Download Wind - UNT Geography

Chapter 6 Atmospheric and Oceanic
Circulations
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Learning Objectives
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Define the concept of air pressure.
Describe instruments used to measure air pressure.
Define wind.
Locate the primary high- and low-pressure areas and
principal winds.
•  Describe upper-air circulation, and define the jet
streams.
•  Explain the regional monsoons and several types of
local winds.
•  Sketch the basic pattern of Earth’s major surface ocean
currents and deep thermohaline circulation.
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Eruption of Mount Pinatubo
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Wind Essentials
Three levels of atmospheric circulation:
•  Primary circulation refers to general
global circulation.
•  Secondary circulation is related to
migratory high-pressure and low-pressure
systems.
•  Tertiary circulation includes local winds
and temporal weather patterns.
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Primary Circulation
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Secondary Circulation
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Tertiary circulation
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Meridional and Zonal Flows
•  Meridional flows move north or south
along meridians.
•  Zonal flows move east or west along
parallels of latitude.
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Meridional and zonal flows
•  Meridional flows move north or south
along meridians.
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Meridional and zonal flows
•  Meridional flows move north or south
along meridians.
•  Zonal flows move east or west along
parallels of latitude.
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Meridional and zonal flows
•  Meridional flows move north or south
along meridians.
•  Zonal flows move east or west along
parallels of latitude.
Meridional flows
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Zonal flows
Air Pressure
•  Atmospheric pressure is the force per unit
area exerted against a surface by the
weight of air above that surface.
•  Air pressure decreases with increasing
altitude.
•  Wind results from a horizontal difference in
air pressure.
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Atmospheric
Pressure
Profile
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Measure Air Pressure—Mercury Barometer
•  Seal the glass tube at
one end.
•  Fill with mercury.
•  Invert into a dish of
mercury.
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Measure Air Pressure—Mercury Barometer
•  Seal the glass tube at
one end.
•  Fill with mercury.
•  Invert into a dish of
mercury.
Water density = 1.0g/cm3
Mercury density = 13.534g/cm3
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Air Pressure Units
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mmHg (millimeter mercury height)
Millibar (mb)
Kilopascal (kPa)
The average air pressure at sea level is
760 mmHg, or 29.92 inchHg, or 101.3
kPa, or 1013.2 mb.
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Aneroid Barometer
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Wind
•  Generally wind refers to the horizontal
motion of air across Earth’s surface.
•  Turbulence adds one vertical
component to wind (e.g., downdraft and
updraft).
•  Two principal properties of wind are
speed and direction.
•  Winds are named for the direction
from which they originate (e.g.,
westerly wind, easterly wind).
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Wind Direction—From Which Wind Originates
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Wind Measurement
Wind vane for
measuring wind
direction
Anemometer
for measuring
wind speed
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Wind Speed Symbol
1-2 knots
5 knots
10 knots
30 knots
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50 knots
1 knot = 1.852 km/hr
1 knot = 1.151 mph
Wind symbols on a weather map
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Driving Forces Within the Atmosphere
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Gravity
Pressure gradient force
Coriolis force
Friction force
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Isobar and contour line
Isobar---equal pressure
Contour line—equal elevation
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Contour lines
Terrain
Pressure Gradient Force
•  Isobar is an equal pressure line.
•  The pressure gradient results in a net force that is directed
from high to low pressure and this force is called the
pressure gradient force.
•  The pressure gradient force is responsible for triggering
the initial movement of air.
•  The closer spacing of isobars, the higher pressure
gradient, the stronger wind.
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Pressure Gradient Determines Wind
Speed
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Wind Direction due to the Pressure
Gradient Force Alone
High pressure center: descending and diverging flow
Low pressure center: ascending and converging flow
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Coriolis Force
•  The Coriolis force is an effect of Earth’s
rotation.
•  The Coriolis force deflects a moving object
to the right of its moving direction in the
Northern Hemisphere, and to its left in the
Southern Hemisphere.
•  The Coriolis force increases as the speed of
the moving object increases.
•  The Coriolis force is zero along the equator,
and reaches maximum near poles.
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Coriolis Force
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Coriolis Force
Northern
Hemisphere:
deflection to
the right
Southern
Hemisphere:
deflection to
the left
Zero at equator,
maximum near
poles
Only affects
large-scale
motions
Geostrophic Wind
Ø  Pressure Gradient Force + Coriolis Force creates
geostrophic wind at upper troposphere.
Ø  Low pressure center: cyclone
Ø  High pressure center: anticyclone
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Geostrophic Wind in Northern and
Southern Hemispheres
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Surface Winds
Near surface: Pressure Gradient Force + Coriolis Force + Friction Force
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Right hand for Northern Hemisphere
Left hand for Southern Hemisphere
January Air Pressure
Four broad pressure areas in Northern Hemisphere and
a similar set exists in Southern Hemisphere.
(1) equatorial low pressure trough (marked by ITCZ line)
(2) subtropical high pressure cells
(3) sub-polar low pressure cells
(4) polar high pressure cells (weak)
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July Air Pressure
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Primary High-Pressure and LowPressure Areas
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Equatorial low-pressure trough
Polar high-pressure cells
Subtropical high-pressure cells
Sub-polar low-pressure cells
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Equatorial Low-Pressure Trough
•  Inter-tropical convergence zone (ITCZ)
•  Heavy rainfall
•  Trade winds
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General Atmospheric Circulation
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Subtropical High-Pressure Cells
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Dry and hot
Westerlies
Trade wind
Bermuda high
Pacific high
Strong in summer
Weak in winter
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Sub-polar Low-Pressure Cells
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Cool and moist
Strong in winter and weak in summer
Aleutian low
Icelandic low
Polar front
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Polar High-Pressure Cells
•  Frigid and dry
•  Descend and diverge clockwise (Northern
Hemisphere) or counterclockwise (Southern
Hemisphere)
•  Polar easterlies
•  Antarctic high is stronger than Arctic high.
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General Atmospheric Circulation
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Jet Streams
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The jet stream is an irregular and concentrated band of westerly wind.
Jet streams are weak during the summer and strong during the winter.
Caused by the Earth’s rotation and atmospheric heating.
The polar jet stream is usually stronger than the subtropical jet stream.
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Local and Regional Winds
•  Land–sea breezes
•  Mountain–valley breezes
•  Monsoonal winds
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Land–Sea Breezes
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Mountain–Valley Breezes
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Monsoonal Winds
Winter
In winter, dry and cold air
flows from north to south.
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Summer
In summer, wet and humid air
flows from south to north.
Monsoon: season
Major Ocean Surface Currents
The frictional drag is the driving force for ocean surface currents.
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Deep-Ocean Thermohaline Circulation
•  Differences in temperature and salinity produce density differences
and thus create deep-ocean thermohaline circulation.
•  What will happen if Greenland’s ice is melting at a fast rate?
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Summary of Chapter 6
•  Atmospheric pressure is the force per unit area exerted against a surface by
the weight of air above that surface. Wind results from a horizontal
difference in air pressure. Winds are named for the direction from which they
originate.
•  There are four driving forces within the atmosphere: gravity, pressure
gradient force, Coriolis force, and friction force. The Coriolis force deflects a
moving object to the right of its moving direction in the Northern
Hemisphere, and to its left in the Southern Hemisphere.
•  Pressure gradient force and Coriolis force create geostrophic wind at the
upper troposphere. Pressure gradient force, Coriolis force, and friction force
create surface wind.
•  Four primary high-pressure and low-pressure areas: equatorial low-pressure
trough, polar high-pressure cells, subtropical high-pressure cells, and subpolar low-pressure cells.
•  Three types of local and regional winds: land-sea breezes, mountain-valley
breezes, and monsoonal winds.
•  The frictional drag is the driving force for ocean surface currents. Differences
in temperature and salinity produce density differences and thus create
deep-ocean thermohaline circulation.
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