The Atmosphere in Motion

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The Atmosphere in Motion

• Chapter 19

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Air Pressure Wind section 1

• Wind
• Horizontal movement of air
• Helps moderate surface temperatures
• Distributes moisture
• cleanses the atmosphere
• Several forces affect direction of movement
• Caused by differences in air pressure

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What is Air Pressure?

• Weight of the air pushing down on Earths surface
• At sea level 14.7 pounds per square inch (psi)
• At sea level, the barometric pressure is 29.92
  inches or 1013.2 millibars (mb).
• As increase elevation, pressure decreases b/c
  less air above
• Decreases 50 every 5 km
• Exerted in all directions

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Air Pressure With Altitude
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MercuryBarometer

• Weight of column of mercury is balanced by the
  pressure exerted on the dish of mercury from air
  above

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Aneroid Barometer Barograph

• Without liquid
• Partially evacuated metal chamber that compresses
  or expands based on outside pressure

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Recording Air Pressure

• Different units can be used to express air
  pressure
• With a mercury barometer
• Inches
• Millimeters
• Meteorologists use
• millibars

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You can easily change from inches to millibars
29.92 inches
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Why Does Air Pressure Change?

• Elevation
• As altitude increases, pressure decreases
• Temperature
• As temperature increases, pressure decreases
• Molecules move further apart as air is heated
• So fewer air molecules than in same volume of
  cool air
• Warm air ? lower pressure, cold air ? high
  pressure
• Humidity
• As humidity increases,
• pressure decreases
• Water molecules have
• less mass than oxygen
• or nitrogen molecules

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Why Does Air Pressure Change?

• Changes in air pressure can aid in predicting the
  weather
• A decrease in pressure often indicates
  approaching warmer, more humid, air along w/ rain
  or snow
• Less dense air ? less pressure exerted
• An increase in pressure often indicates
  approaching cooler, drier air fair weather
• More dense air ? more pressure exerted

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Why Does Air Pressure Change?

• Meteorologists analyze air pressure by plotting
  isobars on weather maps
• Isobar line that joins points of equal
  barometric (air) pressure
• A closed isobar forms a loop on a weather map
• High-pressure area (high)
• Air pressure steadily increases toward the center
  of a set of closed isobars
• Think of a hill
• Low-pressure area (low)
• Air pressure steadily decreases toward the center
  of a set of closed isobars
• Think of a valley

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Isobars
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Pressure Gradient

• Pressure Gradient change in pressure
• change in distance
• The closer the isobars, the steeper the gradient
• Pressure changes quickly
• Faster, stronger winds
• The further the isobars, the more gentle the
  slope
• Pressure changes slowly
• Slower, weaker winds

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Differences in pressure caused by unequal heating
of Earths surface -Winds blow from areas of
High to Low pressure
What Makes the Wind Blow?
H L
WIND
Falling air Rising air (cold, dry
more dense) (warm, moist less
dense) Fair weather stormy weather High
Pressure Low Pressure
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Measuring Surface Wind Direction Speed

• Wind vanes ? instrument to determine the
  direction of wind
• Broad tail
• Resists wind
• Points away from where the wind is blowing from
• Arrowhead
• Points into the wind (where the wind is blowing
  from)
• Winds are named for the direction from which they
  blow from
• Examples
• Blow from west (to east) westerly (or west)
  wind
• Noreaster winds from the northeast

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Measuring Surface Wind Direction Speed

• Anemometer ? instrument used to measure wind
  speeds 10 meters above ground
• Effects on water, smoke, trees, other objects
  can also be used as estimates of wind speed.

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Factors Affecting Winds section 2

• The Coriolis Effect the tendency of an object
  (wind, ocean current, plane, etc.) moving freely
  over the earths surface to curve away from its
  path of travel
• Due to Earths rotation
• Northern Hemisphere ? deflect to right (from
  perspective of object)
• Blow clockwise out of areas of high pressure
• Blow counterclockwise into areas of low pressure
• Diagrams
• Southern Hemisphere ? deflect to left (from
  perspective of object)
• Does not depend on objects direction of movement
• Only noticeable on large scale (winds, planes,
  ocean currents)
• Greatest near poles, least near equator
• Increases if speed of object increases

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Coriolis Effect Animation
Coriolis Effect Wind Direction Visualiziation
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Friction

• Friction between the air ground slows winds
• Changes the impact of the Coriolis effect on
  surface winds
• More friction ? less deflection/curving
• Ex. rough land
• Less friction ? more deflection/curving
• Ex. smooth land or water
• Winds at higher altitudes ? less friction ?
  stronger Coriolis effect

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Friction
Jet Stream Video
Internet Investigation How Does the Jet Stream
Change Through the Year?

• Jet stream Band of very fast winds (120-240
  km/hr) near the top of the troposphere (hardly
  affected by friction)
• Typically 1000s of km long, 100s of km wide,
  about 1 km from top to bottom
• Polar-front jet stream cool polar air joins w/
  warmer air to the south
• Generally flows west to east
• Great effect on weather in the U. S.
• Energy for storms, directs storms paths
• Can reach to central FL in winter, generally over
  Canada and northern U.S. in summer
• Speed depends on pressure gradient in upper
  troposphere (depends on surface temps)
• Fastest in winter
• Tropical-easterly jet stream warm air in tropics
  of N. Hemisphere
• Weaker than polar-front jet stream

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Global Wind Patterns sec 3

• Affected by
• Unequal heating of Earth by sunlight (temp
  diff btw equator poles)
• Earth's rotation (spin) ( Coriolis effect)
• Location of continents
• Time of year
• Local topography (landforms)

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Global Wind Patterns

• What would happen if Earth did not rotate there
  was no Coriolis effect?
• The unequal heating
• makes the tropical equatorial regions warmer than
  the polar regions.
• lower pressure at the (warmer) equator
• Air rises moves toward poles
• higher pressure at the (cold) poles
• Polar air moves toward equator
• heats, rises, continues cycle
• Would result in one large circulation cell in
  each hemisphere

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Effects of Earths Rotation

• B/c Earth rotates
• Coriolis effect prevents air from flowing
  straight from equator to poles
• Air flowing northward from equator is deflected
  to right
• Air flowing southward from equator is deflected
  to left
• Air cools sinks long before reaches polar
  regions
• Air circulation is better represented w/ 3-cells
  in each hemisphere
• Idealized, not 100 accurate, but helpful in
  understanding global wind patterns

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Effects of Earths Rotation

• Three-Celled Circulation Model
• 3 circulation cells in each hemisphere
• 0 (equator)-30
• 30-60
• 60-90 (pole)
• Direction of circulation changes from each cell
  to the next
• Caused by alternating bands of high low
  pressure at Earths surface
• Polar front boundary at 60 where air flows
  away from high pressure poles collides w/
  warmer air moving up from lower latitudes
• As winds blow from high to low pressure, they are
  deflected by the Coriolis effect
• To right in northern hemisphere
• To left in southern hemisphere

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Weaknesses of the Three-Celled Model

• 3 main weaknesses
• Gives a simplified view of circulation btw 30
  60
• Referred to as middle latitudes or mid-latitudes
• Most of the U. S.
• Surfaces winds determined by locations of
  transient high- low-pressure systems
• Change often
• Does not take into account the effects of the
  continents (heat cool more rapidly than oceans)
  or seasons
• Based on a simplified view of upper-level winds
• Impression that generally travel N ? S
• Primarily westerly (except near equator, Coriolis
  is weak)

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Strength of the Three-Celled Model

• Fairly accurate image of general surface winds
  pressures outside the mid-latitudes
• Gives a picture of wind patterns pressure
  systems that is useful for climate studies
• b/c involves averaging patterns over long periods

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Description of Wind Pressure Belts

• Intertropical Convergence Zone (ITCZ) or
  doldrums a low pressure belt at the equator
  where winds from both hemispheres come together
• Little to no wind, hot humid, rain is common
• Tradewinds blow from the NE (N. Hemi) SE (S.
  Hemi) are found at about 30º N S
• Polar highs high-pressure regions where cold air
  sinks at the poles
• Polar easterlies surface winds at poles that
  blow from east
• Prevailing winds winds that usually blow from
  same direction
• Tradewinds
• Polar easterlies
• Prevailing westerlies which blow (SW in N. Hemi
  NW in S. Hemi) in the mid-latitudes.

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Polar Front
Polar Front
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Continental Local Winds Sec 4

• Because of tilt of Earth, relative position of
  sun changes over year
• Causes seasons
• Temperature changes
• Changes in global winds
• Also affected by positions of continents
• Highest temps in N. Hemi often N of equator
• Sun causes air to heat, rise, flow toward poles

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Effects of Seasons Continents

• Summer
• continents hotter than oceans
• heats up ( cools down) faster than water b/c
  absorbs ( radiates) heat better
• hot land heats air above it, becomes less dense,
  rises, causing low pressure
• Oceans cooler than land
• Heats up ( cools down) slower than land b/c does
  not absorb ( radiate) heat quickly
• Cool water, air above cooler, more dense, higher
  pressure
• Highs lows determine direction of prevailing
  winds at various locations
• Winter ? opposite from summer

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• Direction of winds change seasonally ? monsoons
• Most dramatic in southern Asia
• Winter ? cold, dry winds
• Summer ? warm, moist winds heavy rains

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Local Winds

• Extends 100 km or less
• Caused mostly by differences in temperature
• Examples
• Land sea breezes
• Mountain valley breezes

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Developing a Sea Breeze

• Daytime
• Land heats faster creating warm air above it.
• decreases the pressure ( density)
• air rises.
• low pressure develops over land
• Water heats slower, so it has cooler air above
  it.
• Increases pressure ( density)
• Air sinks
• High pressure develops over water causing a
  difference in pressure.
• Wind blows from high (sea) to low (land)

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Developing a Land Breeze

• Nighttime
• Water stays warm longer creating warm air above
  it.
• decreases the pressure ( density)
• air rises.
• low pressure develops over water
• Land cools faster, so it has cooler air above it.
• Increases pressure ( density)
• Air sinks
• High pressure develops over land causing a
  difference in pressure.
• Wind blows from high (land) to low (sea)

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