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Chapter 12Air masses and fronts

Why are cold fronts usually associated with showery weathers?

How ca warm fronts cause freezing rain and sleet to form over a vast area during the winter?

And how can one read the story of an approaching warm front by observing its clouds?

Air Masses• Definition: Large body of air with similar

temperature and moisture characteristics in any horizontal direction at any given altitude.

• Cover many 1000's of square kilometers

• Part of weather forecasting is determining air mass characteristics, how they may be modified, and their movement.

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Air Mass Source Regions• Regions where air mass originate are know as Source

Regions. • Source regions:

– flat, uniform composition – light winds

• So, where are the good source regions?– Arctic– Tropics

• Air masses tend to clash and interact in the middle latitudes.

Air mass source regions and their paths.

Air Mass Classification• Four general categories according to source region (see table)• Extremely cold cP air is sometimes denoted at cA• Extremely hot, humid mT air is sometime denoted by mE• Air Masses on the move:

– if an air mass is colder than the surface over which it is moving, "k" is added

– if an air mass is warmer than the surface over which it is moving, "w" is added

– Example - a cP air mass moving over the great lakes in December becomes cPk

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Air Masses of North America1. Continental Polar/Arctic (cP/cA) air mass

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2. Maritime Polar air mass

4. Continental Tropical (cT) air mass

3. Maritime Tropical (mT) air mass

Continental Polar/Arctic (cP/cA) air mass

• Stable, cold, dry air masses originating over N. Canada and Alaska

• Eventually plunge southward to interior of U.S. as a Shallow dome of high pressure - why?

• Can reach Gulf of Mexico and Florida - freeze crops

• Usually do not move west of Rocky mountains -mountains confine cold air to the east. Upslope precipitation is common east of the rockies as the cP air mass slides to the south.

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Average upper-level wind flow (heavy arrows) and surface position of anticyclones (H) associated with two extremely cold outbreaks of arctic air during December. Numbers on the map represent minimum temperatures (°F) measured

during each cold snap.

•During the summer, cP air mass can bring relief to hot, humid regions.

Typical vertical temperature profile over land for a summer and a winter cP air mass.

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•Produce lake effect snows as they move over the great lakes.

Areas shaded purple show regions that experience heavy lake-effect snows.

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When an air mass moves over a large body of water , its original properties may change considerably.

Cloud Street

mP Air Mass:• West Coast:

– Originate over Asia as cP

– Tends to be unstable – Heavy rains as cool

moist air flows over mountains along west coast

– mP is modified (how?) by time it reaches interior of US, though is milder than cP

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After crossing several mountain ranges, cool moist mP air from off the Pacific Ocean descends the eastern side of the

Rockies as modified, relatively dry Pacific air.

Cold surface

• East Coast:– not as common

as west coast mP

– colder than west coast mP

– usually brought onshore by high pressure to the north of US and/or low pressure to the south moving up the coast.

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• mT Air Mass:– largely originates in Gulf of Mexico, western Atlantic -

affecting eastern 2/3 of country – also originates in tropical eastern pacific (SW monsoons in

summer) – warm, moist, unstable – confined to southern US in winter – important source of moisture feeding storms all year round

Weather conditions during an unseasonably hot spell in the eastern portion of the United States that occurred between the 15th and 20th of April, 1976.

A constant supply of mT air from the Golf of Mexico can bring record-breaking maximum temperatures to the

eastern half of the country!

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• cT Air Mass:– originates over Mexican Plateau region and desert SW – hot, dry, unstable at low levels, stable at upper levels – boundary between cT and mT is often called the dryline

• The dryline is often seen in surface and satellite data and is a favored location for storm initiation:

Atmospheric Fronts - Introduction• Front - boundary, transition zone between

two different air masses • The two air masses have different

densities. Frequently, they are characterized by different temperatures and moisture contents

• Front has horizontal and vertical extent • Frontal boundary/zone can be 1-100 km

wide! • The upward extension of a front is referred

to as a frontal surface, or a frontal zone.

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• Types of synoptic-scale fronts: – stationary fronts – cold fronts – warm fronts – occluded fronts

Stationary Fronts• Has little/no movement • Denoted by alternating cold/warm

frontal symbols – Semicircles face toward cold air– Triangles point toward warmer air

• Associated weather: – clear, partly cloudy, cloudy, light precip– usually nothing severe

• If the stationary front starts moving north in the example to the right it will become a warm fronts

• If the stationary front starts moving south in the example to the right it will become a cold fronts

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Cold Fronts• Zone/boundary between warmer, more moist, unstable air

(usually mT) being replaced by colder, drier, more stable air (usually cP).

• Location of cold front:– leading edge of sharp

temperature change – moisture content (dew point)

changes dramatically – wind shift (direction and speed) – Pressure and pressure change

(pressure tendency is useful!)

Since the cold front is a trough of low pressure, sharp changes in pressure can be significant in locating the front’s position. Figure 11.13

• Location of cold front:– often cloudy/showers/thunderstorms/sometimes severe

A Doppler radar image showing precipitation patterns along a cold front similar to the cold front in Fig. 11.13. Green represents light-to-moderate precipitation; yellow represents heavier precipitation; and red the most likely areas for thunderstorms.

A squall line

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A vertical view of the weather across the cold frontin Fig. 11.13 along the line X–X’.

1. Warm air ahead of front is lifted up and over 2. Can get intense showers/thunderstorms at frontal boundary 3. Cs and Ci clouds are blown ahead of the front by upper level winds 4. Cloud base is generally lower behind the front5. Further behind the front, the air is quite dry, few clouds 6. Steep frontal boundary (1:50), slopes backward into the cold air 7. Frontal speed averages 15-25 knots 8. Temperature and wind profiles on either side of cold front

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The infrared satellite image (a) shows a weakening cold front over land on Tuesday morning, November 21, intensifying into (b) a vigorous front over warm Gulf Stream water on Wednesday morning, November 22.

Frontolysis: a condition cause a front weaken and dissipate –the temperature contrast across a front lessens.Frontogenesis: a condition cause a front to strengthen and regenerate into a more vigorous frontal system – the temperature contrast across a front increases.

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A “back door” cold front moving into New England during the spring. Notice that, behind the front, the weather is cold and damp with drizzle, while to the south, ahead of the front, the

weather is partly cloudy and warm.

In fact, no two fronts are exactly alike.

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Warm Fronts

• Zone/boundary between advancing warmer, more moist air (usually mT) and cooler, drier air (usually mP)

• Average speed is about 10 knots

• Often associated with "overrunning" • Frontal passage: • Clouds associated with warm fronts • frontal surface has a much smaller slope (1:300) than for cold

fronts • Often produces wide-spread nimbostratus precip near front

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• Temperature and wind profiles on either side of warm front?– Frontal inversion

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A dryline moves across Texas and Oklahoma during the late afternoon in May.

Drylines represents a narrow boundary where is a steep horizontal change in moisture.

Radar fine lines

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Dryline and convective initialization !

Occluded Fronts (Occlusion)• Why do they form

– Cold front moves fast -- catches up the warm front

• There are two types of occluded fronts: – cold occlusion – warm occlusion

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Cold Occlusion• Cold front "lifts" the warm

front up and over the very cold air

• Associated weather is similar to a warm front as the occluded front approaches

• Once the front has passed, the associated weather is similar to a cold front

• Most common type of occluded front

• Vertical structure is often difficult to observe

Warm Occlusion• cold air behind cold

front is not dense enough to lift cold air ahead of warm front

• cold front rides up and over the warm front

• upper-level cold front reached station before surface warm occlusion

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Visible satellite image showing a mid-latitude cyclonic storm with its weather fronts over the Atlantic Ocean during March, 2005.

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An idealized vertical view of an upper-air front showing tropopause (heavy red line), isotherms in °C (dashed gray lines),

and vertical air motions. The polar jet stream core (maximum winds) is flowing into the page (from west to east).