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AOSC 200 Lesson 9
54

AOSC 200 Lesson 9

Jan 15, 2016

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AOSC 200 Lesson 9. Clear area in middle of the image is a hot, cloud-free air mass – resulting heat wave caused 200 deaths in the Midwest. Fig. 9.2. Major air masses of the world. Fig. 9.3. AIR MASS. AN AIR MASS IS A BODY OF AIR 1500 KM OR MORE ACROSS AND SEVERAL KM THICK - PowerPoint PPT Presentation
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Page 1: AOSC 200 Lesson 9

AOSC 200Lesson 9

Page 2: AOSC 200 Lesson 9

Fig. 9.2

Clear area in middle of the image is a hot, cloud-free air mass – resulting heat wave caused 200 deaths in the

Midwest

Page 3: AOSC 200 Lesson 9

Fig. 9.3

Major air masses of the world

Page 4: AOSC 200 Lesson 9

AIR MASS

• AN AIR MASS IS A BODY OF AIR 1500 KM OR MORE ACROSS AND SEVERAL KM THICK

• AS THE AIR MASS MOVES IT CARRIES ITS TEMPERATURE AND MOISTURE CONDITIONS WITH IT.

• CAN TAKE SEVERAL DAYS FOR AN AIR MASS TO TRAVERSE AN AREA.

• WHERE THE AIR MASS ORIGINATES IS KNOWN AS THE SOURCE REGION

Page 5: AOSC 200 Lesson 9

• http://www.youtube.com/watch?v=neWN-lEmnbk

• http://www.youtube.com/watch?v=neWN-lEmnbk

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CLASSIFICATION• FOUR BASIC CATEGORIES OF AIR MASSES:

• POLAR - P• ARCTIC - A• TROPICAL - T• EQUATORIAL - E• TWO DESIGNATIONS OF SURFACE IN SOURCE REGION

• MARITIME - m• . CONTINENTAL - c

Page 7: AOSC 200 Lesson 9

CLASSIFICATION

• THUS WE GET:

• cA - CONTINENTAL ARCTIC

• cP - CONTINENTAL POLAR

• cT - CONTINENTAL TROPICAL

• mT - MARITIME TROPICAL

• mP - MARITIME POLAR

• mE - MARITIME EQUATORIAL

Page 8: AOSC 200 Lesson 9

Fig. 9.4

Major air masses that affect North American weather

Page 9: AOSC 200 Lesson 9

CONTINENTAL POLAR (cP) AND CONTINENTAL ARCTIC (cA)

• THESE ARE COLD AND DRY AIR MASSES

• CONTINENTAL POLAR AIR COMES FROM POLEWARD OF THE 50TH PARALLEL.

• CONTINENTAL ARCTIC AIR COMES FROM OVER THE ARCTIC BASIN, AND THE GREENLAND ICE CAP.

• OFTEN CALLED THE SIBERIAN EXPRESS OR ARCTIC CLIPPER

Page 10: AOSC 200 Lesson 9

Siberian Express. An extremely cold arctic air mass covers nearly 90% of the United States.

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Fig. 9.6

Ice in a Florida orange grove as a result of a continental arctic air mass coming down from Canada.

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Fig. 9.8

Temperature at Madison, WI, as a result of a continental arctic

air mass

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MARITIME POLAR (mP) AIR MASSES

• FORM OVER OCEANS AT HIGH LATITUDES• COOL TO COLD AND HUMID. TWO IMPORTANT REGIONS FOR THE US ARE THE NORTH PACIFIC AND NORTHWESTERN ATLANTIC.

• DURING THE WINTER, mP FROM THE PACIFIC USUALLY BEGIN AS cP FROM SIBERIA.

• OROGRAPHIC FORCING PRODUCES HEAVY SNOW OVER THE WEST COAST.

• IF WE HAVE STRONG CYCLONIC FLOW OVER LOWER EASTERN US, THEN UPPER PART BRINGS IN ATLANTIC mP - NOR'EASTER

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Northeaster.

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MARITIME TROPICAL (mT) AIR MASSES

• THOSE AIR MASSES WHICH AFFECT THE US MAINLY ORIGINATE FROM THE GULF OF MEXICO, THE CARIBBEAN SEA, OR THE ADJACENT WESTERN ATLANTIC OCEAN.

• THESE AIR MASSES ARE LARGELY RESPONSIBLE FOR THE HOT AND HUMID WEATHER OF THE SUMMER OVER THE EASTERN AND CENTRAL PARTS OF THE US.

• GIVE US MUCH OF THE WINTERTIME PRECIPITATION OVER THE EASTERN AND CENTRAL STATES, WHEN THE AIR MASS IS FORCED UP OVER COLDER AIR MASSES.

Page 16: AOSC 200 Lesson 9

Fig. 9.9

Maritime Tropical air mass

Page 17: AOSC 200 Lesson 9

CONTINENTAL TROPICAL (cT) AIR MASSES

• NORTH AMERICA NARROWS AS IT EXTENDS SOUTHWARD.

• SO THERE IS NO EXTENSIVE SOURCE REGION FOR THESE AIR MASSES.

• HOT, DRY AIR.• WHEN cT AND mT INTERACT OVER THE SOUTH-CENTRAL US THE CONTRAST CAN BE SO LARGE THAT METEOROLOGISTS LABEL IT AS A DRYLINE.

• THE DRYLINE PROVIDES A FOCUS FOR THUNDERSTORMS JUST LIKE A COLD FRONT

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Box 9.2

Average annual snowfall for the Great Lakes region

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http://www.youtube.com/watch?v=o_ioV4MAUnMhttp://www.youtube.com/watch?v=o_ioV4MAUnM

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Lake Effect Snowfall• Two distinct patterns.• Firs,t the snowfall increases as one goes northward.

• To be expected as the temperature gets colder as one goes northward.

• Second, regions with localized maximum in snowfall are on the Southerly and Easterly side of the Lakes.

• Why?

Page 21: AOSC 200 Lesson 9

Box 9.2

Formation of Lake Effect snowfall

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Lake Effect Snowfall

• As the polar air mass moves over the Lakes the lower layers of the air mass are warmed and moistened by the lake.

• This makes the air mass unstable.• The moist warm air rises and forms clouds – snow.

• Effect can be enhanced if the air mass is lifted by hills.

• Lake effect snow is most prevalent in early winter.

Page 23: AOSC 200 Lesson 9

• http://www.youtube.com/watch?v=MbfCre7-wCw

Page 24: AOSC 200 Lesson 9

FRONTS

• POLAR FRONT THEORY FIRST PUT FORWARD BY BJERKNES AND HIS COLLEAGUES IN. NORWAY, 1914

• FRONTS ARE BOUNDARY SURFACES THAT SEPARATE AIR MASSES.

• IN GENERAL ONE AIR MASS IS WARMER AND OFTEN CONTAINS MORE MOISTURE THAN THE OTHER.

• FRONTS FORM BETWEEN TWO CONTRASTING AIR MASSES.

• IN GENERAL ONE AIR MASS MOVES FASTER THAN THE OTHER.

• THIS PROCESS IS KNOWN AS OVERRUNNING

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Fig. 9.14

Weather associated with a cold front

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Fig. 9.15

Warm, moist air is forced upward by cold front

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COLD FRONT

• COLD AIR MOVING INTO WARM AIR• SHOWN ON WEATHER MAP BY TRIANGLES POINTING INTO THE WARM AIR.

• AVERAGE COLD FRONT MOVES AT 35 KM PER HOUR. WARM FRONT AT 25 KM PER HOUR.

• WARM AIR IS FORCEFULLY MOVED UPWARD. • IF WARM AIR IS MOIST THE LARGE AMOUNTS OF LATENT HEAT IS RELEASED.

• THUNDERSTORMS, HEAVY DOWNPOURS, VIGOROUS WIND GUSTS, TORNADOS

• DARK BAND OF OMINOUS CLOUDS AS FRONT APPROACHES.

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Fig. 9.16

Surface weather associated with a warm front

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Fig. 9.17

Warm, moist air is slowly raised as it flows over the cold air

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WARM FRONT

• WARM AIR MASS MOVING INTO COLD AIR MASS.• SHOWN ON WEATHER MAP BY A LINE WITH SEMICIRCLES EXTENDING INTO THE COOLER AIR

• AS WARM AIR CLIMBS OVER THE RETREATING COLD AIR, IT EXPANDS AND COOLS - CLOUDS - PRECIPITATION

• CIRRUS CLOUDS FOLLOWED BY CIRROSTRATUS AND THEN NIMBOSTRATUS.

• LIGHT TO MODERATE PRECIPITATION OVER A LARGE AREA AND FOR EXTENDED PERIODS.

• IF RAIN EVAPORATES IN COLD AIR MASS THEN HIGH HUMIDITY CAN RESULT - HEAVY FOGS,.

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Fig. 9.18

Occluded Fronts

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OCCLUDED FRONTS

• OCCLUDED FRONTS• WHEN A COLD FRONT OVERTAKES THE WARM FRONT, FORCING THE WARM AIR UP ABOVE THE TWO COLD AIR MASSES.

• WEATHER IS COMPLEX.• CAN HAVE WARM OCCLUDED FRONTS (COLD FRONT IS WARMER THAN FRONT IT OVERTAKES) - PACIFIC COAST.

• COLD OCCLUDED FRONT IS REVERSE OF ABOVE - EAST OF THE ROCKIES.

Page 33: AOSC 200 Lesson 9

Birth of a an Extratropical Cyclone

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LIFE CYCLE OF AN EXTRATROPICAL CYCLONE

• THE LEFT HAND SIDE SHOWS HOW BJERKNES DEPICTED THE LIFE CYCLE.

• FORM ALONG THE LINE BETWEEN THE POLAR AIR MASS AND THE MARITIME TROPICAL AIR MASS.

• SHEARING ACTION OF OPPOSING WINDS PRODUCES CYCLONIC MOTION.

• UNDER SUITABLE CONDITIONS FRONTAL SURFACE WILL ASSUME A WAVE SHAPE.

• OPEN WAVE DEVELOPS COLD AND WARM FRONTS• COLD FRONT CATCHES UP WITH WARM FRONT• CYCLONE DISSIPATES

Page 36: AOSC 200 Lesson 9

Fig. 10-6a, p. 281

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Fig. 10-6b, p. 281

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Box 10.1

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Box 10.1

Page 40: AOSC 200 Lesson 9

Cyclones over the Rockies

• In order for Cyclones formed over the Pacific to each the mid-west they have to go over the Rockies.

• This squeezes the cyclone down, which increases the radius of rotation

• This decreases the rate of rotation (conservation of angular momentum)

• The cyclone appears to weaken.• East of the Rockies the cyclone expands and regains its full rate of rotation

Page 41: AOSC 200 Lesson 9

Fig. 10.11

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Dish-pan Experiment

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Fig. 6.11

Centrifugal Force

Page 45: AOSC 200 Lesson 9

Centrifugal Force

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Fig. 10-12, p. 290

Page 47: AOSC 200 Lesson 9

Formation of Cyclones

• .CLOSE RELATIONSHIP BETWEEN SURFACE DISTURBANCES AND THE FLOW IN THE JET STREAM.

• .FOR A MID-LATITUDE CYCLONE TO FORM:CYCLONIC FLOW MUST BE ESTABLISHED

• .INWARD FLOW OF AIR NEAR SURFACE MUST BE SUPPORTED BY OUTFLOW ALOFT.

• DIVERGENCE AND CONVERGENCE ALOFT• TOTAL SPIN / CYCLONIC HEIGHT = CONSTANT• .VORTICITY - TENDENCY OF AIR TO ROTATE IN A WHIRLPOOL LIKE VORTEX – SPIN AROUND A VERTICAL AXIS

Page 48: AOSC 200 Lesson 9

Fig. 10.13

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