GEO 101: PHYSICAL GEOGRAPHY
Chapter 06: Atmospheric and Oceanic
Circulations
Wind Essentials
Driving Forces
Atmospheric Patterns of Motion
Global Wind Patterns
Local Winds
Oceanic Currents
‰ Weather patterns
‰ Winds spread the natural and
anthropological pollutions worldwide
‰ Winds as a driving mechanism for
ocean currents
Air Pressure and Its Measurement
• Air Pressure:
- the force exerted by the weight of air
molecules.
(air molecules are invisible, they have weight
and take up space)
• Barometers are used to measure the air pressure
Air Pressure and Its Measurement
Mercury
Barometer
Evangelista Torricelli’s observation
Aneroid barometer
Figure 6.2
Air Pressure and Its Measurement
• Pressure Units:
- Millibar (mb)
- Kilopascal (kPa)
- mm (height of mercury)
• Atmospheric Pressure at sea level:
• 1013.2 mb { 101.32 kPa; 760 mm; 29.92 in;
~1.0 kg/cm2 ; 14.7 lb/inch2 }
Winds and Its Measurement
• Winds:
- the horizontal motion of the air across the
earth’s surface.
• Anemometers are used to measure the wind
speed.
Anemometer:
measure the
speed of winds
Wind Vane:
measure the
direction of
winds
10 m (33 ft)
above ground to
reduce local
topographic
effects
Figure 6.4
Figure 6.5
• Winds are named for the direction from
which they originate.
Westerly: a wind from west
Southerly: blows northward
Figure 6.4
Driving Forces
• Circulation of Winds Across Earth
• caused by driving forces within the atmosphere
‰ Pressure gradient
‰ Coriolis force
‰ Friction force
‰ Gravity
• These forces determine both speed and direction of winds.
Pressure Gradient
• The earth surface is unequally heated
• Cold, dense air at poles Æ higher pressure
• Warm, less dense air at equator Æ lower pressure
• Air flows from high pressure to low pressure
Pressure Gradient Force
Figure 6.7
• Isobars: lines connecting points of equal pressure
• Isobar spacing shows steepness of pressure gradient
• Closer isobars denote higher pressure gradient
• Wider isobars indicate more gradual pressure gradient
• Wind speed depends on “steepness” of pressure
gradient
Coriolis Force
•
Coriolis force: a deflective force;
due to earth’s rotation.
•
In northern hemisphere, wind is
deflected to the right.
In southern hemisphere, wind is
deflected to the left.
Effect is greater with greater wind
speed
•
•
Figure 6.9
Coriolis Force
Geostrophic Wind
• The combination of
pressure gradient
force and Coriolis
force.
• They produce
winds that flow
parallel to isobars.
Friction Force
‰ Surface winds experience
friction force
‰ Friction force decreases
wind speed and reduces
Coriolis force
‰
http://physics.uwstout.edu/wx/Notes/ch6notes.htm
Friction force depends on
terrain
Gravity
• Earth’s gravitational force on the atmosphere
• Without gravity, there would be no atmospheric
pressure
• Gravity equally compresses the atmosphere worldwide
Atmospheric Patterns of Motion
Primary High-Pressure and Low-Pressure Areas
Equatorial low-pressure trough (ITCZ-intertropical convergence zone)
Polar high-pressure cells (around 90°N and 90°S, not shown)
Subtropical high-pressure cells (20°-35° N and 20°-35° S)
Subpolar low-pressure cells (around 60°N and 60°S)
Figure 6.11
Atmospheric Circulation and Wind Patterns
http://www.earthsci.org/Flooding/unit1/u1-08-03.html
General Atmospheric Circulation and Wind patterns
Now you should be able to understand this diagram!
Figure 6.13
Upper Atmospheric Circulation
• Constant Isobaric Surface
• Where an air pressure of 500 mb occurs.
• A reference in the upper atmosphere
Figure 6.16
‰ There are
“Ridges” and
“Troughs” at the
middle to upper
atmosphere
‰ These troughs
and ridges are
important for the
cyclones and
anticyclones at
the surface
Figure 6.16
Cyclones, Storms, and Hurricanes
http://www.earthsci.org/Flooding/unit1/u1-08-02.html
Upper Atmospheric Circulation
http://www.earth.rochester.edu
Jet stream at collison zone between cold and warm air masses.
( the best known one is the polar jet stream)
Local Winds
Land-sea breezes
Mountain-valley breezes
Land-Sea
Breezes
Figure 6.19
Mountain-Valley
breeze
Figure 6.20
Regional Scale Winds -- Monsoonal Winds
Summer
Winter
http://www.uwsp.edu/geo/faculty/ritter/geog101/textbook/circulation/regional_scale_wind.htm
•
Monsoon means wind that changes direction with season;
•
•
A typical example is the monsoon of Asia:
Wet Monsoon season: During summer, land heats up fast, warm and
moisture-laden air from the Indian ocean flows toward central Asia Æ
rains in coast and mountain areas
Dry Monsoon season: During winter, drier, colder air of the continent
blows offshore Æ dry conditions coast and mountain areas
Also found in east-central Africa, and southwest desert of U.S.
•
•
Oceanic Currents
Surface Currents
Deep Currents
‰ Important driving force: frictional drag of the winds
‰ Other factors:
ƒ Coriolis force
ƒ Density differences cause by temperature
and salinity
ƒ Configuration of continents and ocean floor
ƒ Astronomical forces (e.g., tides)
Major Ocean Currents
Figure 6.22
Upwelling and Downwelling Oceanic Currents
Upwelling current: surface water is swept away from
the coast (by Coriolis force or offshore winds)
Downwelling current: water gravitates downward
(where there is accumulation of water)
Thermohaline circulation—Deep Ocean Currents
•
•
•
Different temperature and salinity cause density variation
Traveling in lower speed than surface currents, but can transfer a
larger volumes of water
May take 1,000 years for a full cycle from downwelling to
upwelling
Figure 6.23
End of Chapter 6
• Wind Essentials
• Driving Forces
• Atmospheric Patterns of Motion
•Global Wind Patterns
• Local Winds
• Oceanic Currents