Jet stream removes air above the surface cyclone and
supplying air to the surface anticyclone; sinking of cold
air and rising of warm air provide energy for the
developing cyclone as PE is converted into KE
• Summary: For a storm to intensify, there must be an
upper-level counterpart – a trough of low pressure that
lies to the west of the surface low
• Horizontal & vertical motions, cloud patterns and
weather that typically occur with a developing openwave mid-latitude cyclone are shown in the next figure
Summary of clouds, weather, vertical motions and upperair support associated with a developing wave cyclone
Summary – Chapter -8
• Source region of air mass, cP, cA, mT, mP & their
characteristics (temperature, humidity/moisture content,
seasons when they are active, etc)
• Air mass responsible for the summer rainshowers,
thunderstorms in FL, southwest, Sierra Nevada
mountains in CA, and other Gulf Coast regions
• Cold front, warm front, occluded front – how they are
formed, properties; how they are shown in weather maps
• Lake-effect snow – when, where, why?
• Stationary front – why does not move?; thunderstorms
in winter will form along which front?
• Energy for a developing wave cyclone;
• Convergence, divergence, cyclogenesis
Summary – contd.
• Building of a anticyclone
• When will a storm intensify?