Transcript AOSC200_summer_lect9

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
• 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=neWNlEmnbk
• http://www.youtube.com/watch?v=neWNlEmnbk
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
Major air masses that affect North
American weather
Fig. 9.4
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.
Ice in a Florida
orange grove as a
result of a
continental arctic
air mass coming
down from Canada.
Fig. 9.6
Temperature at Madison, WI, as a result of a
continental arctic air mass
Fig. 9.8
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.
Maritime Tropical air mass
Fig. 9.9
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
Average annual snowfall for the Great Lakes
region
Box 9.2
http://www.youtube.com/watch?v=
o_ioV4MAUnM
http://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?
Formation of Lake Effect snowfall
Box 9.2
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=MbfCre
7-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
Weather associated with a cold front
Fig. 9.14
Warm, moist air is forced upward by cold front
Fig. 9.15
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.
Surface weather associated with a warm front
Fig. 9.16
Warm, moist air is slowly raised as it flows over
the cold air
Fig. 9.17
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,.
Occluded Fronts
Fig. 9.18
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
Centrifugal Force
Fig. 6.11
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