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Weather
Weather:
is the short term, day-to-day condition of the atmosphere
Meteorology – the scientific study of the atmosphere
 They focus on physical characteristics and motion and how it
relates to chemical, biological, physical and geological processes
 Study complex linkages of atmospheric systems
 Use this to forecast short-term future conditions of the atmosphere
Mid-Latitude Weather Systems
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Cyclone – Low-pressure system
 Rising air, precipitation
 Relatively small, compact weather system
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Anticyclone – High-pressure system
 Descending air, clear skies
 Relatively large, expansive weather system
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“weather” caused by the interaction of cyclone and
anticyclone
Air Masses:
 each area of the earth’s surface imparts temperature and moisture
characteristics to the air above
 This creates large, regional, homogenous masses of air with
specific temperature, moisture and stability characteristics
Air mass – distinctive body of air with specific temperature and moisture
characteristics
→ air masses interact to produce weather systems
Air masses are labeled by their source region and also
based on thermal and moisture characteristics:
1. Thermal
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Tropical (T) Air Mass – warm
Polar (P) Air Mass – cold
Arctic (A) Air Mass – cold
Antarctic (AA) Air Mass - cold
2. Moisture
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Continental (c) Air Mass – dry
Maritime (m) Air Mass - moist
Continental Polar (cP) – cool dry air masses associated with
anticyclonic conditions
 Form only in the N. Hem b/c S. Hem doesn’t have polar land
masses
 Most developed in winter
 Major players in high and middle latitude weather systems and
climate
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Colder, more dense air pushes warmer air up over it
Maritime Polar (mP) – cool, wet air masses associated with cyclonic
conditions
 Occur over northern oceans
 Subpolar low pressure cells are located within this type of air mass
Maritime Tropical (mT) – warm, humid masses generally associated
with cyclonic activity, however this varies with air mass origin
 There are two that affect N. America:
Gulf/Atlantic – very unstable atmospheric conditions
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Active late spring to early fall (hurricane season)
Responsible for the humidity experienced by the SE U.S.
Tends to be warmer and wetter b/c of the pressure of the warm Gulf
Stream current
Pacific – stable to conditionally unstable
 Cooler and less humid than Gulf/Atlantic
 Reason W U.S. is less humid and receives less rainfall than E
 Cooler and drier due to cooler Pacific currents along the west coast
Continental Tropical (cT) – warm, dry masses associated with
anticyclonic conditions
 Typical of tropical/subtropical continental locations
 Hot, dry air (subtropical high pressure cells
Air Mass modification:
Air masses that move from their source region are modified
by the temperature and mositure characteristics of the
locations they pass
EX: cP air masses that travel south from their Canadian source will bring
frigid temperatures to southern latitudes
- They begin with temps as low as -50 C but are generally warmer by the time
they reach their southernmost extent → warm as they move over warmer
surfaces
- they also can change their moisture characteristics → lake effect snow
Atmospheric Lifting
→ Precipitation
In order for precipitation to occur, an air mass must:
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Cool adiabatically (by expansion)
Reach the dew point temperature = achieving saturation
After saturation, water vapor will condense and form moisture droplets
Moisture droplets form clouds and possibly precipitation
for this to happen and air mass must be lifted!
There are four principle lifting mechanisms:

Convergent lifting → air along the surface flows toward a low
pressure center and rises
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Convectional lifting → local heating differences warm air above
causing it to rise
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Orographic lifting → air is forced over a topographical barrier
(mountain)
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Frontal lifting → air rises along the boundary between contrasting
air masses
Convergent Lifting:
 Air flows from different locations (converges) toward the center of a
low pressure system
 Displacing air upward
 Common in tropical regions where convergent lifting of warm, moist
air leads to the formation of tropical storms
 Within the equatorial region the trade winds converge along the
ITCZ, forcing air up creating large thunderstorms and high
precipitaiton
Convectional Lifting:
 When cooler air masses move over warmer surfaces the warmer
surface heats the air mass from the bottom up (can be large or
local scale)
 Warmer air from the bottom of the air mass rises causing
convection within the air mass
 Convection will continue within the air mass as long as conditions
are unstable
 This lifting will create clouds and convectional precipitation (Atlanta)
Orographic Lifting:
 An air mass is physically forced upward as it pushes past a
mountain
 As the air rises over the mountain it is cooled adiabatically
 When it reaches the lifting condensation level, clouds form
 As it continues up the mountain convectional precipitation will
develop
 Precipitation will continue until the moisture content goes below
saturation or the mass moves over the barrier
Windward slope – is the side of the mountain over which precipitation
occurs
 As the air mass moves up the windward slope it will cool adiabatically and
eventually precipitate
 Much of the air mass’s moisture is lost along the windward slope
Leeward slope – is the opposite side of the mountain
 Descending air is heated by compression as it moves to lower elevations
 Any remaining moisture evaporates
 Air is typically hot and dry
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Chinook winds → warm downslope airflow characteristic of
leeward side
Rainshadow → term applied to leeward side that receive very
little precip due to orographic lifting
Frontal Lifting –
Front: the leading edge of an advancing air mass
 The leading edge of a cold air mass is a cold front
 The leading edge of a warm air mass is a warm front
Cold Front:
• Indicates the advance of a cold air mass;
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cold air is dense and a cold air mass will move as a dense mass of cold
air, hugging the ground
• As a cold front moves into a region occupied by a warmer air mass,
the warm air mass will rise sharply up and over the cold air mass
The warm air mass will cool adiabatically as it rises over a cold air
mass
• This creates instability and cloud/precipitation (cumulonimbus)
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Cold Fronts: ctd
• Cold fronts are recognised by:
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Marked wind shift (usually northerly winds in the N. Hem
Temperature drop
Lower barometric pressure due to rising air along the front
Thick cloud formation and possibly heavy precipitation
Once the front passes, have high pressure and lower temperatures
In the U.S. and Canada, the clash of cold cP and mP air masses from the
north and warmer mT air masses from the south dominate our weather
patterns
Warm Front:
• Indicates the advance of a warm air mass
• The warm air mass is unable to displace the more dense cold air
mass
• Warm air moves gently over the cold air mass
The warm air will cool adiabatically as it rises over the cooler air mass
• Water vapor will condense and clouds and possibly precipitation will
develop (nimbostratus)
• Precipitation is usually the light gentle drizzle rain
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How air masses interact in the mid-latitudes
Midlatitude cyclone - a mid latitude cyclone develops along the polar
front when warm and cold air masses collide resulting in rising warm
air and low pressure development
Development of a midlatitude cyclone includes four stages that take 3-10
days to complete:
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Cyclongensis - development of a low pressure center along the polar
front
Open stage - northward advancement of warm air in front of the front
with cold air advancing south, curculating around a low pressure
center
Occluded stage - cold front overrides the warm front
Dissolving stage - warmer air is pushed aloft and rising stops
Cyclogensis - low pressure systems develop and strengthen
 The polar front presents a discontinuity in temperature, moisture
and wind direction leading to unstable atmospheric conditions
 Air convergence occurs along the cold front but it must be
accompanied by diverging air aloft
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Diverging air aloft allows the surface low to strengthen as more surface
air is forced to rise to fill the void at upper levels
Open stage - surface convergence strengthens the developing low
 Warm air moves north east of the cold front while cold air continues
to push south
 Ccw flow around the low draws more cold air from the north and
west and more warm air from the south
Occluded stage - the cold front overtakes the warm front wedging underneath it
 The cold air mass is more homogenous in cold temperature and high
pressure than the warmer air mass
It moves as a unified mass of colder, more dense air
 It moves faster than the warm front (40 km/hr vs. 16-24 km/hr)
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 Eventually it plows into the warm front forcing the warm front to move over
the cold front
Occluded front - when a cold front overtakes a warm front
Stationary front - stalemate between warm and cold air masses
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Generally you’ll get moderate to heavy precipitation which will eventually taper
off as the cold front continues to force the warm air aloft
Dissolving stage - the low pressure center dissolves
 As the cold front continues to move under the warm front the warm
air behind the warm front continues to rise
 As the cold front completely overtakes the warm front the warm air
is pushed to upper levels above the cold air mass
 With no more warm air to move over the cold front the source of
energy and moisture that feed the cyclone is gone
 As the warm air ceases to rise over the cold front the low will
dissolve
Cold Front
Warm Front
Stationary Front
A sharp cold front boundary can be seen on
both satellite pictures and radar composites
Cyclones
Characteristics of a wave cyclone
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Cyclones normally move eastward, propelled by prevailing
westerlies aloft
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Cold front pushes south and east, supported by a flow of
cold, dry polar air
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The wind direction and temperature change abruptly as the
cold front passes
Example of an Occlusion
Cold Front Approaches a Warm Front
Example of an Occlusion
• Cold front meets the warm front ahead of it
• Warm air is lifted upward, away from the surface
• Occluded front becomes longer  more of the cold front
converges with the warm front
Example of an Occlusion
• Eventually, the cold front completely overtakes the
warm front
• Entire system is occluded
Cold front  cold air moves
toward the warm air mass ahead
Warm front  warm air mass
moving toward a cold air mass
Stationary front  two unlike air
masses remain side by side, but
neither air mass has recently
undergone substantial movement
Occluded front  cold front meets
warm front, warm air gets lifted
upward, away from the surface
When Air Masses Collide…
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Dry Line – a front between warm, moist air and
warm, dry air
 Lifting is extremely strong
 Heavy thundershowers, hail, and possible tornadoes are
characteristic of a dry line
 Seen almost exclusively in the southern states west of the
Mississippi
 cT air versus mT air masses
Dryline Over Texas
Notice the differences in the dew points on both sides of the dryline.
Daily weather forecasting:
Synoptic Analysis: evaluation of weather data
 Numerical weather prediction - data base of weather conditions
used to forecast future weather events
 Weather forecasting is difficult b/c the atmosphere is a nonlinear
(irrational) system and tends toward chaos
Weather forecasts require information on:
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Barometric pressure and pressure changes
Surface air and dew point temperature
Wind speed, direction, and character
Cloud/sky conditions and visibility
Current weather and precipitation record
Today’s weather