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Intro. to Atmospheric Sciences Plymouth State University
AIRMASSES & FRONTS Dr. Sam Miller Intro. to Atmospheric Sciences Plymouth State University 1
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Airmasses
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Airmasses Definition Dimensions:
Large body of air with similar temperature and moisture characteristics Dimensions: Area: 100,000’s km2 or more Depth: Up to three km (surface – 700 mb) Form when a large mass of air spends a lot of time over a source region Source regions are regions of the Earth’s surface with homogeneous characteristics
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Airmass Classification
Based on temperature and moisture content Dry air comes from the middle of continents (c), and moist air comes from the oceans (m) Warm air comes from the tropics (T), and cold air comes from the polar (P) or the Arctic/Antarctic regions (A) Moisture Temperature Dry Continental (c) Moist Maritime (m) Warm Tropical (T) Cold Polar (P) Very Cold Arctic (A)
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Airmass Classification
Based on temperature and moisture content Temperature and moisture are two most important characteristics defining the density of the airmass Cold = Dense Warm = Light Dry = Dense Moist = Light
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Airmass Classification
mT – maritime Tropical moist and warm mP – maritime Polar moist and cold cT – continental Tropical dry and warm cP – continental Polar dry and cold cA – continental Arctic dry and very cold
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Source regions affecting the U.S.
Rocky Mountain states Desert Southwest and Mexican Plateau Northern and interior Canadian provinces Gulf of Mexico Atlantic Ocean Pacific Ocean
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Fronts
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Fronts Front - boundary between two airmasses of differing temperature and/or moisture (i.e. density) characteristics When two airmasses of differing densities collide at a front, the lighter airmass is forced upward
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cP airmass FRONT mT airmass
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Identifying Fronts From Weather Observations
Sharp changes in temperature Changes in moisture content (dew point) Shift in wind direction Organized regions of cloud cover and precipitation Trough (area) of low pressure - Sometimes can find a distinct cyclonic (CCW) bend in the isobars or height contours
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Identifying Fronts From Weather Observations
Sharp changes in temperature Changes in moisture content (dew point) Shift in wind direction Organized regions of cloud cover and precipitation Trough (area) of low pressure - Sometimes can find a distinct cyclonic (CCW) bend in the isobars or height contours PRESSURE FALLS AS FRONT APPROACHES YOUR STATION, AND RISES AFTER FRONT HAS PASSED
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Types of Fronts Cold Fronts Warm Fronts Stationary Fronts
Occluded Fronts Dry Lines
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Types of Fronts Cold Fronts Warm Fronts Stationary Fronts
Occluded Fronts Dry Lines
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Cold Front A transition zone where a cold (polar) airmass advances and replaces a warm (tropical) airmass Colder airmass is denser, so warm airmass is lifted aloft
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Cold Front cP mT
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Cold Front
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Shown as blue line with triangular pips
Point to direction of cold air flow Generally moves toward the E or SE Usually fast moving, esp. in summer May have showers and thunderstorms (cumuliform clouds) On eastern side of front May be severe Band of showers is usually narrow Roughly linear area up to several thousand miles long
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Colder, drier conditions after front passes
Wind shifts from southerly to northwesterly
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Types of Fronts Cold Fronts Warm Fronts Stationary Fronts
Occluded Fronts Dry Lines
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Warm Front A transition zone where advancing warm air (tropical) replaces retreating cold air (polar) Colder airmass is denser, so invading warmer airmass is forced aloft Overrunning
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Warm Front mT cP
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Warm Front mT cP
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Warm Front mT cP
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Warm Front OVERRUNNING mT cP
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Warm Front mT cP RAIN OR DRIZZLE
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Warm Front mT cP FREEZING PRECIP
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Warm Front mT cP ICE PELLETS
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Warm Front mT cP SNOW
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Warm Front
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Red line with semi-circular pips Generally moves toward the N or NE
Point to direction of warm air flow Generally moves toward the N or NE May have precipitation On northern side of front Due to overrunning (warm air sliding over cold air) Steady NS-type (stratiform) precipitation more common than showery CB type (cumuliform) Roughly circular area the size of a state Winter precipitation may include all phases snow, ice pellets (“sleet”), freezing rain
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Warmer, more humid conditions after front passes
Wind shift from easterly to southerly
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There is really only one front – The Polar Front
POLAR AIRMASS TROPICAL AIRMASS
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L The Polar Front is usually associated with areas of low pressure
POLAR AIRMASS L TROPICAL AIRMASS
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Lows rotate in a CCW direction in the Northern Hemisphere
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L Lows rotate in a CCW direction in the Northern Hemisphere
WEST OF LOW CENTER FRONT IS PUSHED TO THE SOUTH
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L Lows rotate in a CCW direction in the Northern Hemisphere
EAST OF LOW CENTER FRONT IS PUSHED TO THE NORTH
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L Lows rotate in a CCW direction in the Northern Hemisphere
WHEN THE POLAR FRONT MOVES SOUTH, WE CALL IT A COLD FRONT
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L Lows rotate in a CCW direction in the Northern Hemisphere
WHEN THE POLAR FRONT MOVES NORTH, WE CALL IT A WARM FRONT
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Eventually… L
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Real world example:
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Types of Fronts Cold Fronts Warm Fronts Stationary Fronts
Occluded Fronts Dry Lines
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Stationary Front Boundary between two airmasses that are not moving with respect to each other. One airmass is usually colder than the other
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Characteristics of a stationary front
mP cP mT
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Alternating red and blue pips on opposite sides of line
Blue triangles away from cold air Red semi-circles away from warm air Little or no movement Stationary boundary between cold and warm air Surface air flow parallel or weak warm air overrunning May have clouds and precipitation If rain happens can cause floods
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Types of Fronts Cold Fronts Warm Fronts Stationary Fronts
Occluded Fronts Dry Lines
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Occluded Front Occurs when a cold front overtakes a warm front
There are three airmasses involved
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Evolution of an Occluded Front
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Before occlusion
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Before occlusion cP mP mT
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Before occlusion cP mP mT
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Before occlusion CROSS-SECTION
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Before occlusion
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Before occlusion cP mT mP
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Occlusion occurs cP mT mP
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Occlusion continues cP mT mP
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Occlusion continues TRIPLE POINT
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WARM SECTOR IS ALOFT NORTH OF TRIPLE POINT
Occlusion continues WARM SECTOR IS ALOFT NORTH OF TRIPLE POINT
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Occlusion continues OCCLUDED FRONT IS THE EXTENSION OF THE COLD FRONT ON THE SURFACE NORTH OF THE TRIPLE POINT
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Evolution of an Occluded Front
Over time, occlusion closes up like a zipper
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Alternating purple triangles and semi-circles
On the same side of the line Pointing in the direction of motion Often has overcast skies Precipitation and clouds have characteristics of both a warm and cold front Stratiform clouds (continuous precip) with embedded cumuliform clouds (showery precip) Colder temperatures after front passes Wind shift from southeasterly to northwesterly
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Types of Fronts Cold Fronts Warm Fronts Stationary Fronts
Occluded Fronts Dry Lines
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Dry Line Boundary between warm, dry air and warm, moist air
Both airmasses are tropical The dry airmass is denser than the moist airmass
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Dry Line cP mT cT
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Solid line with open semi-circles Found in Southern Plains
All brown Found in Southern Plains TX, OK, KS Winds shift from SE to SW Important in tornadic thunderstorm development Found south of low pressure systems Often located between cold and warm fronts
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Review
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Definition of an airmass Horizontal & vertical scales
- Normal range at sea level Horizontal & vertical scales Source regions & naming conventions - cA - cP - mP - cT mT
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Definition and characteristics of a front
How to identify front from plotted observations How to identify front from satellite and radar data When airmasses meet, the lighter one is forced aloft Types of fronts - Cold - Warm - Stationary - Occluded – THREE AIRMASSES; TIME EVOLUTION Dry Line – TWO TROPICAL AIRMASSES
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