Transcript Document

Chapter 5
Winds and Global Circulation
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.
Atmospheric Pressure
Atmospheric pressure: pressure exerted by
the atmosphere because of the force of
gravity acting upon the overlying column
of air
• Atmospheric pressure at a single location varies
slightly from day to day
Barometer: instrument that
measures atmospheric pressure
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.
Atmospheric Pressure
Air Pressure and Altitude
Atmospheric pressure
decreases with altitude
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.
Local Wind Patterns
• Winds are identified by
the direction from
which the wind comes
• Wind vane?
• Anemometer?
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.
Local Wind Patterns
Pressure Gradients
Wind is caused by differences in atmospheric pressure from one place to
another
Air tends to move from regions of high pressure to regions of low pressure
Isobars ?
Pressure gradient: change
of atmospheric pressure
measured along a line at
right angles to the isobars
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.
Pressure Gradients develop because of unequal heating in the
atmosphere
Local Wind Patterns
Local Winds
• Santa Ana winds
• Sea and land Breezes
• Mountain and valley winds
Cyclones and Anticyclones
Coriolis Effect: effect of the Earth’s rotation that acts like a force to
deflect a moving object on the Earth’s surface to the right in the
northern hemisphere and to the left in the southern hemisphere
• Acts at right angles to direction of motion
• Deflects to right in the northern hemisphere, left in the southern hemisphere
• No deflection at equator, maximum deflection at poles
Cyclones and Anticyclones
Coriolis Effect
Balance of Forces on a parcel of surface air:
• Pressure Gradient
• Coriolis Effect
• Strength increases with speed of
motion
• Strength decreases with latitude
• Friction
• Exerted by ground surface
• Proportional to wind speed
• Acts opposite to direction of
motion
Cyclones and Anticyclones
Cyclone: center of low atmospheric pressure
 Air spirals inward (convergence) and upward
 Associated with cloudy, rainy weather
Anticyclone: center of high atmospheric pressure
 Air spirals downward and outward (divergence)
 Associated with fair weather
Global Wind and Pressure Patterns
Subtropical high-pressure belts:
belts of persistent high
atmospheric pressure centered
at about lat. 30º N and S
Polar Front: front lying between cold polar
air masses and warm tropical air masses
Intertropical
Convergence
Zone (ITCZ):
zone of
convergence of
air masses
along the
equatorial
trough
Hadley Cell: lowlatitude atmospheric
circulation cell with
rising air over the
equatorial trough and
sinking air over the
subtropical highpressure belts
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.
Global Wind and Pressure Patterns
ITCZ and Monsoon
Circulation
•ITCZ and Hadley cells shift with the
seasons
•Shift is very large in Asia
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.
ITCZ and Monsoon Circulation
Monsoon: seasonal reversal of winds
Winds Aloft
Pressure decreases less rapidly with height in warmer air than in colder air
There is a temperature gradient from the equator to the poles
So, at high altitudes there is a pressure gradient from the equator to the
polesstrong winds at high altitudes
Winds Aloft
The Geostrophic Wind
Forces acting on an upper air parcel:
1. Pressure gradient force:
Moves from high pressure to low pressure
2. Coriolis force
Deflects to right in NH, to left in SH
Pressure gradient force balances coriolis force
Wind blows parallel to isobars
Geostrophic Wind: wind at high levels above
the Earth’s surface blowing parallel with a
system of straight parallel isobars
Winds Aloft
Global Circulation at Upper Levels:
Winds Aloft
Rossby Waves, Jet Streams, and the Polar Front
Rossby waves:
• Arise in the polar front
• Contribute to variable weather in
mid-latitudes
Rossby waves: horizontal
undulations in the flow path
of the upper-westerlies;
upper-air waves
Polar front: zone where cold
polar air meets warm tropical
air
Flow of air along front smooth for days or weeks.
Waves stronger. Tongue of cold air
Brough south and warm north.
Undulations begin and
Become stronger. Rossby
Wave forms. Warm air
Pushed poleward,
cold air south
Tongue pinched off.
Pool of cold air
farther
south than originally.
Become
cyclones of cold air
Persist for days of
weeks.
Winds Aloft
Jet streams: high-speed air flow in narrow bands
within the upper-air westerlies and along certain
other global latitude zones at high levels
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.
• Polar-front jet stream
•Westerly;
associated with
Rossby waves
• Subtropical jet stream
•Westerly; above
subtropical highpressure cells
• Tropical easterly jet
stream
•Runs east to west
•Summer only
•Develops in Asia
Ocean Currents
Oceanic circulation:
Ocean current: persistent,
dominantly horizontal flow
of water
• Currents exchange heat between high and low latitudes
Ocean Currents
Large-Scale Circulation of Ocean Waters
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.
Ocean Currents
Current Patterns
During an El Niño event:
• Upwelling along Peruvian coast ceases
• Trade winds weaken
• Weak equatorial eastward current develops
• Moisture and temperature
patterns alter