atmospheric forces - uclaclass.atmos.ucla.edu/as3/scrns/pdf_files/04forces.aos3.notes.pdf ·...
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Atmospheric Forces
Makin’ the wind go ‘round
Ch. 4: Atmospheric Pressure
• Vertical Pressure Profile
• Barometers
• Maps of Pressure Fields
Isobars
Heights of Pressure Surfaces vs. Temperature
Height Contours
Ch. 4: Forces and Wind
• Atmospheric Forces
Pressure Gradient Force
Coriolis Force
Friction
• Winds and Balance of Forces
Hydrostatic Balance
Winds Aloft—Geostrophic Balance
Surface Winds
• Wind Measurement
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Force (F)
Sea level pressures:
• 1 atmosphere (atm)
• 1013.2 millibars (mb)
• 14.7 lb./sq. in. (psi)
Weight ofAtmos.
z
P
Measurement: Barometers
Mercury Barometer
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Aneroid Barometer
Reduction of Pressureto Sea Level
990
950
975
870
1000SL
Charting Pressure
• Isobars: lines connecting points of equal pressure
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Heights of Pressure Surfaces
• Pressure surface: a surface where the atmospheric pressure is the same everywhere
• Height contours: lines connecting points of equal heights of some pressure surface
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Pressures and Heights
500
600
700
800
900
500
600
700
800
900
500 mb surface
500-mbHeight
Pressures and Heights
500
600
700
800
900
500
600
700
800
900
Cold Warm
500 mb surface
Heights of pressure surfaces aloft are
higher when the air is warmer.
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Forces
F→F→
PGF ∝Δp
d
100410081012
1004
1008
1012 More closely spaced pressure contour lines: higher pressure gradient force
Pressure Gradient Force
Pressure force is proportional to the difference in pressures
Coriolis Force
View from above
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Coriolis Force
View on turntable
Coriolis Force
Coriolis force magnitude Fcor ∝Object’s speed
Earth’s rotation speed
Latitude
Direction: right angle (90°) to direction of motion
Northern Hemisphere Southern Hemisphere
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Friction
• Frictional force opposes motion
• Magnitude directly proportional to object’s speed
Motion(wind)
Friction
f ∝ –V
f→
V→
Hydrostatic Balance
Balance of forces in the vertical direction
Gravity
Vertical Pressure Gradient Force
Geostrophic Balance
Vg
→
PGF
Fcor
L
H
Geostrophic wind
blows parallel to
isobars or height
contours
Geostrophic balance: steady-state wind
where the PGF is exactly balanced by the
Coriolis force
Northern Hemisphere: lower pressure is to
the left of the wind motion
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Circulation AroundHighs and Lows
L H
Counterclockwise Clockwise
Northern Hemisphere
Circulation AroundHighs and Lows
L H
Clockwise Counterclockwise
Southern Hemisphere
Vg→Vg→
Surface Winds
—Add friction
L
H
PGF→
Fcor→
Fcor
→
f→ V
→
Fcor→
Surface winds blow at an angle to the isobars, turned slightly toward lower pressure
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Circulation at the Surface
L H
Counterclockwise Clockwise
Northern Hemisphere
Counterclockwise,spiral inward
Clockwise,spiral outward
Ridges and Troughs
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Wind Instruments
• Direction: Wind Vane
• Speed: Anemometer
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