Download Ch.8 Weather - UNT Geography

Chapter 8 Weather
Learning Objectives
•  Define the concept of air mass and describe air masses
that affect North America, and relate their qualities to
source regions.
•  Identify and describe four types of atmospheric lifting
mechanisms.
•  Explain the formation of orographic precipitation, and
review an example of orographic effects in North
America.
•  Identify various forms of violent weather by their
characteristics (thunderstorms, tornados, hurricanes),
and review several examples of each.
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Weather and Climate
•  Weather is the short-term, day-to-day
condition of the atmosphere.
•  Weather is a “snapshot” of atmospheric
conditions.
•  Climate is the long-term average (e.g.,
over decades) of weather conditions and
extremes in a region.
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Elements Contributing to Weather
•  Temperature
•  Air pressure
•  Relative humidity
•  Wind speed and direction
•  Seasonal factors such as insolation and
Sun angle
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Air Masses and Earth’s Surface
•  Each area of Earth’s surface imparts its
temperature and moisture characteristics
to overlaying air.
•  The effect of the surface on the air creates
regional air masses with a homogenous
mix of temperature, humidity, and stability.
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Air Masses
•  Air mass is a distinctive body of air with
homogenous mix of temperature,
humidity, and stability, and it initially
reflects the characteristics of its source
region.
•  The interactions of air masses produce
weather patterns.
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Classification of Air Masses
•  Classifying air masses according to the
moisture and temperature
characteristics of their source regions:
•  Moisture – “m” for maritime (wet) and
“c” for continental (dry)
•  Temperature – “A” for arctic, “P” for
polar, “T” for tropical, “E” for
equatorial, and “AA” for Antarctic.
•  mT: maritime tropical
cP: continental polar
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Moist Air Density
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Density=mass/volume
Molecular weight of dry air Md=29
Molecular weight of H2O Mw=18
Density of air is proportional to RH*Mw+(1-RH)* Md
RH is relative humidity between 0 and 1.
Moist Air
Dry Air
water
vapor
RH=0.7
Water vapor
RH=0.2
Density ~ 0.7*18+(1-0.7)*29=21.3 < Density ~ 0.2*18+(1-0.2)*29=26.8
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Moist air is less dense than dry air!
Air Masses Affecting North America
in Winter
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Global January Barometric Pressure
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Air Masses Affecting North America
in Summer
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Global July Barometric Pressure
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Continental Polar (cP)
•  Continental polar (cP) air masses form
only in the Northern Hemisphere and are
most developed in winter and cold-weather
conditions.
•  The cold, dense cP air displaces moist and
warm air in its path, producing lifting,
cooling, and condensation.
•  An area covered by cP air in winter
experiences cold, stable air, clear skies,
high pressure, and anti-cyclonic wind flow.
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Maritime Polar (mP)
•  Maritime polar (mP) air masses in the
Northern Hemisphere exist over the
northern oceans.
•  Within mP air masses, cool, moist,
unstable conditions prevail throughout
the year.
•  The Aleutian and Icelandic sub-polar lowpressure cells reside within these mP air
masses, especially during winter.
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Maritime Tropical (mT)
•  Two maritime tropical (mT) air masses—
the mT Gulf/Atlantic and the mT Pacific—
influence North America.
•  The mT Gulf/Atlantic air mass is unstable
and active from late spring to early fall.
•  The mT Pacific is stable to conditionally
unstable and generally lower in moisture
content and available energy than the
mT Gulf/Atlantic air mass.
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Ocean surface current
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Air Mass Modification
•  As air masses migrate from source
regions, their temperature and moisture
characteristics modify and slowly take on
the characteristics of the land over which
they pass.
•  A typical example is the lake-effect snow
belt of the Great Lakes.
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Lake-Effect
Snowbelts
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Four Atmospheric Lifting Mechanisms
•  To form precipitation, air masses must lift
and rise in altitude to reach the dew-point
temperature, condense, and form clouds.
•  Convergent lifting
•  Convectional lifting
•  Orographic lifting
•  Frontal lifting (cold and warm fronts)
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Convergent Lifting
•  Air flows toward an area of low pressure.
•  Low pressure center: air converging and ascending,
cooling and condensation occurring.
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Convectional Lifting
•  The air above the warmer surfaces is heated and rises.
•  The warmer surfaces produce convectional lifting
due to local heating.
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Convectional Lifting Process
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Orographic Lifting
•  Oro means “mountain.”
•  Orographic lifting occurs when air is forced to ascend
upslope as it is pushed against a mountain.
Dry, hot
Chinook wind
Moist, warm
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Rain shadow
Rain Shadow
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Frontal Lifting
•  A front is the transition zone between two air
masses of different densities. Since density
differences are most often caused by temperature
differences, fronts usually separate air masses
with contrasting temperatures.
•  The leading edge of a cold air mass is a cold front.
•  The leading edge of a warm air mass is a warm
front.
Cold Front
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Warm Front
Cold Front
•  Cold air forces
warm air aloft.
•  400 km wide
•  Precipitation
behind the cold
front
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Warm Front
• Warm air moves up and over cold air.
• 1000 km wide
• Precipitation ahead of the warm front
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Violent Weather
•  Thunderstorms
•  Tornadoes
•  Tropical cyclones
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Thunderstorms
•  The energy source of thunderstorms is from the condensation of
a large amount of water vapor in clouds.
•  Thunderstorms are associated with heavy precipitation, lightning,
thunder, hail, strong winds, and maybe tornadoes.
Average annual number of days experiencing thunderstorm
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Tornado
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Tornado
Tornado Spatial and Temporal
Distribution in the United States
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Tropical Cyclones
•  The tropics extend from 23.5°N to 23.5°S.
•  About 80 tropical cyclones occur annually
worldwide.
•  About 45 tropical cyclones are powerful
enough to be classified as hurricanes (North
America), typhoons (台⻛风) (China, Japan,
Philippines), and cyclones (Australia, India,
Indonesia) per year.
•  The warmer the ocean and the atmosphere,
the more powerful the storm.
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Tropical Cyclone Classification
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The Mechanism of Tropical Cyclones
Sea-surface temperature must
be greater than 26°C for triggering
a tropical cyclone.
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Profile of a Hurricane
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Physical Structure of a Hurricane
•  The central area of a hurricane is called the eye,
which is quiet and warm.
•  Around the eye swirls a thunderstorm cloud called
the eyewall, which is the area of most intense
precipitation.
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Worldwide Pattern of the Most Intense
Tropical Cyclones
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Hurricanes Gilbert (Northern Hemisphere)
and Catarina (Southern Hemisphere)
Tropical Cyclones
Count-clockwise rotation in northern hemisphere
Clockwise rotation in southern hemisphere.
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Storm Surge and High Waves
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Hurricane Katrina, August 29, 2005
•  One of the five deadliest hurricanes in the history of the
United States.
•  At least 1800 people lost their lives in the hurricane and
the subsequent floods.
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Summary of Chapter 8
•  Weather is the short-term condition of the atmosphere.
Meteorology is the scientific study of the atmosphere.
•  An air mass is a regional volume of air that is homogenous in
humidity, stability, and cloud coverage and that may extend
through the lower half of the troposphere.
•  Air masses are categorized by their moisture content—“m” for
maritime (wetter) and “c” for continental (drier)—and their
temperature, which is a function of latitude: “A” (arctic),
“P” (polar), “T” (tropical), “E” (equatorial), and “AA” (Antarctic).
•  There are four atmospheric lifting mechanisms: convergent
lifting, convectional lifting, orographic lifting, and frontal lifting.
•  A tornado is a violently rotating column of air in contact with the
ground surface.
•  A tropical cyclone becomes a hurricane, typhoon, or cyclone
when winds exceed 65 knots (119 kmph, 74 mph).
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