Foundations of Earth Science
The Atmosphere in Motion
Chapter 13 Lecture
Natalie Bursztyn
Utah State University
Foundations of Earth Science
Eighth Edition
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Define air pressure.
Describe the instruments used to measure these weather elements.
Focus Questions 13.1
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Air pressure is the force exerted by weight of air above
Weight of the air at sea level
14.7 psi or 1 kg/cm2
Decreases with increasing altitude
Units of measurement
Millibar (mb)
Standard sea level pressure is 1013.2 mb
Inches of mercury
Standard is 29.92 inches of mercury
Understanding Air Pressure
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Understanding Air Pressure
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Understanding Air Pressure
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Instruments for measuring
Barometer
Mercury barometer
Invented by Torricelli in 1643
Uses a glass tube filled with mercury
Aneroid barometer
“Without liquid”
Uses an expanding chamber
Barograph
Continuously records the air pressure
Understanding Air Pressure
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Understanding Air Pressure
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Understanding Air Pressure
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Understanding Air Pressure
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Discuss the three forces that act on the atmosphere to either create or alter winds.
Focus Question 13.2
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Wind – horizontal movement of air
Out of areas of high pressure
Into areas of low pressure
Controls of wind
Pressure gradient force (PGF)
Isobars
Lines of equal air pressure
Pressure gradient
Pressure change over distance
Factors Affecting Wind
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Factors Affecting Wind
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Factors Affecting Wind
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Coriolis effect
Apparent deflection in wind direction due to Earth’s rotation
Deflection to the right in Northern Hemisphere
To the left in Southern Hemisphere
Friction
Only important near the surface
Acts to slow the air’s movement
Factors Affecting Wind
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Factors Affecting Wind
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Factors Affecting Wind
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Upper air winds
Generally blow parallel to isobars
Geostrophic winds
Jet stream
“River” of air
High altitude
High velocity (120 to 240 kph)
Factors Affecting Wind
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Factors Affecting Wind
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Contrast the weather associated with low-pressure centers (cyclones) and high-pressure centers (anticyclones).
Focus Question 13.3
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Cyclone
A center of low pressure
Pressure decreases toward the center
Winds associated with a cyclone
In the Northern Hemisphere
Inward (convergence)
Counterclockwise
In the Southern Hemisphere
Inward (convergence)
Clockwise
Highs and Lows
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Anticyclone
Winds associated with an anticyclone
In the Northern Hemisphere
Outward (divergence)
Clockwise
In the Southern Hemisphere
Outward (divergence)
Counterclockwise
Associated with subsiding air
Usually bring “fair” weather
Highs and Lows
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Highs and Lows
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Highs and Lows
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Highs and Lows
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Summarize Earth’s idealized global circulation.
Describe how continents and seasonal temperature changes complicate the idealized pattern.
Focus Questions 13.4
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Caused by unequal surface heating
3 pairs of atmospheric cells redistribute heat
Idealized global circulation
Equatorial low pressure zone
Rising air
Abundant precipitation
Intertropical convergence zone
Subtropical high pressure zone
Subsiding, stable, dry air
Near 30º latitude
Location of great deserts
General Circulation of the Atmosphere
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General Circulation of the Atmosphere
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Air traveling to equator from subtropical high produces the trade winds
Air traveling to poles from subtropical high produces the westerly winds
General Circulation of the Atmosphere
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Subpolar low-pressure zone
Warm and cool winds interact
Polar front: an area of storms
Polar high-pressure zone
Cold, subsiding air
Air spreads to equator and produces polar easterly winds
Polar easterlies collide with the westerlies along the polar front
General Circulation of the Atmosphere
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Influence of continents
– Seasonal temperature differences disrupt the
Global pressure patterns
Global wind patterns
– Influence is greatest in N. Hemisphere
Monsoon
Seasonal change in wind direction occurring over land
During warm months
– Air flows onto land
– Warm, moist air from the ocean
Winter months
– Air flows off the land
– Dry, continental air
General Circulation of the Atmosphere
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General Circulation of the Atmosphere
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The Westerlies
Complex pattern
Air flow is interrupted by cyclones
Cells move west to east in the N. Hemisphere
Create anticyclonic and cyclonic flow
Paths of cyclones and anticyclones are associated with the upper-level airflow
General Circulation of the Atmosphere
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List three types of local winds.
Describe their formation.
Focus Questions 13.5
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Produced from temperature differences
Small scale winds
Land and sea breezes
Mountain and valley breezes
Chinook and Santa Ana winds
Local Winds
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Land and sea breezes
A sea breeze develops because cooler air over the water moves toward the land
Reaches greatest intensity during the mid- to late afternoon
At night it reverses, and a land breeze develops
Local Winds
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Mountain and valley breezes
Air on mountain slopes is heated more than air at the same elevation over the valley floor
Glides upslope and generates a valley breeze
Cool air is denser than warm air and drains downslope into the valley as a mountain breeze
Local Winds
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Chinook and Santa Ana winds
Chinooks
Warm, dry winds moving down the east slopes of the Rockies
Santa Anas
Chinook like wind that occurs in southern California
Local Winds
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Local Winds
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Describe the instruments used to measure wind.
Explain how wind direction is expressed using compass directions.
Focus Questions 13.6
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Two basic measurements:
Direction
Winds are labeled from where they originate
North wind blows from the north
Instrument for measuring wind direction is the wind vane: direction indicated by either
Compass points
Scale of 0º to 360º
Prevailing wind comes more often from one direction
Speed
Often measured with a cup anemometer
Measuring Wind
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Measuring Wind
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Changes in wind direction
Associated with locations of
Cyclones
Anticyclones
Often bring changes in
Temperature
Moisture conditions
Measuring Wind
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Measuring Wind
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Discuss the major factors that influence the global distribution of precipitation.
Focus Question 13.7
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Regions influenced by high pressure experience relatively dry conditions
Regions influenced by low pressure receive ample precipitation
Tropical regions (equatorial low) are the rainiest
Subtropical deserts (subtropical high) are arid
Global Distribution of Precipitation
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Other factors influencing precipitation
Nature of the air
Moisture capacity
Latitude
Distribution of continents and oceans
Distribution of mountains
Global Distribution of Precipitation
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Global Distribution of Precipitation
[insert Figure 13.24 here]
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