AOSC 200Lesson 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

Fig. 9.3

Major air masses of the world

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

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

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

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

CLASSIFICATION

• THUS WE GET:

• cA - CONTINENTAL ARCTIC

• cP - CONTINENTAL POLAR

• cT - CONTINENTAL TROPICAL

• mT - MARITIME TROPICAL

• mP - MARITIME POLAR

• mE - MARITIME EQUATORIAL

Fig. 9.4

Major air masses that affect North American weather

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

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

Fig. 9.6

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

Fig. 9.8

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

air mass

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

Northeaster.

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.

Fig. 9.9

Maritime Tropical air mass

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

Box 9.2

Average annual snowfall for the Great Lakes region

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

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?

Box 9.2

Formation of Lake Effect snowfall

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.

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

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

Fig. 9.14

Weather associated with a cold front

Fig. 9.15

Warm, moist air is forced upward by cold front

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.

Fig. 9.16

Surface weather associated with a warm front

Fig. 9.17

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

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,.

Fig. 9.18

Occluded Fronts

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.

Birth of a an Extratropical Cyclone

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

Fig. 10-6a, p. 281

Fig. 10-6b, p. 281

Box 10.1

Box 10.1

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

Fig. 10.11

Dish-pan Experiment

Fig. 6.11

Centrifugal Force

Centrifugal Force

Fig. 10-12, p. 290

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

Fig. 10.13