Chapter 7: Atmospheric Disturbances – p. 1 of 10
Chapter 7: Atmospheric Disturbances
I.
II.
The Impact of Storms on the Landscape
Air Masses
air masses: large, variable parcels of air that are distinguishable from one
another
A. Characteristics
1. large: 1,000 miles across, several miles deep
2. uniform properties in the horizontal dimension: temperature,
humidity, and stability are relatively homogenous at any given altitude
3. recognizable entity and travel as one: distinct from surrounding air
B. Origin
1. most air masses are associated with anticyclonic conditions: must
remain stable long enough to develop characteristics
2. source regions
a. characteristics of Earth’s surface needed to generate air masses
1) extensive
2) physically uniform
3) associated with air that is stationary or anticyclonic
b. areas particularly well suited:
1) ocean surfaces
2) extensive flat land areas with uniform covering of snow,
forest, or desert
c. major source regions for air masses that affect North America (Fig. 7-1)
1) warm-air masses
a) southern North Atlantic
b) Gulf of Mexico/Caribbean Sea
c) southern North Pacific
d) deserts of southwestern US and northwestern Mexico – in
summer time
2) cold-air masses
a) northern Atlantic and Pacific Oceans
b) snow covered lands of north-central Canada
d. air masses can originate almost anywhere in the low or high
latitudes but rarely in the midlatitudes due to prevailing westerlies
C. Classification
1. air masses are classified on the basis of source region
2. latitude of source region correlates with temperature of the air mass
a. low latitudes = warm or hot
b. high latitudes = cool or cold
3. nature of the surface influences humidity content of the air mass
a. continental = dry
b. ocean = moist
4. 6 basic categories (Table 7-1)
a. A = Arctic/Antarctic
b. cP = continental polar
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III.
c. mP = maritime polar
d. cT = continental tropical
e. mT = maritime tropical
f. E = equatorial
D. Movement and Modification
1. air mass is modified as it leaves source region
2. air mass modifies the weather of the regions it passes through
3. characteristics affected: temperature, humidity, and stability
E. North American Air Masses
1. North America is prominent area of air mass interaction
a. lack of east-west trending mountains in the eastern 2/3 allows polar
air to flow southward and tropical air to flow northward
b. north-south trending mountains in west impede movement of Pacific
air masses
2. major North American air masses
a. cP and A air masses originating in central and northern Canada
1) dominant feature in winter
2) cold, dry, stable
b. mP air masses originating in the northern Pacific
1) cool, moist, unstable in lower layers
2) coastal regions
a) widespread cloudiness and heavy precipitation in mountainous
regions
b) fog in summer
3) interior: moderate temperatures and clear skies
c. mT air masses originating over Atlantic/Caribbean/Gulf of Mexico
1) warm, moist, unstable
2) principal precipitation source for regions east of Rockies
3) especially dominant in summer
d. mT air masses originating over the Pacific
1) areas of anticyclonic subsidence  cooler, drier, more stable than
Atlantic mT
2) influences southwestern US
e. cT air masses originating over northern Mexico/southwestern US
1) summer only
2) hot, dry, unstable  heat waves and drought in southern Great Plains
f. E air masses affect North America only in association with hurricanes
Fronts
A. intro
1. front: boundary between 2 unlike air masses
a. air masses do not mix easily
b. relatively narrow, 3 dimensional zone of discontinuity
c. most conspicuous difference between air masses is usually
temperature  abrupt temperature gradient
d. cold front forms where an advancing cold air mass meets warmer air
e. warm front forms where an advancing warm air mass meets colder air
Chapter 7: Atmospheric Disturbances – p. 3 of 10
IV.
f. warmer air always rises over cooler air
g. fronts “lean”/slope upwards from the surface
1) allows uplifting and adiabatic cooling
2) cold front: leans backwards with a steep leading edge
3) warm front: leans forward at a gentle slope
B. Cold Fronts (Fig 7-3)
1. cold front: a front that brings cold air
2. characteristics
a. steep front
b. faster advance
c. rapid lifting  unstable air
d. vertically developed clouds; cumulonimbus clouds common
e. clouds and precipitation concentrated along and immediately
behind ground level position of front
f. precipitation: turbulent, high intensity, short duration
3. portrayed on weather maps as blue line or black line w/solid
triangles extending in the direction toward which front is moving
C. Warm Fronts (Fig 7-4)
1. warm front: a front that brings warm air
2. characteristics
a. gentle slope
b. warm air ascends over retreating cold air
c. clouds and precipitation precede ground level position of the
advancing front
d. broad, protracted, gentle precipitation
e. cirrus clouds  altocumulus or altostratus
3. portrayed on weather maps as red line or black line w/semicircles
extending in the direction toward which front is moving
D. Stationary Fronts
1. when neither air mass displaces another
2. weather unpredictable
3. weather map: combination of warm and cold front symbols
E. Occluded Fronts
1. when a cold front overtakes a warm front
Atmospheric Disturbances
A. intro
1. 2 types of disturbances: stormy and calm
2. characteristics of atmospheric disturbances
a. variable in size but smaller than components of the general
circulation
b. migratory and transient
c. relatively brief duration
d. produce relatively predictable weather conditions
B. Midlatitudes Disturbances
1. midlatitudes are principal battleground of tropospheric phenomena
2. polar and tropical air masses converge
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V.
3.  where most fronts occur
4. where weather is most dynamic and changeable season to season and
day to day
C. Tropical Disturbances
1. low latitude weather characterized by monotony
2. disturbances: tropical cyclones and easterly waves
D. Localized Severe Weather
1. short-lived but sometimes severe
2. thunderstorms and tornadoes
Midlatitude Cyclones
A. Intro
1. midlatitude cyclones are most significant of all atmospheric
disturbances
a. dominate weather maps in midlatitudes
b. responsible for most day-to-day weather changes
c. bring precipitation
2. large, migratory, low pressure systems
3. also called depressions, lows, or wave cyclones
4. midlatitudes (35o to 70o) in zone of westerlies so movement is from
west to east (“…weather forecasting in the midlatitudes is essentially a
west-facing vocation.”)
B. Characteristics
1. typical general characteristics
a. large: diameter 1,000 miles
b. low pressure: 990 – 1,000 mb
c. oval shape w/long axis trending NE-SW
d. clear-cut pressure trough extends southwesterly from center
2. circulation pattern
a. converging, counterclockwise in northern hemisphere: attracts
cool air from north and warm air from south
b. two fronts
1) cold front extends southwest from cyclone center and runs
along the pressure trough
2) warm front extends eastward from center along another
pressure trough
c. temperature
1) cool sector north and west (larger sector at ground level)
2) warm sector south and east
d. two zones of cloudiness and precipitation along the fronts
1) most precipitation originates in warm air rising above fronts and
reaches the ground in cool sector
2) much of cool sector: clear, cold, stable air
3) warm sector: moist, unstable
C. Weather Changes with the Passing of a Front
1. weather changes associated with a cold front:
a. sharp temperature decrease
Chapter 7: Atmospheric Disturbances – p. 5 of 10
VI.
b. wind shift: from southerly ahead of front to northwesterly behind
c. pressure falls as front approaches, rises after passage
d. clear skies ahead of front → cloudiness and precipitation at the
front → clear skies as front moves away
2. similar changes of lesser magnitude occur with passage of warm front
D. Movements
1. midlatitude cyclones are transient features
2. 4 kinds of movement:
a. whole system moves in the westerlies at 20-30 mph
a. can cross U.S. in 3-4 days
b. moves faster in winter than in summer
b. cyclonic air circulation: converging, counterclockwise (in northern
hemisphere)
c. cold front travels faster than storm so swings counterclockwise
around center pivot
d. warm front travels more slowly than the storm so appears to move
clockwise toward the cold front
E. Life Cycle
1. Cyclogenesis
a. origin to maturity: 3-6 days
b. maturity to dissipation: 3-6 days
c. cyclogenesis (birth of cyclones) causes:
1) wave along polar front
2) upper airflow meridional (meandering N/S) rather than zonal (E/W)
often along polar jet stream
3) converging air at ground level; divergence aloft
4) leeward side of mountains, especially in Colorado in winter
2. Occlusion
a. storm dissipates when the cold front overtakes the warm front
 occluded front
b. warm sector forced aloft, reducing the pressure gradient
F. Occurrence and Distribution
1. 6-15 midlatitude cyclones in northern and southern hemisphere at
any one time
2. more numerous, better developed and faster moving in winter
because temperature contrasts are greater
Midlatitude Anticyclones
A. midlatitude anticyclone: extensive, migratory, high-pressure cell of
the midlatitudes; referred to as a high
B. Characteristics
1. clear and dry weather: no surface convergence, no front, no clouds
2. may be strong winds along leading edge
3. in winter: very cold temperatures
4. move from west to east
5. clear, stable, dry weather
6. stagnation  pollution
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C. Relationship of Cyclones and Anticyclones
1. midlatitude cyclones and anticyclones alternate with one another
2. wind diverging from eastern margin of anticyclone fits into flow of air
converging into western side of low
3. anticyclone is a polar air mass having cold front of the cyclone as its
leading edge
VII. Minor Tropical Disturbances: Easterly Wave
A. characteristics
1. common tropical disturbance
2. long, weak, migratory, low pressure system
3. occurs between 5o and 30o latitude
4. drift westward in the trade winds
5. fair weather ahead; stormy weather behind
6. small percentage develop into hurricanes
VIII. Major Tropical Disturbances: Hurricanes
A. intro
1. tropical cyclone: intense, low pressure disturbances that develop
in the tropics and occasionally move into the midlatitudes
2. terminology
a. hurricanes – in North and Central America
b. typhoons in western north Pacific
c. baguios in Philippines
d. cyclones in Indian Ocean and Australia
3. diameter: 100-600 miles (smaller than midlatitude cyclones)
4. categories based on wind speeds
a. tropical depression: < 33 knots/38 mph
b. tropical storm: 34-63 knots/39-73 mph (named at this stage)
c. hurricane: 64+ knots/74+ mph
B. Characteristics
1. low pressure systems
2. circular
3. steep pressure gradient  strong inspiraling winds that rise in
intense updrafts
4. Eye of a Hurricane
a. singular area of calm at center with downdraft w/diameter 10-25 miles
b. eye wall: winds reach highest speeds with strong updrafts
C. Origin
1. where
a. over warm, tropical oceans
b. just poleward of ITCZ
1) none within 3o of equator because of minimal Coriolis effect
2) no hurricane has been known to cross equator
2. easterly waves catalyze development of hurricanes
D. Movement
1. common occurrences: (Fig 7-19)
a. east of the Philippines
Chapter 7: Atmospheric Disturbances – p. 7 of 10
b. west of southern Mexico and Central America
c. west central North Atlantic/Caribbean/Gulf of Mexico
d. western South Pacific
e. South Indian Ocean
f. North Indian Ocean
2. very rare in South Atlantic and southeastern Pacific – waters too cold
and high pressure dominates
3. strongest/largest hurricanes occur in the western Pacific Ocean
4. generally travel east to west following the trades
a. some curve poleward ending up in the westerlies
b. hurricanes, diminished, can survive off east coasts of continents in
midlatitudes but not along west coasts where ocean currents are
cooler
5. temporal
a. most last 1 week; maximum = 4 weeks
b. dies as loses energy source:
1) over land
2) cooler environment of midlatitudes
c. seasonality
1) late summer and fall
2) when ocean temperatures are highest and ITCZ is shifted
farthest poleward
E. Damage and Destruction
1. greatest damage and loss of life from flooding brought by high seas;
90% of hurricane related deaths are drownings
a. extreme low pressure  ocean surface bulge in center
b. wind driven ocean storm surge
2. greatest US hurricane disaster
a. Galveston Island, TX 1900
b. 8,000 deaths
3. chronic disaster area: Bangladesh’s Ganges and Brahmaputra
river delta from Indian Ocean cyclones
4. Saffir-Simpson Hurricane Scale ranks intensity (Table 7-2)
a. based on central eye pressure, wind speed and storm surge
height
b. ranges from 1 to 5 (most severe)
F. People and the Environment: Hurricane Katrina
1. description
a. Aug 28, 2005 – category 5 over Gulf of Mexico
1) winds > 150 knots/170 mph
2) central pressure 902 mb (6th lowest on record)
b. strong category 3 when made landfall southeast of New Orleans on
Aug 29, 2005 (sustained winds > 108 knots/125 mph)
c. average 10” rainfall
d. 26-28 foot storm surge
2. major causes of damage
Chapter 7: Atmospheric Disturbances – p. 8 of 10
a. levee failure allowed water from Lake Pontchartrain to pour into
central city; 80% New Orleans underwater
b. New Orleans sits in a shallow bowl -- much of the city lies below
sea level protected by a series of levees
c. flood control measures upstream had damaged protective wetlands
3. impacts
a. more than 1,200 deaths
b. tens of thousands displaced
G. People and the Environment: Hurricanes and Global Warming
1. 2005 hurricane season in North Atlantic most active on record:
a. 28 named storms
b. 3 of the most powerful hurricanes ever measured in terms of minimum
atmospheric pressure in the eye occurred that year (Table 7-A):
1) Wilma #1 (at 882 mb)
2) Rita (#4)
3) Katrina (#6)
2. debate about connection with global warming:
a. there has been a general increase in the annual number of
hurricanes in the North Atlantic over the last 15 years
b. many meteorologists think increased frequency is part of normal
multi-decadal cycle of hurricane activity
c. possible global warming links:
1) greenhouse gas emission scenario models predict increase in
intensity of tropical cyclones during this century
2) global warming has resulted in an increase in sea surface
temperatures
IX.
Localized Severe Weather
A. Thunderstorms
1. thunderstorm: violent convective storm accompanied by thunder
and lightning
2. characteristics
a. localized and short-lived
b. high humidity, instability, vertical development  towering
cumulonimbus clouds with showery precipitation
c. uplift associated with thermal convection, hurricanes, tornadoes,
fronts and orographic lifting
3. stages
a. cumulus stage: updraft prevails and cloud grows
b. mature/most active stage: updrafts and downdrafts coexist; cloud
continues to enlarge
c. dissipating stage: downdrafts dominate
4. occurrence
a. where there’s high temperatures, high humidity, high instability
b. most prevalent: ITCZ; none poleward of 60o latitude
c. greater frequency over land than water (convectional heating)
d. mostly summer
Chapter 7: Atmospheric Disturbances – p. 9 of 10
e. in US greatest activity is in Florida and the Gulf Coast; least along
Pacific coast (cool water and subsidence)
B. Lightning
1. average 65 deaths/year in the U.S.; most dangerous places to be:
under a tree, in a boat, on a tractor, and playing golf
2. development but not cause is understood
3. development of large cumulonimbus clouds → separation of electrical
charges:
a. positive charge in upper layers of cloud
b. negative charge at cloud base
c. positive charge builds on Earth’s surface
4. thunder: lightning heat produces instantaneous expansion of air 
creates a shock wave  sound wave
5. because light travels faster than sound, we see the lightning flash
before we hear the thunder; a 5 second delay indicates lightning is
about 1 mile away
C. Tornadoes
1. tornado: deep low pressure cell surrounded by a violently whirling
cylinder of wind
2. most destructive of all atmospheric disturbances
a. most intense vortex in nature
b. most extreme pressure gradient (100 mb) in nature
c. maximum wind speed estimates up to 300 mph
3. track southwest to northeast in US
4. travel speed ~30 mph
5. Enhances Fujita tornado intensity scale ranges from F0 to F5 (most
intense) (table 7-3)
6. average annual death toll in U.S. = 60
7. Formation
a. warm, moist unstable air
b. midlatitude cyclone
c. in squall line preceding rapidly advancing cold front
d. vertical wind shear: significant change in wind speed or direction from
the bottom to top of storm – basic tornado requirement
e. mesocyclone: circular rotation (6 miles) of air in severe
thunderstorm
8. temporal
a. spring and early summer: considerable air mass contrast in
midlatitudes
b. midafternoon: maximum heating
9. geographical locations
a. 90% of reported tornadoes occur in the US (about 1,000 sighted
annually)
b. relatively flat terrain – central and southeastern US
c. contact zone between cP and mT air masses
d. over land
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X.
XI.
Focus: Forecasting Severe Storms with NEXRAD
A. Oklahoma sky: world’s most prolific breeder of tornadoes
1. combination of mT (Gulf), cP (Canada), cT (desert) and polar front jet
stream
B. NEXRAD (next-generation radar)
1. reflected microwaves, analyzed on principle of Doppler effect, reveal
storm intensity
2. can detect motion toward and away from radar site: strong
counterclockwise rotation is the hallmark of a mesocyclone
Focus: Forecasting Severe Storms with the Vertical Wind Profiler and the
Oklahoma Mesonetwork
A. Vertical Wind Profiler
1. Doppler radar unit
2. sends radar energy upward to sense wind speed and direction in the
first few kilometers of the atmosphere
B. Mesonet:
1. observes mesoscale features (1.2 – 12.5 miles)
2. 120 automated weather station radio readings every 5 minutes
3. Mesonetwork locates dryline:
a. boundary between mT and cT air
b. zone in which surface air streams are forced to converge and lift
c. generates some of the largest tornadic thunderstorms on Earth