Transcript Fig. 19-25, p.465

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Chapter 19
Objectives:

Describe how humidity is
measured.
Explain how
condensation occurs.
List and describe the
three basic types of
clouds.
Discuss how winds form.
Explain vertical and
horizontal air motions.
Describe frontal weather.
Describe long and short
term weather
disturbances.
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Fig. 19-CO, p.446
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Moisture in the Air:
Warm air can hold more
water vapor than cold air.
Humidity is the amount of
water vapor in the air.
Absolute humidity is the
mass of wv in a given
volume of air (g/cubic
meter); relative humidity is
the amount of wv in air
relative to the maximum it
can hold at a given
temperature.
If you cool air with a given
amount of wv, does RH
increase or decrease?
When relative humidity
reaches 100% the air is
saturated (it has reached its
dew point).
Fig. 19-1, p.447
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Cooling and Condensation:
Three atmospheric processes cool air to its dew point and cause
condensation: radiation cooling, contact cooling and cooling of rising air.
Ice crystals condense on a window on a frosty morning, an example of
contact cooling.
Fig. 19-2, p.448
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Most clouds form as rising air cools. The cooling causes
invisible water vapor to condense as visible water droplets or
ice crystals, which we see as a cloud.
Fig. 19-3, p.449
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Cooling of Rising
Air:
a rising air mass
initially cools
rapidly at the dry
adiabatic lapse
rate (10 C/1000
meters or 5.5
F/1000 ft). Then,
after condensation
begins, is cool
more slowly at the
wet adiabatic lapse
rate (variable but
less than dry rate).
Fig. 19-4, p.450
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Rising Air and Precipitation:
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Three mechanisms cause air to
rise and cool: a) orographic
lifting, b) frontal wedging and c)
convection-convergence.
Fig. 19-5, p.450
Fig. 19-5a, p.450
Fig. 19-5b, p.450
Fig. 19-5c, p.450
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Convective
Processes and
Clouds:
A) as dry air rises, it
expands and cools at
the DALR. When it
cools to temp of
surrounding air, it
stops rising.
B) as moist air rises,
it initially cools at
the DALR. It soon
cools to its dew
point and clouds
form. It then cools
more slowly (at the
WALR). As a result,
it remains warmer
than surrounding air
and rises for 1000s
of meters. It stops
rising when all
moisture has
condensed, and then
it again cools at the
DALR.
Fig. 19-6, p.451
Focus on Inversion Layers and Air Pollution.
Normally, the temperature in the atmosphere decreases with
altitude…
p.452a
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Here, polluted air
rises and mixes with
cooler air above and
disperses the
pollutants…
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At night, the ground
cools by radiation
and may be cooler
than the air above (a
stable
condition)…this is an
atmospheric or
temperature
inversion…
p.452b
Pollutants concentrate in the stagnant layer of cool air near the
ground…warm air cannot rise above the inversion layer.
Usually the morning Sun breaks the inversion (how?); or storm
winds can dissipate the layer.
p.453a
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Early morning inversion layer in Poland; clouds, steam and
pollutants are concentrated close to the ground.
p.453b
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Types of Clouds: Cirrus clouds are high, wispy clouds
composed of ice crystals. They form at 20-50,000 feet above
SL.
Fig. 19-7, p.453
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Stratus clouds spread out across the sky in a low, flat layer.
Form when condensation occurs at the same elevation at which
air stops rising. Can bring steady rain.
Fig. 19-8, p.454
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Cumulus clouds are fluffy white clouds that typically display
flat bottoms and billowy tops. Nimbo refers to a cloud that
precipitates. Cumulonimbus clouds are towering rain clouds,
commonly produce intense storms…see book for other names.
Fig. 19-9, p.454
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Cloud names are based on the shape and altitude of the clouds.
Fig. 19-10, p.454
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Types of Precipitation:
Rain
Snow, Sleet and Glaze
Hail
Snow blankets the ground
during the winter in
temperate regions. Snow
and ice cover the ground
year-round in the high
mountains and at the poles.
Fig. 19-11, p.455
Precipitation (cont)
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Rain: when clouds form there isn’t always rain. It takes about
1 million droplets to coalesce in a cloud to form one rain drop!
Large drops can form in towering clouds (where ice forms first,
and warms as it reaches the ground).
Snow: if the temperature of cloud formation is below freezing
and the temperature near the ground is below freezing.
Sleet/Glaze: Raindrops form in a warm cloud, and fall through a
layer of cold air at lower elevation. The drops freeze into small
sphere of ice (sleet). If the freezing zone is thin and the drops
don’t have time to freeze, but land on subfreezing surfaces, can
form a coating of ice (glaze)…this can be very dangerous.
Hail: vary in size from 5mm in diameter to 14 cm in diameter
(weighing 1.5 pounds!)…fall only from cumulonimbus clouds.
Hailstones are layered.
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Glaze forms when rail falls on
a surface that is colder than
the freezing temperature of
water.
Fig. 19-12, p.456
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Fog: fog is a cloud that forms at or near the ground, but by
processes different than those that create higher level clouds.
Advection (warm, moist air from sea blows onto cool land),
Radiation (Earth’s surface and air near surface cools at night),
Evaporation (air cooled by evaporation from water) and
Upslope fog (when air cools as it rises along land surface).
Fig. 19-13, p.457
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Pressure and
Wind
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Warm air rises (low
pressure region), cool
air sinks (high
pressure region).
Wind is generated.
Winds near the Earth’s
surface always flow
away from a region of
high pressure and
toward a low-pressure
region, and are caused
by pressure
differences from
unequal heating of the
atmosphere.
Fig. 19-14, p.458
Fig. 19-14a, p.458
Fig. 19-14b, p.458
Fig. 19-15, p.459
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Winds blow, as stated
earlier, in response to
differences in pressure.
Fig. 19-16, p.459
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Wind speed
determined by
magnitude of
pressure differences
over distance, called
pressure gradient.
Points of equal
pressure are
connected by map
lines called isobars.
Note in this pressure
map steep pressure
gradients in the NE
and NW cause high
winds that spiral CCW
into the low pressure
zones.
Fig. 19-17, p.459
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The Coriolis Effect
deflects winds to the right
in the NH (and to the left
in the SH). Only winds
blowing due east or west
are unaffected.
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Friction also plays a role,
as wind speed generally
increase with elevation.
Fig. 19-18, p.460
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Cyclones and
Anticyclones.
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A) in the NH, a
cyclone consists of
winds spiraling
CCW into a low
pressure region.
B) an anticyclone
consists of winds
spiraling CW out
from a highpressure zone.
What type of
weather
predominates
these zones and
why?
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Fig. 19-19, p.461
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Fronts and
Frontal
Weather:
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Air masses are
classified by their
source region.
Polar (P) am
originate in the
high latitudes and
are cold; Tropical
(T) am originate in
the low latitudes
and are warm;
Continental (C) am
originate over land
(dry); Maritime (M)
am originate over
water and are
moist.
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Fig. 19-20, p.462
Table 19-1, p.462
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Warm Fronts and Cold Fronts: a warm front forms when
moving warm air collides with stationary (or slow moving)
cold air; a cold front forms when moving cold air collides with
stationary or slower moving warmer air.
Above are symbols commonly used in weather maps.
Fig. 19-21, p.463
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In a warm front, moving warm air rises gradually over cold
air and cools adiabatically, generating clouds and usually light
precipitation.
Fig. 19-22, p.463
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In a cold front, moving cold air slides abruptly beneath warm
air, forcing it to rise steeply upwards; leading edge of a cold
front is much steeper than a warm front, causing warm air to
rise rapidly, creating a narrow band of violent weather.
Fig. 19-23, p.463
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An occluded front forms where warm air is trapped and lifted between two
cold air masses. What happens here? Inclement weather from two fronts,
generally short-lived (warm air mass cut off from moisture evaporating from
Earth’s surface).
A stationary front is boundary between two stationary air masses. Can last
for several days, similar to warm front weather (rain, drizzle, and fog may
Fig. 19-24, p.464
occur).
Fig. 19-24a, p.464
Fig. 19-24b, p.464
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Middle-Latitude Cyclone…
Fig. 19-25, p.465
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A front develops…
Fig. 19-25a, p.465
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A small disturbance creates a kink in the front…
Fig. 19-25b, p.465
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A low pressure region and cyclonic circulation
develop…
Fig. 19-25c, p.465
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An occluded front forms.
Fig. 19-25d, p.465
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Most North American cyclones follow certain
paths called storm tracks from west to east
(from jet stream and westerlies)…
Fig. 19-26, p.466
Mountains, Oceans, Lakes and Weather
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Mountain ranges and rain-shadow deserts.
Fig. 19-27, p.466
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Rain-shadow
deserts lie east of
the California
mountain ranges.
Rainfall is shown in
cm/yr.
Fig. 19-28, p.467
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Sea and Land Breezes: a) sea breezes blow inland during the day. b) land
breezes blow out to sea at night.
A monsoon is a seasonal wind and weather system caused by uneven
heating and cooling of continents and oceans. More than half the
inhabitants of the Earth depend on monsoons because the predictable heavy
summer rains bring water to the fields of Africa and Asia.
Fig. 19-29, p.467
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Thunderstorms: a typical t-storm develops in three stages:
1) air rises, cools and condenses, creating a cumulus cloud; 2)
latent heat of condensation energizes the storm, forming
heavy rain and violent wind; 3) the cloud cools, convection
weakens and the storm wanes.
Fig. 19-30, p.468
Fig. 19-31, p.469
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Two hypothesis for the
origin of lightning: 1)
friction between intense
winds and ice particles
generates charge
separation; 2) charged
particles are produced from
above by cosmic rays and
below by interactions with
the ground. The particles
are then distributed by
convection currents.
Fig. 19-32, p.470
Fig. 19-32a, p.470
Fig. 19-32b, p.470
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Tornadoes: most violent of all storms. Protrude from cumulonimbus clouds,
and can travel 40-65 km/hr (even 110 km/hr). 75% are concentrated in the
Great Plains (east of the Rockies), 700-1000 occur in the U.S. each year.
During Spring/Early Summer, cold air from Canada collides with warm air
from the Gulf of Mexico.
Fig. 19-33a, p.470
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Tornadoes form Hurricane Andrew destroyed homes in LaPlace,
Louisiana (1992).
Fig. 19-33b, p.470
Fig. 19-34, p.471
Table 19-2, p.471
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Tropical Cyclone: hurricane in North America and the Caribbean, typhoon in
western Pacific and cyclone in Indian Ocean). Avg. 600 km in diameter, persist for
days or weeks. Low pressure in center of a hurricane generate winds of 120-300
km/hr. LP can raise sea level several meters and cause a storm surge. Form only
over warm oceans (need moist, warm air). In late summer, tropical air rises creating
a belt of LP over the tropics. A disturbance can create spiraling winds (inward);
warm, moist air rises, condenses, latent heat is released causing more air to rise,
more winds blow in, more condensation and precipitation occur and more heat is
released…the eye is where vertical air flow occurs (calm)…hurricanes then are
powered by latent heat, pushed by prevailing winds and deflected by Coriolis effect.
How do they dissipate?
Fig. 19-35, p.472
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Hurricane Origins:
 About 84 tropical cyclones (hurricanes, typhoons,
cyclones) form each year
– About 10 in north Atlantic Ocean, Caribbean Sea, Gulf of
Mexico
Hurricane Origins
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Strength assessed by Saffir-Simpson scale
– Category 1: wind damages trees and unanchored mobile homes
– Category 2: winds blow down trees, major damage to mobile
homes, some roofs
– Category 3: winds blow down large trees, strip foliage, destroy
mobile homes, damage small buildings
– Category 4: all signs blown down, heavy damage to buildings,
major damage to coastal buildings, flooding extends inland
– Category 5: severe damage to buildings, major damage to
buildings less than 5 m above sea level and within 500 m of
shoreline, small buildings overturned and blown away
Table 19-4, p.473
19.11 Hurricane Katrina
Formed 8/23/2005 – passed over FL as a
Category 1
 Grew to a category 5 in Gulf of Mexico
 Made landfall as a 3-4
 8.5m storm surge inundated New Orleans
and the MI coast
 Death toll between 1,300 and 4,000
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19.11 Hurricane Katrina
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Gulf coast and hurricanes
– Long history of tropical storms
– 10 deadliest storms – before 1955
– 10 costliest – after 1955, more than half after
1989
 More people live on coasts now – property damage
 Earlier warning – people get out of the way more
efficiently
– So why was Katrina so devastating?
19.11 Hurricane Katrina
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Barrier islands, costal wetlands, delta lands,
all absorb a storm’s energy
– Over the past century, 1/3 of the coastal
wetlands has disappeared
– Human activity has reduced sediment flow to
the delta (chap 11)
– The delta is massive enough to depress the
crust – less sediment leads to net subsidence
– Canals have killed plants with salt water
19.11 Hurricane Katrina
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Levee failure – portions of the levee system
were built atop peat soils
– Peat is weak and transmits water readily
– Lead to catastrophic failure
 Add in that much of the city is below sea level
– 2005 had a record number of tropical storms
and hurricanes
 A portent of things to come?
Fig. 19.35, p.498
Fig. 19.34a, p.496
Fig. 19.34b, p.496
Table 19.4a, p.497
Table 19.4a, p.497
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Since 1900, about 1/3 of wetlands (shown in blue) have been lost to
erosion, subsidence and dredging of canals.
Fig. 19.36, p.498
Fig. 19.37, p.499
Fig. 19.38a, p.500
Fig. 19.38b, p.500
19.12 El Niño
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An anomalous current that brings warmer
water to the west cost of South America
– Occurs every 3-7 years for a year
– Weakens trade winds which reduces cold
upwelling
 This creates warmer surface water
 Depresses fisheries
 Changes weather patterns in may places
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El Nino: in a normal year, trade winds drag warm surface
water westward across the Pacific and pile it up in a low mound
near Indonesia and Australia, where the warm water causes
rain. The surface flow creates upwelling of cold, deep,
nutrient-rich waters along the coast of SA.
Fig. 19-36, p.474
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In an El Nino year, the trade winds slacken and the warm water
flows eastward toward SA, causing storms and rain to move
over SA, and diminishing the upwelling currents.
Fig. 19-37, p.475
p.477