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AOSC 200
Lesson 15
Computer generated image of Hurricane Mitch –
October 26, 1998. Near Honduras
Note the distinct ‘eye’ and the large extent of the hurricane
Fig. 8-18, p. 223
Damage from hurricane Andrew, August 1992
Fig. 8-19, p. 224
Fig. 8-23, p. 228
Fig. 8.29
TROPICAL CYLONES
• THESE INTENSE TROPICAL STORMS ARE
KNOWN BY DIFFERENT NAMES IN VARIOUS
PARTS OF THE GLOBE:
•
HURRICANES - ATLANTIC
•
TYPHOONS - WESTERN PACIFIC
•
CYCLONES - INDIAN OCEAN
•
• MOST FORM BETWEEN THE LATITUDES OF 5
TO 20 DEGREES.
• LESS THAN 5 DEGREES, THE CORIOLIS FORCE
IS TOO SMALL
• MORE THAN 20 DEGREES, THE TEMPERATURE
OF THE OCEAN IS TOO COLD
• MUST HAVE WIND SPEEDS OF MORE THAN 119
KM PER HOUR AND HAVE A ROTARY
CIRCULATION TO BE A HURRICANE/TYPHOON
TROPICAL CYCLONE FORMATION
• HURRICANES DEVELOP MOST OFTEN WHEN
OCEAN WATERS HAVE REACHED MAXIMUM
TEMPERATURES - 25 C OR HIGHER.
• INITIAL STAGE IS SOME FORM OF
DISTURBANCE - NAMED BY THE WEATHER
SERVICE AS TROPICAL DISTURBANCES.
• ONLY A FEW TROPICAL DISTURBANCES
DEVELOP 119 KM PER HOUR WINDS
• IF WINDS <119 BUT > 61 - TROPICAL STORM
• NAMES GIVEN WHEN STORM REACHES
TROPICAL STORM STATUS
TROPICAL CYCLONES
• MATURE TROPICAL CYCLONES AVERAGE
ABOUT 600 KM ACROSS
• BAROMETRIC PRESSURE CAN DROP ACROSS
CYCLONE FROM 1010 TO 950 MILLIBARS
• THIS GENERATES RAPID, INWARD SPIRALING
WINDS.
• AS AIR MOVES CLOSER TO CENTER ITS
VELOCITY INCREASES
• CONSERVATION OF ANGULAR MOMENTUM.
• IN THE CENTER OF THE EYE THE AIR IS
DESCENDING – HENCE IT IS FREE OF CLOUDS.
TROPICAL CYCLONES
• MOIST SURFACE AIR IS TURNED UPWARD AND
ASCENDS.
• AS MOIST AIR MOVES UP IT COOLS AND WATER
VAPOR CONDENSES.
• THIS PROVIDES LATENT HEAT TO INCREASE
BUOYANCY OF THE RISING AIR.
• NEAR THE TOP OF THE HURRICANE THE
AIRFLOW IS OUTWARD.
• AIR MUST BE PULLED AWAY FROM HURRICANE
AT THE TOP – REINFORCEMENT FROM ABOVE.
• THE CENTRAL DOUGHNUT AREA HAS
DOWNWARD MOTION, IS CALLED THE EYE OF
THE STORM
• EYE WALL CONTAINS STRONGEST WINDS.
Fig. 8-24, p. 229
Schematic of a Hurricane
TROPICAL CYCLONES
• TROPICAL CYCLONES DIMINISH WHEN
• THEY MOVE OVER COLDER OCEAN WATERS
• MOVE ONTO LAND
• REACH A LOCATION WHEN LARGE-SCALE FLOW ALOFT IS
UNFAVORABLE
Fig. 8.27
STORM SURGE
Fig. 8.35
TROPICAL CYCLONE DAMAGE
• WIND DAMAGE
• STORM SURGE
• INLAND FLOODING
Fig. 8-39, p. 247
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
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
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.
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.