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Winds
Chapter 2, Section 3, p.46-52
What Is Wind?
• Air is a fluid so it can move easily
– Winds are caused by differences in air pressure
• Wind: horizontal movement of air from areas of high
pressure to areas of lower pressure
• Unequal heating of atmosphere causes differences in
air pressure
– Convection currents form when surface is heated by sun
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Air over heated surface expands – becomes less dense
Less dense air has less pressure
Cooler areas nearby have more dense air
Cooler, denser air flows under warmer, less dense air
Warm air forced to rise
Measuring Wind
• Winds are described by their direction and speed
– Wind vanes determine wind direction
• Spins and one end points into the wind
– Winds are named by where they come from
• North wind – blowing from north to south
• Southeast wind – blowing from southeast to northwest
– Anemometer: an instrument with 3-4 cups mounted
on the ends of spokes that spin on an axle and can
measure wind speed
• As wind speed increases; the anemometer will spin __?__
• As wind speed decreases; the anemometer will spin __?___
Wind-Chill Factor
• Wind-chill factor: the increased cooling a
wind can cause
– Wind blowing over your skin removes body heat
• The stronger the wind blows; the cooler/hotter you feel
• Weather report:
– “The temperature outside is 20°F, but the 30 mph wind
makes the wind-chill factor seem like 1°F
Local Winds
• Local winds are caused by the unequal
heating of Earth’s surface within a small area
– Local winds: winds that blow over short distances
– Local winds only form when large scale winds are
weak
Sea Breeze
• Unequal heating along the shore of a body of
water
– Sun heats Earth’s surface during day
• Land warms up faster than water
– More energy needed to heat up body of water than body of
land
• Air over land becomes warmer than air over water
• Warm air expands & rises creating low pressure
• Cooler, more dense air over water blows in under rising
air
– Sea breeze: a local wind that blows from an ocean or lake
Land Breeze
• Unequal cooling along the shore of a body of
water
– Land cools more quickly than water at night
• Warmer air over the water expands and rises
• Cooler, more dense air over land blows in under rising
air
– Land breeze: the flow of air from land to a body of water
Global Winds
• Global winds: winds that blow steadily from
specific directions over long distances
– Global winds are caused by the unequal heating of
Earth’s surface similar to local winds
– Global winds occur over a large area
• Sun’s radiation hitting Earth
– Most direct over equator
» Direct rays heat Earth’s surface intensely
» Temperatures are higher near equator
– Low angle at the poles
» Sun’s energy spread out over larger area heating surface less
» Temperatures are lower near poles
Global Convection Currents
• Temperature differences between the equator
and poles produce giant convection currents in
the atmosphere
– Global winds produced by this air movement pattern
• Difference in air pressure causes winds on Earth’s surface to
blow from poles to equator
– Warm air rises at equator
» Lower air pressure
– Cool air sinks at poles
» Higher air pressure
• Higher in atmosphere air flows away from equator
The Coriolis Effect
• Coriolis effect: the way Earth’s rotation makes
winds curve
– Global winds would move in a straight line from
poles to equator ff Earth didn’t rotate
• Earth rotates from west to east
• Making it seem like winds have curved
– Northern hemisphere – wind blowing south turns toward
southwest
» Facing equator – turns right
– Southern hemisphere – wind blowing north turns toward
northwest
» Facing equator – turns right
Global Wind Belts
• Pattern of calm areas and wind belts around
Earth
– Caused by Coriolis effect and other factors
– Calm areas:
• Doldrums
• Horse latitudes
– The major wind belts are:
• Trade winds
• Prevailing westerlies
• Polar easterlies
Global Winds
• Active Art Activity
– (cfp-4023)
Doldrums
• Sun heats surface strongly at equator
– Warm air rises creating area of low pressure
– Cool air moves into area but is heated before it
can move far
• Little horizontal movement of air
• Weak winds near equator
• Doldrums – regions with little to no win dnear the
equator
Horse Latitudes
• Horse latitudes – 30°N & 30°S latitudes
– Latitude: distance from the equator, measured in
degrees
– Warm air rises at equator flows north & south
– 30°N & S, air stops moving toward poles & falls
• Area of calm air located here
• Hundreds of years ago sailors became stuck in area
– Ran out of food and water for horses
– Threw them over board
– “Horse latitudes”
Trade Winds
• Winds blowing from 30°N & 30°S to equator
– Cold air over horse latitudes sinks producing area of
high pressure
– Causes surface winds to blow away from 30°N & S –
toward equator & away from it
• Winds blowing toward equator turned west by Coriolis effect
• Northern hemisphere – trade winds blow southwest
• Southern hemisphere – trade winds blow northwest
– Hundreds of years – sailors used these winds to carry cargo from
Europe to West Indies and South America
– Steady winds called trade winds
Prevailing Westerlies
• Winds between 30° and 60° N & S
– Winds blow toward poles
• Coriolis effect turns them east
• Westerlies because they blow from westerly direction
– Northern hemisphere – blow from southwest
– Southern hemisphere – blow from northwest
• Prevailing westerlies play important part in weather of
U.S.
Polar Easterlies
• Winds blowing from poles to 60°N or 60°S
– Cold air sinks at poles and flows toward lower
latitudes
– Coriolis effect shifts winds to the west = “polar
easterlies”
– Polar front
• Polar easterlies meet prevailing westerlies
• Warm & cold air mix
• Major effect on U.S. weather
Jet Streams
• Jet streams: bands of high speed winds 10 km
above Earth’s surface
– Hundreds of km wide
– Blow west to east
• 200 – 400 km / hr
– Wander north and south along wavy path
• Polar Jet Stream
• Subtropical Jet Stream
Homework
• Read Chapter 2, Section 3, p46-52
• Answer ?s 1-3 p52