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
Figure 7-1
• 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
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—Tstorms & tornadoes
– Tropical disturbances— easterly
waves & hurricanes
Midlatitude Cyclones
Figure 7-6
• Midlatitude Cyclone
– Large migratory L-pressure
system in mid-latitudes (3060° 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-today weather changes
– Bring precipitation to much of
world’s population
Midlatitude Cyclones
• 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
Note: the shift in winds
& change in precipitation
at the frontal boundaries
mP
cP
mT
Midlatitude
Cyclones
• Life Cycle
– Cyclogenesis
• Birth of midlatitude
cyclone
– Occlusion
• Death of
midlatitude cyclone
Figure 7-9
Midlatitude Cyclones
Midlatitude Cyclones
• Upper level divergence & convergence related to
cyclogenesis
Figure 7-10
Midlatitude Cyclones
• 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
Midlatitude Cyclones
Warm front
Wraparound
precipitation
west of
low
Cold
front
Midlatitude Cyclones
Mid-latitude Cyclone 8:27a.m. 11-28-05
Midlatitude Cyclones
Mid-latitude Cyclone 12:47p.m. 11-28-05
Midlatitude Cyclones
Surface Temperatures associated with
Mid-latitude cyclone (11-28-05)
Midlatitude Cyclones
Pressure and
wind associated
with 11-28-05
mid-latitude
cyclone
Midlatitude Cyclones
Surface
winds
associated
with
11-28-05
mid-latitude
cyclone
Nor’easters
• 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
Famous 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
Rare Nor’easter Eye; Nor’easter center
Note: counterclockwise flow around center
Nor’easters:
Blizzard of 2006 Snow Totals
• 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
NYC
National Archives
Skiing to Central Park
Times Square
Nor’easters: December 2009
• 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”
Thundersnow
• 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
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