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AMS Weather Studies
Introduction to Atmospheric Science, 5th Edition
Chapter 8
Wind & Weather
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Driving Question
 What forces control the speed and direction of the wind?
 This chapter covers:
 The various forces that either initiate or modify
atmospheric circulation
 Each force examined separately
 Then combined to show how together they drive
atmospheric circulation.
 Monitoring wind speed and direction
 Scales of atmospheric circulation
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Case-in-Point
Sinking of the Edmund Fitzgerald
 In 1975, the Edmund Fitzgerald was
the largest ore carrier in the Great
Lakes at 222 m (729 ft)
 Intense low-pressure system moved
over the Great Lakes
 Wind speeds estimated at 94 km/hr
(58 mph), gusting to 137 km/hr (85
mph) with waves 3.5-5 m (12-16ft)
 Recent studies show structural deficiencies and poor ship conditions
played a large roll in sinking the ship
 Gordon Lightfoot memorialized the Edmund Fitzgerald in song
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Sinking of the Edmund Fitzgerald
http://cimss.ssec.wisc.edu/wxwise/fitz.html
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Forces Governing the Wind
 Force
 Push or pull that causes an object at rest to move, or alters movement
of an object already in motion
 Has both direction and magnitude (vector quantity)
 Newton’s second law of motion
Force = mass x acceleration
 Acceleration (a change in velocity) is a response to a force
 Apply each force governing wind to parcel that is a unit mass
or air
 Forces acting on wind
 Air pressure gradient, centripetal force (consequence of other forces),
Coriolis Effect (not a true force), friction, gravity
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Forces Governing the Wind
 Air pressure gradient
 Exist whenever air pressure varies between locations
 Horizontal pressure gradient: air pressure change along
constant altitude
 Determined on weather maps from isobar patterns, drawn at 4-mb
intervals; interpolation between stations is always necessary
 Vertical air pressure gradient: exists over a certain point
 Permanent feature of the atmosphere
 Measured in the direction of greatest change
 Perpendicular to the isobars
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Forces Governing the Wind
 Closely spaced isobars (A)
 Air pressure changes rapidly
with distance
 Strong pressure gradient
 Widely spaced isobars (B)
 Air pressure changes
gradually with distance
 Weaker pressure 7gradient
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Forces Governing the Wind
Sloshing water back and forth in a tub creates pressure gradients along the tub
bottom, analogous to a horizontal air pressure gradient in the atmosphere.
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Forces Governing the Wind
 Centripetal Force
 Isobars a surface weather map curve,
so wind blows in curved paths
 Indicates influence centripetal force
 Center-seeking force
 String exerts inward force on rock, confining it to a curved path (see image)
 Force directed perpendicular to direction of the rock’s motion, toward center of
circular orbit
 To increase rotation rate, or shorten string, requires large centripetal force
 Not an independent force
 Tension in the string responsible for the centripetal force
 If string is cut, centripetal force no longer operates, the rock flies off in a straight line
 As described by Newton’s first law of motion (an object moving in straight line remains
so unless acted on)
 Arises from an imbalance in other forces operating in the atmosphere
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Forces Governing the Wind
 Coriolis Effect
 From space, a storm system on Earth appears to move in a
straight line; an observer on Earth sees the storm center
following a curved path
 Curved motion implies an unbalanced force is operating;
unaccelerated, straight motion implies balance
 Unbalanced force operates on Earthbound rotating coordinate
system
 Forces are balanced in the non-rotating system fixed in space
 The net force responsible for curved motion is the Coriolis
Effect.
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Coriolis Effect
http://www.classzone.com/books/earth_science/terc/content/visu
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alizations/es1904/es1904page01.cfm
Forces Governing the Wind
The familiar north-south, east-west frame of reference rotates
eastward in space as Earth rotates on its axis. Rotation of the
coordinate system gives rise to the Coriolis Effect.
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Forces Governing the Wind
 Coriolis Effect
 Deflection
 Right in Northern Hemisphere
 Left in the Southern Hemisphere
 Strongest at the poles, decreases
moving away from poles, zero at
the equator.
 Fast-moving objects deflected more
than slower because faster objects
cover greater distances
 Longer the trajectory, greater the
shift of the rotating coordinate
system with respect to the moving
air parcel
 Coriolis Effect only significantly
influences the wind in large-scale
weather systems
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Coriolis Effect?
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Forces Governing the Wind
 Friction
 Resistance an object or medium encounters as it moves in
contact with another object or medium
 Viscosity – resistance of fluid (liquid and gas) flow
 Molecular viscosity: the random motion of molecules in the fluid
 Eddy viscosity (important): arises from much larger irregular motions,
called eddies
 Atmospheric boundary layer – zone to which frictional resistance
(eddy viscosity) confined
 Above 1000 m (3300 ft), friction is a minor force
 Turbulence – fluid flow characterized by eddy motion
 We experience turbulent eddies as
gusts of wind
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Examples of Eddy Viscosity
Stream Example:
Rocks in a streambed cause the
current to break down into eddies
that tap some of the stream’s
energy so that the stream slows.
Snow Fence Example:
A snow fence taps some of the wind’s
kinetic energy by breaking the wind into
small eddies. Wind speed diminishes,
causing loss of snow-transporting ability.
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Examples of Eddy Viscosity
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Forces Governing the Wind
 Gravity
 Force that holds objects to the Earth’s surface
 Net result of gravitation and centripetal force
 Gravitation is the force of attraction between the Earth and an object
 Magnitude directly proportional to the product of the masses of Earth and the object
 Inversely proportional to the square of the distance between both centers of mass
 Much weaker centripetal force is caused by the Earth’s rotation
 Always acts directly downward
 No influence on horizontal wind
 Only influences ascending or descending air
 Accelerates object downward toward Earth’s surface at 9.8 m per sec
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Forces Governing the Wind
 Horizontal pressure gradient force: responsible for initiating air motion
 Accelerates air parcels perpendicular to isobars, away from high pressure, toward
low pressure
 Centripetal force: imbalance of actual forces
 Exists when wind has a curved path
 Changes wind direction, not wind speed
 Always directed inward toward center of rotation
 Coriolis Effect: arises from the rotation of Earth
 Deflects winds to the right in the Northern Hemisphere
 Deflects winds to the left in the Southern Hemisphere
 Friction: acts opposite to the wind direction
 Increases with increasing surface roughness
 Slows horizontal winds within about 1000 m (3300 ft) of the surface
 Gravity: accelerates air downward
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 It does not modify horizontal winds
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Wind: Joining Forces
 Newton’s first law of motion
 When the forces acting on a parcel of air are in balance, no net force
operates, and parcel either remains stationary or continues to move
along a straight path at a constant speed
 Interaction of forces control vertical and horizontal air flow
through:




Hydrostatic equilibrium
Geostrophic wind
Gradient wind
Surface winds and horizontal winds within the atmospheric boundary
layer
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Wind: Joining Forces
 Hydrostatic equilibrium
 Air pressure always declines with
altitude
 Vertical pressure gradient force is
upward
 Were this the only force, air would
accelerate away from Earth
 Counteracting downward force is
gravity
 Balance between is hydrostatic
equilibrium
 Slight deviations from hydrostatic
equilibrium cause air parcels to
accelerate vertically.
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Wind: Joining Forces
 Geostrophic wind
 Winds blowing on large scale
parallel isobars with low pressure
on the left (in the Northern
Hemisphere)
 Geostrophic wind is horizontal
movement of air that follows a
straight path at altitudes above
the atmospheric boundary layer
 Caused by a balance between the
horizontal pressure gradient force
and Coriolis Effect
 Develops only where the Coriolis
Effect is significant (large-scale
weather systems)
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Wind: Joining Forces
Gradient Wind
 Gradient Wind
 Similar to geostrophic wind
 Large-scale, frictionless, blows parallel to the isobars
 The path is curved
 Forces not balanced because a net centripetal force constrains air
parcels to curved trajectory
 Occurs around high and low pressure centers above the
boundary layer
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Wind: Joining Forces
 Gradient Wind
 High in N. Hemisphere
 Coriolis Effect slightly
greater than pressure
gradient force, inwarddirected centripetal
force
 Wind is clockwise
 Low in N. Hemisphere
 Pressure gradient force
slightly greater than
Coriolis Effect, inwarddirected centripetal
force
 Wind is
counterclockwise
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Wind: Joining Forces
 Surface Winds
 Friction slows wind and interacts
with other forces to change wind
direction
 Friction combines with the Coriolis
Effect to balance the horizontal
pressure gradient force
 Friction acts directly opposite the
wind direction
 Coriolis Effect always at right angle
to wind direction
 Winds cross isobars at an angle
that depends on roughness of
Earth’s surface.
 Angle varies from 10-45 degrees
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Wind: Joining Forces
 Surface Winds
 The closer to Earth’s surface
the winds are, the more
friction comes into play
 For the same horizontal air
pressure gradient, the angle
between the wind direction
and isobars decreases with
altitude in the atmospheric
boundary layer
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Wind: Joining Forces
 Surface Winds
 High (anticyclone): surface
winds blow clockwise and
outward
 Low (cyclone): surface winds
blow counterclockwise and
inward
 In Southern Hemisphere,
 Cyclone: surface winds blow
clockwise and inward
 Anticyclone: winds blow
counterclockwise and outward
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Wind: Joining Forces
On a typical surface weather map, isobars exhibit clockwise (anticyclonic)
curvature (ridges) and counterclockwise (cyclonic) curvature (troughs).
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Continuity of Wind
 Horizontal and vertical components of the wind are linked
 Surface winds follow Earth’s topography
 Uplift occurs along frontal surfaces
 In a surface high, horizontal winds diverge from the center
 Vacuum does not develop because air descends to replace air at surface
 Aloft, horizontal winds converge above the center of surface high
 Anticyclones typically have clear skies and a weak horizontal pressure gradient
H
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Continuity of Wind
 In a surface low, horizontal winds converge toward the center.
 Air ascends in response to converging surface winds and diverging winds
aloft
 Cyclones are typically stormy weather systems with cloud and
precipitation development
L
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Continuity of Wind
Surface winds accelerate
and undergo horizontal
divergence when blowing
from a rough surface to a
smooth surface.
Surface winds undergo
horizontal convergence
when blowing from a
smooth to a rough surface.
Divergence of surface
winds causes air to
descend, whereas
convergence of surface
winds causes air to ascend.
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Continuity of Wind
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Monitoring Wind Speed and Direction
 Wind velocity is vector quantity
 Has both magnitude (speed) and direction
 Wind distinguished between horizontal and vertical
components
 Magnitude of vertical air motion typically only 1% to
10% of horizontal wind speed
 Wind direction is the direction it is coming from,
not blowing to
 Most common instruments only measure
horizontal wind
 Wind vane: freely rotating with counterweighted
arrow that points into the wind (top image)
 Windsock stretches downwind (bottom image)
 Wind speed can be estimated by its effect
on water
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using Beaufort scale
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Beaufort Scale
of Wind Force
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Monitoring Wind
Speed and Direction
 Instruments that measure wind speed:
 Cup anemometer - speed of spinning cups
translated into wind speed (top)
 Hot-wire anemometer - measures loss of
heat from heated wire, translates into wind
speed
 Aerovanes - a 3 or 4 blade propeller spins at
a rate of wind speed, fin on the back turns it
into the wind, indicating direction; electric
sensor connected to computer (middle)
 Sonic anemometer - consists of 3 arms that
send and receive ultrasonic pulses; Sound
wave travel times are translated into wind
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speed and direction (bottom)
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Monitoring Wind Speed and Direction
 Instruments should be mounted
10 m (33 ft) above the ground
 Rooftop locations should be
avoided
 Radiosondes, satellites, and wind
profilers measure winds aloft
Time variations in wind speed
and direction over a six-hour
period.
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Scales of Weather Systems
 Planetary-scale systems: large-scale wind belts encircling the planet
(midlatitude westerlies, trade winds)
 Synoptic-scale systems: continental or oceanic in nature
(migrating cyclones, hurricanes, and air masses)
 Mesoscale-scale systems: circulation systems that influence weather in part
of a large city or county (thunderstorms, sea breeze)
 Microscale systems: weather system covering a very small area such as
several city blocks (weak tornado) 37
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