Ch 7

Title Page Photo

“Sunshine is delicious, rain is refreshing, wind
braces us up, snow is exhilarating; there is really
no such thing as bad weather, only different kinds
of good weather.”—John Ruskin
(Brainyquote.com)
1

Chapter Seven Vocabulary
• air mass (p. 179) • occluded front (p. 188)
• cold front (p. 183) • occlusion (p. 188)
• easterly wave (p. 191) • Saffir-Simpson Hurricane Scale
• eye (of a hurricane) (p. 195) • (p. 198)
• eye wall (p. 195) • stationary front (p. 185)
• front (p. 182) • storm surge (p. 198)
• Fujita tornado intensity scale (p. • thunder (p. 203)
204) • thunderstorm (p. 200)
• funnel cloud (p. 204) • tornado (p. 204)
• hurricane (p. 193) • tropical cyclone (p. 193)
• lightning (p. 201) • tropical depression (p. 193)
• mesocyclone (p. 207) • tropical storm (p. 193)
• midlatitude anticyclone (p. 191) • warm front (p. 183)
• midlatitude cyclone (p. 185) • waterspout (p. 207)
• wind shear (p. 195)

2

The Impact of Storms on the
Landscape

 Storms are phenomena that are more
limited than the broad-scale wind and
pressure systems.
 They are transient and temporary.
 Storms involve the flow of air masses
as well as a variety of atmospheric
disturbances.

3

1

The Impact of Storms on the
Landscape
 They have short-run and long-run impacts.
 In some parts of world, have major influence
on weather, some on climate.
 Long-run includes both positive and negative
impacts on landscape.
 Positive: promote diversity in vegetative
cover, increase size of lakes and ponds, and
stimulate plant growth

4

 Air Masses
• Air mass—a large
parcel of air that has
relatively uniform
properties in the
horizontal dimension
and moves as an entity.
Such extensive bodies
are distinct from one
another and compose
the troposphere.

Fig. 7-1

5

Air Masses
 Characteristics
 Air mass must meet three requirements:
1. Must be large (horizontal and vertical).
2. Horizontal dimension must have uniform
properties (temperature, humidity, and stability).
3. Must be distinct from surrounding air, and when
moves, must retain that distinction (not be torn
apart).

6

2

Air Masses
 Origin
 Formation occurs if air
remains over a uniform land
or sea surface long enough
to acquire uniform
properties.
 Source regions—parts of
Earth’s surface that are
particularly suited to
generate air masses
because they are
1. Extensive
2. Physically uniform
3. Associated with air that is
stationary or anticyclonic.
7

Air Masses
 Classification
 Because source region determines properties of air masses, it is
the basis for classifying them.
 Use a one- or two-letter code.
 Table 7–1 provides a simplified classification of air masses, along
with the properties associated with each.

8

• Classification
– Letter System
First Letter – Second Letter (capitalized) –
Humidity indicator Temperature indicator

Land or water Latitude
c = continental E = 0º  10º Equatorial
(dry air) T = 10º  35º Tropical
m = maritime P = 55º  70º Polar
(moist air) A = 70º  90º Arctic / Antarctic

* Middle latitudes (35º  55º) not a major source region

9

3

– Types
• E – Equatorial
• mT – maritime Tropical
• cT – continental Tropical
• mP – maritime Polar
• cP – continental Polar
• A – Arctic / Antarctic

10

Air Masses
 Movement and
Modification
 Some air masses remain in
source region indefinitely.
 Movement prompts
structural change:
 Thermal modification—
heating or cooling from
below;
 Dynamic modification—
uplift, subsidence,
convergence, turbulence;
 Moisture modification—
addition or subtraction of
moisture.
 Moving air mass modifies
the weather of region it
moves through.
11

Air Masses
 North American Air Masses
 Physical geography of U.S.
landscape plays a critical role in
air-mass interaction.
 No east–west mountains to
block polar and tropical air flows,
so they affect U.S.
weather/climate.
 North–south mountain ranges in
west modify the movement,
therefore the characteristics, of
Pacific air masses.
 North American Air Masses
 cA and cP
 mP
 mT
 cT
12

4

Air Masses
 North American Air
Masses (con’t)
 Maritime tropical (mT) air
from the Atlantic,
Caribbean/Gulf of Mexico
strongly influences
climate east of the
Rockies in the United
States, southern Canada,
and much of Mexico.
 Primary source of
precipitation. Also brings
periods of uncomfortable
humid heat in summer. 13

Air Masses
 North American Air
Masses (con’t)
 Continental tropical
(cT) air has
insignificant influence
on North America,
except for bringing
occasional heat waves
and drought conditions
to the southern Great
Plains.
 Equatorial (E) air
affects North America
only through
hurricanes. 14

Fronts
 Front—a zone of
discontinuity between
unlike air masses
where properties of
air change rapidly.
 It’s narrow but three-
dimensional.
 Typically several
kilometers wide (even
tens of kilometers
wide).
 Functions as a barrier
between two air
masses, preventing
15
their mingling except
in this narrow

5

Fronts
Fronts (con’t)
 Though all primary
physical properties are
involved in a front,
temperature provides the
most conspicuous
difference.
 Fronts lean, which
allows air masses to be
uplifted and adiabatic
cooling to take place.
 Lean so much, closer to
horizontal than vertical.
 Always slopes so that
warmer air overlies
cooler air.
 Fronts move in
association with the
16
direction of the more
active air mass, which

Fronts
Warm Fronts
 Warm Front—the leading edge of an advancing warm air
mass.
 Brings warm air.
 Results in clouds and precipitation, usually broad, protracted, and
gentle, without much convective activity.
 Unstable rising air can result in showery and even violent
precipitation.
 Weather maps show ground-level position of warm front;
precipitation usually falls ahead of this position.

17

Fronts
 Cold Fronts
 Cold Front—the leading edge of a cool air mass actively
displacing warm air mass.
 Brings cold air.
 Leads to rapid lifting of warm air, which makes it unstable and thus
results in blustery and violent weather along cold front.
 Weather maps show ground-level position of cold front (usually has
a protruding “nose”); clouds and precipitation tend to be
concentrated along and immediately behind the ground-level
position.

18

6


Fronts
Stationary Fronts
 Stationary Front—the
common “boundary”
between two air masses
in a situation in which
neither air mass displaces
the other.
 Occluded Front
 Occluded Front—a
complex front formed
when a cold front
overtakes a warm front.

19


Atmospheric Disturbances
Two types of
disturbances: stormy
and calm.
 Both types have
common characteristics:
1. Smaller than components
of general circulation, but
extremely variable in size;
Source: NOAA Photo Library
2. Migratory and transient; http://www.photolib.noaa.gov/collections.ht
3. Relatively brief in ml
duration;
4. Produce characteristic
and relatively predictable
weather conditions
20

 Midlatitude
Disturbances
 Many kinds of
atmospheric disturbances
are associated with
midlatitudes, which are
principal battleground for
tropospheric phenomena.
August 7, 2005
 Midlatitude cyclones and Source: http://www.nnvl.noaa.gov/
midlatitude anticyclones Middle Latitudes (35º  55º)
are more significant Battleground between
because of size and tropical and polar air masses
prevalence. 21

7

 Tropical
Disturbances
 Low latitudes are
characterized by
monotony of weather
with the same
consistent weather.
 The only breaks in this
pattern are provided
Katrina by transient
August 28, 2005
Source: NOAA, http://www.nnvl.noaa.gov/ disturbances such as
hurricanes. 22


 Localized Severe Weather
 Occur in many parts of the world.
 Constitute short-lived but severe weather phenomena
such as thunderstorms and tornadoes.
 Thunderstorms and Tornadoes

23

Midlatitude Cyclones
 Midlatitude cyclone—large migratory
low-pressure system that occurs within
the middle latitudes and moves generally
with the westerlies; also called lows or
wave cyclones, depressions.
 Probably most significant of all atmospheric
disturbances.
 Basically responsible for most day-to-day
weather changes.
 Bring precipitation to much of the world’s
populated regions.

24

8

 Characteristics
 Typical mature midlatitude
cycle is 1,600 kilometers
(1,000 miles) in diameter;
has oval shape.
 Patterns of isobars, fronts,
and wind flow in Southern
Hemisphere are mirror
images of those in
Northern Hemisphere.
 In Northern Hemisphere:
 Circulation pattern
converges
counterclockwise;
 Wind-flow pattern attracts
cool air from north and
warm air from south;
creates two fronts.

25

 These two fronts divide the cyclone into a cool
sector north and west of center and a warm
sector south and east.
 Size of sectors varies with location: on ground,
cool sector is larger, but in atmosphere, warm
sector is more extensive.
 Warm air rises along both fronts, causing
cloudiness and precipitation, which follows
patterns of cold and warm fronts.
 Much of cool sector is typified by clear, cold,
stable air, while air of warm sector is often moist
and tending toward instability, so may have
sporadic thunderstorms. May have squall fronts
of intense thunderstorms.
26

 Weather Changes with a
Passing Front
 With a passage of a cold front,
the following changes typically
occur:
 The temperature decreases
sharply.
 Winds shift from southerly
ahead of the front to
northwesterly following it (in
the Northern Hemisphere)
 The pressure falls as the front
approaches and then rises
after it passes.
 Generally clear skies are
replaced by cloudiness and
precipitation at the front. 27

9

• Movements
– Midlatitude cyclones
move throughout their
existence.

28

Midlatitude Anticyclones
 An extensive migratory
high-pressure cell of the
midlatitudes that moves
generally with the
westerlies.
 Characteristics
 Typically larger than a
midlatitude cyclone, but also
moves west to east.
 Travels at same rate, or little
slower, than midlatitude
cyclone.
 Is prone to stagnate or
remain over same region
(while cyclones do not).
 Can cause concentration of
29
air pollutants.

• Relations of Cyclones and Anticyclones
– Often occur in next to each other in midlatitudes
– Anticyclone forms a cold front on its leading edge

• Fig. 7-18

30

10

Major Tropical Disturbances:
Hurricanes

 Tropical cyclone—a storm most significantly
affecting the tropics and subtropics, which is
intense, revolving, rain-drenched, migratory,
destructive, and erratic. Such a storm system
consists of a prominent low-pressure center that
is essentially circular in shape and has a steep
pressure gradient outward from the center.
 Tropical cyclones provide the only break in weather
in low latitudes.
 Also called
1. Hurricanes in North and Central America
2. Typhoons in western North Pacific
3. Baguios in Philippines
4. Tropical cyclones in Indian Ocean and Australia

31

Hurricane Katrina,
August 29, 2005
• Fig. 7-20 32

Hurricanes
 Having diameters of between 160
and 1000 kilometers, tropical
cyclones are smaller than
midlatitude cyclones.
 Three categories of tropical cyclones:
1. Tropical depression—winds of 33 knots
(61 kilometers or 38 miles) per hour or
less.
2. Tropical storm—winds between 34 and
63 knots ([63 and 117 kilometers] or [39
and 73 miles]) per hour.
3. Hurricane—winds of 64 knots (119
kilometers or 74 miles) per hour or more;
can double and even triple that
minimum.

33

11

Hurricanes
 Characteristics
 The hurricane pulls in warm, moist air for fuel,
and this air rises and cools adiabatically.
 This causes condensation and this in turn
releases heat, which further increases the
instability of the air.

34

Hurricanes
 Eye of a Hurricane
 Eye—the nonstormy center of a tropical cyclone, which has a
diameter of 16 to 40 kilometers (10 to 25 miles). In the eye, there
are no updrafts, but instead a downdraft that inhibits cloud
formation.
 Eye wall—peripheral zone at the edge of the eye where winds
reach their highest speed and where updrafts are most
prominent.
 Weather pattern within a hurricane is symmetrical.
 Comprised of bands of dense cumulus and
cumulonimbus clouds called spiral rain bands.
 Eyewall replacement—the process in which a new wall of
storms surrounds the wall of storms circling the hurricane’s eye.
When this occurs, the inner wall disintegrates so the new wall
replaces it. This process tends to weaken the storm
35

Hurricanes
 Origin
 Form only over warm oceans and where there is
no significant wind shear.
 Coriolis effect plays key role: it’s at minimum at
equator, and no hurricane has been observed to
form within 3˚ of equator or cross over it.
 Rare to have hurricane closer than 8˚ to 10˚ of
equator.
 The exact mechanism of formation is not clear,
but they always grow from some preexisting
disturbance 36

12

Hurricanes
 Movement
 Most common in North Pacific basin
(origination in Philippines and west of
southern Mexico and Central America):
 West central portion of the North Atlantic
basin, extending into Caribbean and Gulf of
Mexico is third in prevalence.
 Totally absent from the South Atlantic and from the
southeastern part of the Pacific.
 Absent apparently because the water is too cold and
because high pressure dominates.
37

•
Hurricanes
General pattern of movement is highly predictable:
– About one-third travel east to west without much latitudinal change.
– About two-thirds start off on an east–west path and then curve poleward.
• Exception occurs in southwestern Pacific Ocean north and northeast of New
Zealand, where general circulation pattern steers hurricanes, so they travel west
to east.
• Average hurricane lasts a week; those that remain over tropical oceans
can live up to 4 weeks.
– Dies down over continents because energy source of warm, moist air is cut
off.
– Dies down in midlatitudes because cooler environment.
• In midlatitudes, can diminish in intensity but grow in size and become a
midlatitude cyclone.

38

Hurricanes
 Damage and Destruction
 High seas, or storm surge cause the most damage.
 Storm size is key to how much damage is
caused, then physical configuration of
landscape and population size and density of
affected area.
 Saffir-Simpson Hurricane Scale has been established
to rank the intensity of hurricanes.
 Ranges from 1 to 5, with 5 being the most
severe

39

13

Saffir-Simpson Hurricane Scale

40

Source Areas

41

Pattern of Movement

42

14

– Greatest disasters
• Galveston, TX (1900)
• Ganges-Brahmaputra delta
• U.S. Gulf Coast (Katrina, 2005)
– Fig. 7-27

43

• Hurricanes and Global Warming
– Number of hurricanes increasing

• Fig. 7-C

44

– Intensity of hurricanes increasing

• Table 7-A

45

15

• Hurricane Wilma, strongest North Atlantic-Caribbean
hurricane on record

• Fig. 7-D, Hurricane Wilma (Oct. 20, 2005)
46

 Localized Severe Weather

• Occur on a more
localized scale than
do tropical and
midlatitude cyclones
and anticyclones.

• Fig. 7-31

47

Thunderstorms
 Thunderstorm—violent convective storm
accompanied by thunder and lightning; usually
localized and short lived.
 Vertical air motion, considerable humidity, and
instability combine to create towering cumulonimbus
clouds, so thunderstorms are always associated with this
combination.
 Frequently occur in conjunction with other kinds of
storms (hurricanes, tornadoes, fronts [especially cold
fronts]) in midlatitude cyclones, and orographic lifting.
 Associated with other mechanisms that can trigger
unstable uplift.

48

16

Thunderstorms
 Mechanism triggers uplift of warm, moist
air.
 Cumulus stage—updrafts prevail and clouds
grow. Rise to above freezing level, where
supercooled water droplets and ice crystals
coalesce, then fall. Initiate a downdraft.
 Mature state—updrafts and downdrafts coexist
as cloud continues to enlarge (but precipitation
is leaving bottom of cloud). Most active time.
 Dissipating state—downdrafts dominate, and
turbulence ceases.
 Virtually unknown poleward of 60˚ of latitude.

49

– Sequential Development

• Fig. 7-28

50

– Frequency by latitude (per year)

• Fig. 7-29

51

17

– Frequency of hailstorms in the United States (per year)

• Fig. 7-30
52

Lightning
 More than 8.5 million lightning
bolts daily in world.
 Most frequently, lightning
occurs as exchanges between
adjacent clouds or between
the upper and lower portions
of the same cloud; it also
occurs as an electrical
connection of ionized air from
cloud to ground.
 The sequence that leads to
lightning discharge is known,
but the mechanism for
electrification is not.

53

Lightning
 Sequence:
 Large cumulonimbus cloud
experiences a separation of
electrical charges.
 Positively charged particles are
mostly high in cloud, while
negatively charged particles tend
to concentrate in base.
 Growing negative charge in base
attracts a growing positive charge
on Earth’s surface immediately
below cloud.
 An insulating barrier lies between
cloud base and surface.
 Contrast between cloud base and
surface builds to tens of million
volts and overcomes the insulating
barrier.
 Finger of negative current flicks
down from cloud and meets a
positively charge darting upward
from the ground, causing lightning.
54

18

Lightning
 Cause is unknown;
different theories.
 Most popular theory:
updrafts carry
positively charged
particles to top, while
falling ice pellets
gather negative
charges and transport
them downward.
55

Thunder

 Thunder—an instantaneous expansion of air caused
by the abrupt heating that lightning bolt produces.
This expansion creates a shock wave that becomes
a sound wave.
– Can time the distance that lightning is away because of the
different rates thunder and lightning travel at (speed of
sound vs. speed of light).
– Five-second interval equals about a mile; three-second
interval equals about a kilometer.

56

Tornadoes

57

19

Tornadoes
 Tornado—a localized cyclonic low-pressure cell surrounded by a
whirling cylinder of wind spinning so violently that partial vacuum
develops within the funnel.
 Has the most extreme pressure gradients known (as much as 100-
millibar difference between tornado center and air immediately
outside funnel).
 Extreme pressure difference produces winds of extraordinary
speed.
 How fast are winds?
• No one knows, because tornadoes blow to bits the anemometer
(instrument for measuring speed). Maximum estimates range from 320
to 800 kilometers (200 to 500 miles) per hour.
 The strength of a tornado is described using the Fujita tornado
intensity scale (Table 7-3).
58

– Classification
• Fujita tornado intensity scale

– Table 7-3 59

Tornadoes
 Formation
 Exact mechanism of formation is unknown.
 Usually develops in warm, moist, unstable air associated
with midlatitude cyclone.
 High wind shear (horizontally rotating air) may cause
strong updrafts to form in a supercell thunderstorm.
 The rotating air may then be tilted vertically forming a
mesocyclone.
• About 50% of all mesocyclones formed result in tornadoes.
 Waterspouts occur over ocean; have less pressure
gradient, gentler winds, and reduced destructive
capability.
60

20

Tornadoes: Formation
(con’t)
 Most often develops along a squall line that preceded
a rapidly advance cold front, or along the cold front.
 Spring and early summer are favorable for
development because there’s considerable air-mass
contrast present in the midlatitudes at that time.
• Most occur in midafternoon, at time of maximum heating.
• More than 90% of all reported tornadoes occur in United
States,
• Reflects optimum environmental conditions:
• Relatively flat terrain of central and southeastern U.S. provides
uninhibited interaction of Canadian cP and Gulf mT air masses.

61

Tornadoes

 Waterspouts occur over ocean; have
less pressure gradient, gentler winds,
and reduced destructive capability

62

21

Ch 7

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