Download Lesson 15-3

Lesson 15-3
Global Winds &
Local Winds
Why Air Moves
• Wind-movement of
air caused by
differences in air
pressure
• Greater pressure
difference-faster
wind movement
• Fig. 1, p. 458
Air Rises at the Equator and
Sinks at the Poles
• Differences in air pressure – caused by unequal
heating of the Earth
• Equator-receives more direct solar energy than
other latitudes, so air is warmer & less dense and
rises to form area of low pressure
• This warm, rising air flows toward the poles
• Poles- air is colder, denser than surrounding air,
so it sinks, creating an area of high pressure
around poles.
• This cold, polar air flows toward the equator
• Together, these actions form pressure belts
around the earth that cause winds
Pressure Belts are Found
Every 30°
• Wind moves as air in many large, circular
patterns – convection cells
• Convection cells-separated by pressure
belts(bands of high pressure and low pressure at
every 30° of latitude
• Warm air rises over equator toward poles, then
cools
• At 30°N & 30°S latitude, some cool air sinks
forming high pressure belts moving towards
equator, then warms again (cycle)
• At 60°N & 60°S latitude, this warmer air rises to
create low pressure belts, flows back to poles
Pressure Belts
The Coriolis Effect
• Pressure differences cause
air to move between
equator & poles
• Since Earth is rotating,
winds don’t travel directly
N or S
• Coriolis effect-apparent
curving of path of winds &
ocean currents due to
Earth’s rotation
• Northern hemispherewinds traveling N curve to
the east, and winds
traveling S curve to the
west (Fig. 3, p. 460)
Global Winds
• Global winds-blow steadily from
specific directions over long
distances; caused by the
combination of convection cells found
at every 30°N latitude & the Coriolis
effect. (Fig. 4, p. 461)
Local Winds
• Local winds- blow over short
distances from any direction
• Local landforms/features-can
create temperature
differences producing winds
• Ex: Sea and land breezes;
mountain and valley breezes
• Mountain & valley- during the
day, sun warms air on
mountain slopes, it rises up
the mountain slope, creating
a valley breeze. At night, air
along slope cools, moves
down the mountain slope,
creating a mountain breeze.
• Fig. 6, p. 462
Lesson
16-1
Water in the
Air
The Water Cycle
• Water cycle-continuous movement of water from
sources on Earth’s surface into the air, onto and
over land, into the ground, and back to the
surface
• Weather – the condition of the atmosphere at a
certain time and place; it’s affected by the
amount of water in the air
• As water on Earth evaporates, it becomes water
vapor (moisture in the air).
• Humidity – the amount of water vapor in the air
• As evaporation increases-humidity of air
increases
The Water Cycle
• Air’s ability to hold water vapor changes as the
temperature changes (Fig. 2, p. 483)
• Humidity can be expressed as relative humidity
(amount of vapor in air compared to maximum
amount of vapor air can hold at a given
temperature; given as a percentage
• Saturated- relative humidity is 100%; when air
holds all of the vapor it can at a given
temperature
• Relative humidity – affected by (2) factors:
amount of vapor & temperature, which governs
the weather experienced on Earth
Lesson 16-2
Air Masses
Air Masses
• Changes in weather are caused by
movement & interaction of air masses
• Air mass-large body of air where
temperature & moisture content are
similar; characterized by moisture content
& temperature
• Moisture & content temperaturedetermined by area over which air mass
forms (source regions)
• Ex. Gulf of Mexico – warm, wet because
area is warm and has a lot of water vapor
Air Masses That Affect
Weather in N. America
• Maritime (m)-forms over water; wet
• Continental (c)-forms over land; dry
• Polar (P)-forms over the polar
regions; cold
• Tropical (T)-forms over the Tropics;
warm
• Describe: mp? mt? cp? ct?
• Fig. 1, p. 490
Air Masses
Air Masses that Influence
the U.S.
Cold Air Masses
• Most of cold winter weather in
US is due to (3) polar air
masses
– cP over Northern Canada;
brings extremely cold
weather(in summer-brings
cool, dry weather)
– mP over N. Pacific Ocean;
cool & very wet; brings rain &
snow to Pac. Coast in winter;
cool, foggy weather in
summer
– mP over N. Atlantic Ocean;
brings cool, cloudy weather &
precipitation to New England
in winter; cool, foggy weather
in summer
Warm Air Masses
• Most of warm weather in US
is due to (4) warm air masses
– mT over warm areas in the
Pac. Ocean (milder than mP
mass over the P.O.)
– mT over Gulf of Mex. & Atl.
Ocean-move north across
East Coast into Midwest.
Summer: bring hot & humid
weather, hurricanes,
thunderstorms, etc. Winter:
bring mild, cloudy weather
– cT over deserts of northern
Mexico & southwestern US.
Move northward bringing
clear, dry, hot weather in
summer
Fronts
• Front – the area in which two types of air
masses meet
• There are (4) types:
– Cold front
– Warm front
– Occluded front
– Stationary front
• Fronts are associated with weather in
middle latitudes
Fronts
• Cold Front– forms where cold air (more
dense) moves under warm air (less dense)
and pushes the warm air up
• Can move quickly & bring:
– Thunderstorms
– Heavy rain
– Snow
• Cooler weather usually follows it because
the air mass behind the cf is cooler and
drier than air mass it isreplacing
Fronts
• Warm Front – forms where warm air (less
dense) moves over cold air (more dense)
• This movement is gradual: it brings
– Drizzly rain
• Followed by clear and warm weather
Fronts
• Occluded Front – forms when a warm air
mass(less dense) is caught between two
colder air masses (more dense)
• The coldest air mass moves under &
pushes up the warm air mass.
• Coldest air mass then moves forward until
it meets another cold air mass that is
warmer & less dense
Fronts
• The colder of these two air masses pushes
up the warmer air mass
• Sometimes two cold air masses will mix
• Typically brings cool temperatures, rain,
and snow
Fronts
• Stationary Front– forms when a cold air
mass (more dense) meets a warm air
mass.
• Neither cold air mass has enough force to
lift the warm air mass over itself
• They stay separated
• Often brings many days of cloudy, wet
weather
Cold & Warm Fronts
References
• Holt Science & Technology: GA Earth
Science. pp. 458-463; 482-493.