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AMS Weather Studies
Introduction to Atmospheric Science, 4th Edition
Chapter 8
Wind and Weather
© AMS
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Case-in-Point
 This Case-in-Point talks of the sinking of the Edmund
Fitzgerald in Lake Superior in 1975
– At the time, it was the largest ore carrier in the Great Lakes at 222 m (729 ft)
– An intense low-pressure system
moved over the Great Lakes
– Wind speeds were estimated
to be 95 km/hr (58 mph) gusting to
137 km/hr (85 mph) with waves of
3.5 to 5 m (12 to 16 ft)
– Recent studies show that structural deficiencies
and poor ship condition played a large role in the sinking of the ship
 This is the shipwreck that was memorialized in a song by Gordon Lightfoot
 The Case-in-Point serves as a reminder that even the largest ships are
vulnerable to wind and weather
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Driving Question
 What forces control the speed and direction of the
wind?
– Different weather systems bring different types of
weather depending on the air circulation (wind) that
characterizes each system
– Wind is the local motion of air measured relative to the
rotating Earth
– In this chapter we will:
 Investigate the various forces that either initiate or modify
atmospheric circulation
– First, each force will be examined separately
– Then, they will be combined to show how together they drive
atmospheric circulation
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Forces Governing the Wind
 A force is a push or pull that can cause an object at
rest to move, or that alters the movement of an object
already in motion
– Force has both direction and magnitude (vector quantity)
– It is useful to apply each force governing the wind to a parcel
that is a unit mass (e.g., single kilogram) or air
– Newton’s second law of motion
 Force = mass x acceleration
 Acceleration (a change in velocity) is a response to a force
 Forces that act on wind are a consequence of:
–
–
–
–
–
An air pressure gradient
Centripetal force (occurs as a consequence of other forces)
Coriolis Effect (an apparent, but not a true force)
Friction
Gravity
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Forces Governing the Wind
 Pressure Gradient Force
– A gradient is a change in some property over
distance
– Air pressure gradients exist whenever air
pressure varies from one place to another
 A horizontal pressure gradient refers to air pressure
change along a constant altitude surface
– They can be determined on weather maps from isobar
patterns. By U.S. convention, isobars are drawn at 4-mb (4hPa) intervals and interpolation between stations is always
necessary.
 A vertical air pressure gradient exists over a certain
point and is a permanent feature of the atmosphere
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Pressure Gradient Force
 Example: 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. In response to a pressure
gradient, water (or air) flows from an area of
higher pressure to an area of lower pressure.
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Forces Governing the Wind
 Centripetal Force
– Isobars plotted on a surface weather map are almost always curved; the
wind blows in curved paths. Curved motion indicates the influence of the
centripetal force.
– Center-seeking force; the string exerts a net force on the rock by
confining it to a curved path
– Increasing the rotation rate or shortening the string requires a large
centripetal force
– Not an independent force; the tension in the string is responsible for the
centripetal force
 If the string is cut, the centripetal force no longer operates and the rock flies
off in a straight line as described by Newton’s first law of motion (an object in
straight-line, unaccelerated motion remains that way unless acted upon by
an unbalanced force).
– Results from an imbalance in other forces operating in the atmosphere
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Forces Governing the Wind
 Coriolis Effect
– Frame of reference example: In looking at the Earth
from space, a storm system appears to move in a
straight line at constant speed. Meanwhile, an observer
on Earth observes the storm center following a curved
path.
– Curved motion implies that a net (or unbalanced) force
is operating and unaccelerated, straight motion implies
a balance of forces
– A net force operates on the Earthbound rotating
coordinate system whereas forces are balanced in the
non-rotating system fixed in space
– The net force responsible for curved motion is the
© AMS Coriolis Effect
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Coriolis Effect
 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, continued
– Deflection is to the right in
Northern Hemisphere and to
the left in the Southern
Hemisphere
– Deflection is strongest at the
poles, decreases moving away
from poles, and is zero at the
equator
– Fast-moving objects are
deflected more than slower
ones because faster objects
cover greater distances. The
longer the trajectory, the
greater is the shift of the
rotating coordinate system with
respect to the moving air parcel
– Coriolis Effect only significantly
influences the wind in largescale weather systems
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Forces Governing the Wind
 Friction
– The resistance an object or medium encounters as
it moves in contact with another object or medium
– The resistance of fluid (liquid and gas) flow is
termed viscosity
 Two types:
– Molecular viscosity: the random motion of molecules in the fluid
– Eddy viscosity (more important): arises from much larger irregular
motions, called eddies
 Atmospheric boundary layer: the zone to which frictional
resistance (eddy viscosity) is essentially confined
– Above 1000 m (3300 ft), friction is a minor force
 Turbulence: fluid flow characterized by eddy motion
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– 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
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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 snow13
transporting ability
Forces Governing the Wind
 Gravity
– The 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 some
object
– It’s magnitude is directly proportional to the product of the masses of
Earth and the object
– It is inversely proportional to the square of the distance between their
centers of mass
 The much weaker centripetal force is caused by the Earth’s rotation
 Gravity always acts directly downward
– It does not influence horizontal wind
– It only influences air that is ascending or descending
– Accelerates a unit mass downward toward Earth’s surface at 9.8 m per
sec each second
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Forces Governing the Wind
 Summary
– Horizontal pressure gradient force is responsible for initiating
almost all air motion
 Accelerates air parcels perpendicular to isobars, away from high
pressure and toward low pressure
– Centripetal force is an 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
 It increases with increasing surface roughness
 Slows horizontal winds within about 1000 m (3300 ft) of the surface
– Gravity accelerates air downward
 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 the 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:
–
–
–
–
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Hydrostatic equilibrium
The geostrophic wind
The gradient wind
Surface winds, 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 the two forces
is hydrostatic equilibrium
– Slight deviations from
hydrostatic equilibrium cause air
© AMS parcels to accelerate vertically
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Wind: Joining Forces
 Geostrophic wind
– Winds blowing at a large scale
tend to parallel isobars with low
pressure on the left in the
Northern Hemisphere
– Geostrophic wind is a 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 the Coriolis Effect
– Develops only where the Coriolis
Effect is significant (i.e., in largescale weather systems)
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Wind: Joining Forces
 Gradient Wind
– Shares many characteristics with the
geostrophic wind
 Large-scale, frictionless, and blows parallel
to the isobars
– The path of the gradient wind is curved
 Forces are not balanced because a net
centripetal force constrains air parcels to
a curved trajectory
– Occurs around high and low pressure
centers above the boundary layer
– High (anticyclone) in N. Hemisphere
 Coriolis Effect is slightly greater than the
pressure gradient force giving rise to an
inward-directed centripetal force
 Wind is clockwise
– Low (cyclone) in N. Hemisphere
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 Pressure gradient force is slightly greater
than the Coriolis Effect giving rise to an
inward-directed centripetal force
 Wind is counterclockwise
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Wind: Joining Forces
 Surface Winds
– Friction slows the wind and
interacts with the 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 whereas the Coriolis
Effect is always at a right angle to
the wind direction
– Winds now cross isobars at an
angle, which depends on
roughness of Earth’s surface
 Angle varies from 10 degrees or
less to 45 degrees
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Wind: Joining Forces
 Surface Winds, cont.
– The closer to the 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 in the Northern
Hemisphere
– Surface winds blow clockwise
and outward in a high
(anticyclone)
– Surface winds blow
counterclockwise and inward in
a low (cyclone)
– In the Southern Hemisphere,
surface winds in a cyclone blow
clockwise and inward; in an
anticyclone winds blow
counterclockwise and outward
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Wind: Joining Forces
On a typical surface weather map, isobars exhibit clockwise curvature (ridges)
and counterclockwise 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.
A vacuum does not develop
because air slowly descends
to replace diverging air at the
surface. Aloft, horizontal winds
converge above the center of
the surface high.
 Anticyclones typically have
clear skies and a weak
horizontal pressure gradient
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Warming &
evaporation
H
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Continuity of Wind
 In a surface low,
horizontal winds converge
toward the center. Air
does not pile up at the
center, but ascends in
response to converging
surface winds and
diverging winds aloft.
 Cyclones are typically
stormy weather systems
with cloud and
precipitation development
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Cooling &
Condensation
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Continuity of Wind
Surface winds accelerate and undergo horizontal divergence when blowing from
a rough surface to a smooth surface (e.g., from land to water). Surface winds
undergo horizontal convergence when blowing from a smooth to a rough surface
(e.g.’ from water to land). Divergence of surface winds causes air to descend,
convergence of surface winds causes air to ascend.
©whereas
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Monitoring Wind Speed and Direction
 Wind velocity is a vector quantity
– Meaning it has both magnitude (speed) and
direction
 Wind is usually distinguished between
horizontal and vertical components
– The magnitude of vertical air motion is typically
only 1% to 10% of the horizontal wind speed
– The most common instruments only measure the
horizontal wind
– A wind vane consists of a free rotating and
counterweighted arrow that points into the wind
 Wind direction is the direction it is coming from, not
blowing to
– A windsock stretches downwind
– Wind speed can be estimated by its effect on
water using the Beaufort scale
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Beaufort Scale
of Wind Force
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Monitoring Wind Speed and Direction
 Wind speed is measured using a cup anemometer
– The speed at which the cups spin is translated into wind speed
 A hot wire anemometer measures the loss of heat from a
heated wire, and translates that into wind speed
 Aerovanes measure wind speed and direction
– A 3 or 4 blade propeller spins at a rate proportional to the wind speed
and a fin on the back turns it into the wind, indicating direction.
Electronic sensors are connected to a recording computer or digital
display.
 A sonic anemometer consists of 3 arms that send and receive
ultrasonic pulses. Sound wave travel times are translated into
wind speed and direction
– Scheduled to replace cup anemometers in NWS ASOS
 Instruments should be mounted at 10 m (33 ft) above the
ground. Rooftop locations should be avoided.
 Radiosondes, satellites, and wind profilers measure winds aloft
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Monitoring Wind Speed and Direction
Cup Anemometer
Time
variations in
wind speed
and
direction
over a sixhour period
Sonic
Anemometer
aerovane
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Scales of Weather Systems
 Planetary-scale systems: large-scale wind belts encircling the planet
(e.g., midlatitude westerlies, trade winds)
 Synoptic-scale systems: continental or oceanic in nature (e.g.,
migrating cyclones, hurricanes, and air masses)
 Mesoscale-scale systems: circulation systems that influence weather in
part of a large city or county (e.g., thunderstorms, sea breeze)
 Microscale systems: weather system covering a very small area such
as several city blocks (e.g., weak tornado)
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Conclusions
 Unequal rates of radiational heating and cooling
within the Earth-atmosphere system are
responsible for temperature gradients
 The atmosphere circulates in response and heat
energy is converted to kinetic energy
 Various forces studied in this chapter shape
atmospheric circulation (the wind)
 This chapter built a realistic model of atmospheric
motion that demonstrates why and how winds
circulate around high and low pressure systems
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