Atmospheric Forces
and Winds
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Atmospheric pressure
Measuring air pressure
Surface and upper-air charts
Why the wind blows
Surface winds
Atmospheric
Pressure
Atmospheric Pressure
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air pressure - definition
air pressure and temperature
pressure gradient force
• Air pressure is, quite literally,
the weight of the atmosphere
above us.
Stepped Art
Fig. 6-2, p. 143
Measuring air
Pressure
Barometers
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mercury barometer
aneroid barometer
altimeter
barograph
Pressure Readings
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station pressure and sea-level pressure
isobars
Surface and Upper Air
Charts
Surface and Upper Air Charts
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isobaric maps
contour lines
ridges
troughs
• Color-filled contour maps are the same as ordinary
contour maps, except that the area between adjacent
lines is filled in with color.
Filled Contour Maps
Why the Wind Blows
Newton’s Laws of Motion
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Newton’s first law: “An object at rest will remain at
rest and an obect in motion will move in a straight line at
constant speed unless acted on by an unbalanced
force.”
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Newton’s second law:
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Newton’s third law:
and opposite reaction.”
“Every action has an equal
Forces that Influence the
Wind
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net force and fluid movement
• Wind is the result of a balance of several forces.
Pressure Gradient Force
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pressure gradient
pressure gradient force
strength and direction of the pressure
gradient force
• The horizontal (rather than the vertical) pressure
gradient force is responsible for causing air to move horizontally.
Fig. 6-11, p. 151
Coriolis Force
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real and apparent forces
Coriolis force
strength and direction of the Coriolis force
factors that affect the Coriolis force
• It is sometimes claimed that “water swirls down a bathtub
drain in opposite directions in the northern and southern
hemispheres”. This is not true.
Fig. 6-14, p. 153
Straight-line Flow Aloft
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combination of the pressure gradient and
Coriolis forces
geostrophic wind
• Geostrophic winds can be
observed by watching the
movement of clouds.
Curved Winds Around Lows
and Highs Aloft
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cyclonic and anticyclonic flow
centripetal force
gradient wind
Winds on Upper-level Charts
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gradients in contour lines
meridional and zonal winds
• Height contours on upper-level charts are interpreted
in the same way as isobars on surface charts.
Stepped Art
Fig. 6-19, p. 158
Surface Winds
Surface Winds
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planetary boundary layer
friction
frictional effects on the wind
• Most people rarely venture out of the planetary
boundary layer.
Winds and Vertical
Motions
Winds and Vertical Motions
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divergence and convergence
hydrostatic equilibrium
Summary of Atmospheric Forces
“True” forces:
•Gravity
•Pressure Gradient
•Friction
“Ficticious” forces:
•Coriolis force
•Centrifugal force
Summary of Atmospheric Force
Balances
Vertical:
•Hydrostatic Balance
Horizontal:
•Geostrophic Balanice
•Gradient Balance
•Ekman Balance
(see Table 6-1 in Ackerman and Knox)
Atmospheric
Circulations
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Scales of atmospheric motions
Eddies - big and small
Local wind systems
Global winds
Global wind patterns and the oceans
Scales of
Atmospheric Motions
Scales of Atmospheric
Motions
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scales of motion
microscale
synoptic scale
planetary scale
• Lots of important weather events occur on microscales,
like evaporation of liquid water molecules from the
earth’s surface.
Eddies - Big and
Small
Eddies - Big and Small
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eddy
rotor
wind shear
turbulence
• Wind shear can sometimes be observed by watching the
movement of clouds at different altitudes.
Local Wind Systems
Thermal Circulations
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isobars and density differences
thermal circulations
Stepped Art
Fig. 7-4, p. 172
Sea and Land Breezes
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sea breeze
land breeze
• Sea and land breezes also
occur near the shores of large
lakes, such as the Great
Lakes.
Stepped Art
Fig. 7-5, p. 174
Seasonally Changing Winds the Monsoon
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Monsoon wind system
Asian monsoon
other monsoons
Mountain and Valley Breezes
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valley breeze
mountain breeze
• The nighttime mountain breeze is sometimes called
gravity winds or drainage winds, because gravity
causes the cold air to ‘drain’ downhill.
Katabatic Winds
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drainage winds
bora
• Katabatic winds are quite fierce in parts of Antarctica,
with hurricane-force wind speeds.
Chinook Winds
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Chinook winds
compressional heating
chinook wall cloud
• In Boulder, Colorado, along the eastern flank of the
Rocky Mountains, chinook winds are so common that
many houses have sliding wooden shutters to protect
their windows from windblown debris.
Fig. 7-14, p. 180
Santa Ana Winds
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Santa Ana wind
compressional heating
wildfires
• Many Southern California
residents regularly hose
down
their roofs to prevent fires
during Santa Ana wind
season.
Desert Winds
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dust storms
dust devils
Global Winds
General Circulation of the
Atmosphere
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cause: unequal heating of the earth’s
surface
effect: atmospheric heat transport
• Ocean currents also transport heat from the equator to
the poles and back.
Single-cell Model
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basic assumptions
Hadley cell
why the single-cell model is wrong
• One of the world’s
premier atmospheric
science research
facilities,the Hadley
Centre for Climate
Research, is
named after
George Hadley.
Three-cell Model
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model for a rotating earth
Hadley cell
doldrums
subtropical highs
trade winds
intertropical convergence
zone
• Many global circulation terms,
westerlies
including ‘trade winds’ and
polar front
‘doldrums’, were named by
polar easterlies
mariners who were well acquainted
with wind patterns.
Fig. 7-21, p. 185
Average Surface Winds and
Pressure: The Real World
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semipermanent highs and lows
Bermuda high & Pacific high
Icelandic low & Aleutian low
Siberian high
• The Bermuda High frequently brings hot, muggy
weather to the eastern US.
Fig. 7-22a, p. 188
Fig. 7-22b, p. 189
The General Circulation and
Precipitation Patterns
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major controls
ITCZ, midlatitude storms, polar front
• Most of the world’s
thunderstorms are found
along the ITCZ.
Westerly Winds and the Jet
Stream
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jet streams
subtropical jet stream
polar front jet stream
Global Wind Patterns
and the Oceans
Winds and Upwelling
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upwelling
wind flow parallel to the coastline
• Upwelling frequently occurs along the
coast of California.
El Niño and the Southern
Oscillation
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El Niño events
Southern Oscillation
La Niña
teleconnections
• ENSO is an example of a global-scale weather
phenomenon.
Fig. 7-32, p. 196
Other Atmosphere-Ocean
Interactions
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North Atlantic Oscillation
Arctic Oscillation
Pacific Decadal Oscillation
• Other atmosphere-ocean interactions may very well be
discovered in the coming years.