Chapter 7: Atmospheric Disturbances Part I: Midlatitude Disturbances

Chapter 7: Atmospheric Disturbances

Part I: Midlatitude Disturbances

The Impact of Storms on the Landscape

• Immediate storm impacts– Widespread or local damage

• Thunder & lightning• Strong winds• Precipitation• Flooding

• Long-term storm impacts– Water supply

• Lakes/ponds

– Diversity of vegetation

Figure 7-B

Air Masses

• Properties of air masses– Large

• Diameter >1600 km

– Uniform horizontal properties

– Travels as 1 entity– 3 requirements:

• Large• Uniform properties• Distinct from surrounding

air

Figure 7-2

Air Masses

• Source Regions: areas that generate air masses– Remain over uniform surface

long enough to acquire uniform characteristics

– Extensive– Physically uniform– Stationary or H pressure– Continental or Maritime – Latitude– Affects:

• Humidity • Temperature • Stability

Figure 7-1

Air Masses

• Air mass classification– 2 letter classification system– Lowercase letter = moisture

content• c—continental, dry• m—maritime, humid

– Uppercase letter = source region

• P—polar source region• T—tropical source region• A—arctic source region• E—equatorial source region

Air Masses

• U.S. Air Masses– cP– mP– mT– cT

• Physical geography

of U.S.– No E-W mountains– Air mass clashes

• Violent weather

Fronts

• Front: zone of discontinuity between unlike air masses– AKA Barrier between 2 air masses– Rapid change in air properties

• Temperature is most conspicuous

– Move in the direction of the more active air mass

• 4 primary frontal types:– Cold front– Warm front– Stationary front– Occluded front

Figure 7-5

Fronts

• Cold Front: cold air advancing; cold air is agressor– Faster than warm fronts– Lift warm air ahead of cold fronts– Brings colder temperatures– Heavy precipitation falls ALONG cold front

Figure 7-3

Fronts

• Warm Front: warm air advancing; warm air is aggressor– Gentle slope of warm air rising above cool air

• Slow cloud formation

– Brings warmer temperatures– Gentle precipitation falls AHEAD of warm front Figure 7-4

Cold Fronts and Warm Fronts

Fronts

• Stationary front: no advance of either air mass– No aggressor air mass

• Occluded front: cold air overtakes warm air– Complex

Figure 7-11

Atmospheric Disturbances

• 3 Types– Midlatitude disturbances—

midlatitude cyclones– Localized severe weather—T-

storms & tornadoes– Tropical disturbances— easterly

waves & hurricanes

Midlatitude Cyclones

• Midlatitude Cyclone– Large migratory L-pressure

system in mid-latitudes (30-60° N/S)

– Converge counterclockwise in N Hemisphere

• Circulation creates fronts• Winds pull cool air from N &

warm air from S

– Moves with westerlies – Most significant atmospheric

disturbance– Responsible for most day-to-

day weather changes– Bring precipitation to much of

world’s population

Figure 7-6


• Weather changes behind front– Temperature: decreases as cold front passes– Winds: change from S before cold front to NW after it passes– Pressure: decreases as cold front nears & rises after it passes

• Cyclone movement– Steered by jet stream– Cyclonic wind

circulation– Cold front advances

faster than warm front

mT

mPcP

Note: the shift in winds & change in precipitation at the frontal boundaries

• Life Cycle– Cyclogenesis

• Birth of midlatitude cyclone

– Occlusion• Death of

midlatitude cyclone


Figure 7-9



• Upper level divergence & convergence related to cyclogenesis

Figure 7-10


• Occurrence and distribution– Typically 6–15 cyclones exist worldwide– More numerous & better developed in winter than in summer– Move more equatorward during summer

Figure 7-13

Wrap-around precip-itation west of low

Warm front

Cold front


Mid-latitude Cyclone 8:27a.m. 11-28-05


Mid-latitude Cyclone 12:47p.m. 11-28-05


Surface Temperatures associated with Mid-latitude cyclone (11-28-05)


Pressure and wind associated with 11-28-05 mid-latitude

cyclone


Surface winds

associated with

11-28-05 mid-latitude

cyclone


• Cyclonic storm along E coast of N America; named so because winds over the area preceding the storm are from the NE

• 2 Components– Gulf Stream L-pressure– Arctic H-pressure

• 2 Types– Off-shore forming– On-shore forming

• Nor’easter season– October – April

• May dump several inches of rain and/or feet of snow

• May last several days

• Waves cause flooding, beach erosion & structural damage

• Low temperatures & wind gusts may exceed hurricane force

Nor’easters

Hurricane Nor'easter

Temperature Warm Cold

Size 200-300 miles across Up to 1000 miles across

Shape Symmetrical Irregular

Duration 6-8 hours Up to a week

Frequency Don’t occur every year 100% chance every year

Intensity 74+ mph 35-50 mph onshore; higher offshore

Season June to November October to April

Damage May level an area, but limited in size

Spreads damage around a greater area

Geography South North

Names Officially named Tie occurrence to date or use superlative

Press Coverage

Extensive media coverage Less news coverage; few know what they are or their effects

Nor’easters

• Blizzard of 1888 • Ash Wednesday Storm of 1962 • Groundhog Day gale of 1976 • Blizzard of 1978 ("Blizzard of '78") • Halloween Nor'easter of 1991

– ("Perfect Storm") • Great Nor’easter of December 11,

1992• Super Storm of March 13, 1993 • Blizzard of 1996 • Blizzard of 2006 • December 2009 Nor’easter• Blizzards of 2010• 2011 Halloween Nor’easter

Famous Nor’easters

http://upload.wikimedia.org/wikipedia/en/9/9c/Storm_of_the_century_satellite.gif

Rare Nor’easter Eye; Nor’easter centerNote: counterclockwise flow around center

• New York City – 26.9” in Central Park– Snow fell 2-5+ in/hr– Lightning/thundersnow

• Washington D.C. – 8-10”

• Baltimore, MD – 13-15”

• Boston, MA – 15-20”

• Newark, NJ (airport) – 21.3”

• Fairfield, CT – 30”• Winds 20-30 mph, gusts 40-60mph

Nor’easters: Blizzard of 2006 Snow Totals

National Archives

Skiing to Central Park Times Square

NYC

• Snow Totals– Reagan National Airport (Washington, D.C.) – 16.4”– Brookhaven, NY – 26.3”– Philadelphia, PA – 23.2”– Boone, NC – 18”– Asheville, NC – 12”– Norwich, CT – 20”– Boston, MA – 11”

Nor’easters: December 2009

• Thundersnow– T-storm with snow

instead of rain – 2 mechanisms:

• Elevated instability• Strong lifting

– Rare– Associated with

intense snowfalls • Severe thundersnow

– Snow with hail 3/4" or larger in diameter or if winds are 50+ mph

Thundersnow

Thundersnow

Lake-effect Snow

Midlatitude Anticyclones

• Midlatitude Anticyclones—H pressure system– Subsiding, diverging windsat– Clockwise flow around anticyclone– Move with the westerlies– Larger than cyclones, but move

slightly slower• Often become stationary

• Relationship to cyclones– Occur independently, but have

functional relationship– Anticyclone follows cyclone

Figures 7-12 & 7-14

Documents

Chapter 7: Atmospheric Disturbances Part I: Midlatitude Disturbances