© AMS 1 Chapter 8 Wind and Weather AMS Weather Studies Introduction to Atmospheric Science, 4 th Edition

© AMS© AMS 11

Chapter 8Chapter 8

Wind and WeatherWind and Weather

AMS Weather StudiesAMS Weather Studies Introduction to Atmospheric Science, 4Introduction to Atmospheric Science, 4 thth Edition Edition

© AMS© AMS 22

Case-in-PointCase-in-Point This Case-in-Point talks of the sinking of the This Case-in-Point talks of the sinking of the Edmund Edmund

Fitzgerald Fitzgerald in Lake Superior in 1975in Lake Superior in 1975– At the time, it was the largest ore carrier in the Great Lakes at 222 m (729 ft)At the time, it was the largest ore carrier in the Great Lakes at 222 m (729 ft)

– An intense low-pressure system An intense low-pressure system

moved over the Great Lakesmoved over the Great Lakes

– Wind speeds were estimated Wind speeds were estimated

to be 95 km/hr (58 mph) gusting to to be 95 km/hr (58 mph) gusting to

137 km/hr (85 mph) with waves of 137 km/hr (85 mph) with waves of

3.5 to 5 m (12 to 16 ft)3.5 to 5 m (12 to 16 ft)

– Recent studies show that structural deficiencies Recent studies show that structural deficiencies

and poor ship condition played a large role in the sinking of the shipand 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 LightfootThis 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 The Case-in-Point serves as a reminder that even the largest ships are

vulnerable to wind and weathervulnerable to wind and weather

© AMS© AMS 33

Driving QuestionDriving Question

What forces control the speed and direction of the What forces control the speed and direction of the wind?wind?– Different weather systems bring different types of Different weather systems bring different types of

weather depending on the air circulation (wind) that weather depending on the air circulation (wind) that characterizes each systemcharacterizes each system

– Wind is the local motion of air measured relative to the Wind is the local motion of air measured relative to the rotating Earthrotating Earth

– In this chapter we will:In this chapter we will: Investigate the various forces that either initiate or modify Investigate the various forces that either initiate or modify

atmospheric circulationatmospheric circulation– First, each force will be examined separatelyFirst, each force will be examined separately

– Then, they will be combined to show how together they drive Then, they will be combined to show how together they drive atmospheric circulationatmospheric circulation

© AMS© AMS 44

Forces Governing the WindForces Governing the Wind A force is a push or pull that can cause an object at A force is a push or pull that can cause an object at

rest to move, or that alters the movement of an object rest to move, or that alters the movement of an object already in motionalready in motion– Force has both direction and magnitude (vector quantity)Force has both direction and magnitude (vector quantity)– It is useful to apply each force governing the wind to a parcel It is useful to apply each force governing the wind to a parcel

that is a unit mass (e.g., single kilogram) or airthat is a unit mass (e.g., single kilogram) or air– Newton’s second law of motion Newton’s second law of motion

Force = mass x accelerationForce = mass x acceleration Acceleration (a change in velocity) is a response to a forceAcceleration (a change in velocity) is a response to a force

Forces that act on wind are a consequence of:Forces that act on wind are a consequence of:– An air pressure gradientAn air pressure gradient– Centripetal force (occurs as a consequence of other forces)Centripetal force (occurs as a consequence of other forces)– Coriolis Effect (an apparent, but not a true force)Coriolis Effect (an apparent, but not a true force)– FrictionFriction– GravityGravity

© AMS© AMS 55

Forces Governing the WindForces Governing the Wind

Pressure Gradient ForcePressure Gradient Force– A gradient is a change in some property over A gradient is a change in some property over

distancedistance– Air pressure gradients exist whenever air Air pressure gradients exist whenever air

pressure varies from one place to anotherpressure varies from one place to another A horizontal pressure gradient refers to air pressure A horizontal pressure gradient refers to air pressure

change along a constant altitude surfacechange along a constant altitude surface– They can be determined on weather maps from isobar They can be determined on weather maps from isobar

patterns. By U.S. convention, isobars are drawn at 4-mb patterns. By U.S. convention, isobars are drawn at 4-mb (4-hPa) intervals and interpolation between stations is (4-hPa) intervals and interpolation between stations is always necessary.always necessary.

A vertical air pressure gradient exists over a certain A vertical air pressure gradient exists over a certain point and is a permanent feature of the atmospherepoint and is a permanent feature of the atmosphere

© AMS© AMS 77

Pressure Gradient ForcePressure Gradient Force

Example: Sloshing water back and forth in a tub Example: Sloshing water back and forth in a tub creates pressure gradients along the tub bottom, creates pressure gradients along the tub bottom, analogous to a horizontal air pressure gradient in analogous to a horizontal air pressure gradient in the atmosphere. In response to a pressure the atmosphere. In response to a pressure gradient, water (or air) flows from an area of gradient, water (or air) flows from an area of higher pressure to an area of lower pressure.higher pressure to an area of lower pressure.

© AMS© AMS 88


Centripetal ForceCentripetal Force– Isobars plotted on a surface weather map are almost always curved; the Isobars plotted on a surface weather map are almost always curved; the

wind blows in curved paths. Curved motion indicates the influence of the wind blows in curved paths. Curved motion indicates the influence of the centripetal force.centripetal force.

– Center-seeking force; the string exerts a net force on the rock by Center-seeking force; the string exerts a net force on the rock by confining it to a curved pathconfining it to a curved path

– Increasing the rotation rate or shortening the string requires a large Increasing the rotation rate or shortening the string requires a large centripetal forcecentripetal force

– Not an independent force; the tension in the string is responsible for the Not an independent force; the tension in the string is responsible for the centripetal forcecentripetal force If the string is cut, the centripetal force no longer operates and the rock flies 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 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 straight-line, unaccelerated motion remains that way unless acted upon by an unbalanced force).unbalanced force).

– Results from an imbalance in other forces operating in the atmosphereResults from an imbalance in other forces operating in the atmosphere

© AMS© AMS 99


Coriolis EffectCoriolis Effect– Frame of reference example: In looking at the Earth Frame of reference example: In looking at the Earth

from space, a storm system appears to move in a from space, a storm system appears to move in a straight line at constant speed. Meanwhile, an observer straight line at constant speed. Meanwhile, an observer on Earth observes the storm center following a curved on Earth observes the storm center following a curved path. path.

– Curved motion implies that a net (or unbalanced) force Curved motion implies that a net (or unbalanced) force is operating and unaccelerated, straight motion implies is operating and unaccelerated, straight motion implies a balance of forcesa balance of forces

– A net force operates on the Earthbound rotating A net force operates on the Earthbound rotating coordinate system whereas forces are balanced in the coordinate system whereas forces are balanced in the non-rotating system fixed in spacenon-rotating system fixed in space

– The net force responsible for curved motion is the The net force responsible for curved motion is the Coriolis EffectCoriolis Effect

© AMS© AMS 1010

Coriolis EffectCoriolis Effect

The familiar north-south, east-west frame of reference The familiar north-south, east-west frame of reference rotates eastward in space as Earth rotates on its axis. rotates eastward in space as Earth rotates on its axis. Rotation of the coordinate system gives rise to the Coriolis Rotation of the coordinate system gives rise to the Coriolis Effect.Effect.

© AMS© AMS 1111

Forces Governing the WindForces Governing the Wind Coriolis Effect, continuedCoriolis Effect, continued

– Deflection is to the right in Deflection is to the right in Northern Hemisphere and to Northern Hemisphere and to the left in the Southern the left in the Southern HemisphereHemisphere

– Deflection is strongest at the Deflection is strongest at the poles, decreases moving away poles, decreases moving away from poles, and is zero at the from poles, and is zero at the equatorequator

– Fast-moving objects are Fast-moving objects are deflected more than slower deflected more than slower ones because faster objects ones because faster objects cover greater distances. The cover greater distances. The longer the trajectory, the longer the trajectory, the greater is the shift of the greater is the shift of the rotating coordinate system with rotating coordinate system with respect to the moving air parcelrespect to the moving air parcel

– Coriolis Effect only significantly Coriolis Effect only significantly influences the wind in large-influences the wind in large-scale weather systemsscale weather systems

© AMS© AMS 1212

Forces Governing the WindForces Governing the Wind FrictionFriction

– The resistance an object or medium encounters as The resistance an object or medium encounters as it moves in contact with another object or mediumit moves in contact with another object or medium

– The resistance of fluid (liquid and gas) flow is The resistance of fluid (liquid and gas) flow is termed viscositytermed viscosity Two types:Two types:

– Molecular viscosity: the random motion of molecules in the fluidMolecular viscosity: the random motion of molecules in the fluid– Eddy viscosity (more important): arises from much larger Eddy viscosity (more important): arises from much larger

irregular motions, called eddiesirregular motions, called eddies

Atmospheric boundary layer: the zone to which frictional Atmospheric boundary layer: the zone to which frictional resistance (eddy viscosity) is essentially confinedresistance (eddy viscosity) is essentially confined

– Above 1000 m (3300 ft), friction is a minor forceAbove 1000 m (3300 ft), friction is a minor force

Turbulence: fluid flow characterized by eddy motionTurbulence: fluid flow characterized by eddy motion– We experience turbulent eddies as gusts of windWe experience turbulent eddies as gusts of wind

© AMS© AMS 1313

Examples of Eddy ViscosityExamples of Eddy Viscosity

Stream Example: Rocks in a Stream Example: Rocks in a streambed cause the current to streambed cause the current to break down into eddies that tap break down into eddies that tap some of the stream’s energy so that some of the stream’s energy so that the stream slowsthe stream slows

Snow Fence Example: A snow Snow Fence Example: A snow fence taps some of the wind’s fence taps some of the wind’s kinetic energy by breaking the wind kinetic energy by breaking the wind into small eddies. Wind speed into small eddies. Wind speed diminishes, causing loss of snow-diminishes, causing loss of snow-transporting abilitytransporting ability

© AMS© AMS 1414

Forces Governing the WindForces Governing the Wind GravityGravity

– The force that holds objects to the Earth’s surfaceThe force that holds objects to the Earth’s surface– Net result of gravitation and centripetal forceNet result of gravitation and centripetal force

Gravitation is the force of attraction between the Earth and some Gravitation is the force of attraction between the Earth and some objectobject

– It’s magnitude is directly proportional to the product of the masses of It’s magnitude is directly proportional to the product of the masses of Earth and the object Earth and the object

– It is inversely proportional to the square of the distance between their It is inversely proportional to the square of the distance between their centers of masscenters of mass

The much weaker centripetal force is caused by the Earth’s rotationThe much weaker centripetal force is caused by the Earth’s rotation Gravity always acts directly downwardGravity always acts directly downward

– It does not influence horizontal windIt does not influence horizontal wind

– It only influences air that is ascending or descending It only influences air that is ascending or descending

– Accelerates a unit mass downward toward Earth’s surface at 9.8 m per Accelerates a unit mass downward toward Earth’s surface at 9.8 m per sec each secondsec each second

© AMS© AMS 1515

Forces Governing the WindForces Governing the Wind SummarySummary

– Horizontal pressure gradient force is responsible for initiating Horizontal pressure gradient force is responsible for initiating almost all air motionalmost all air motion Accelerates air parcels perpendicular to isobars, away from high Accelerates air parcels perpendicular to isobars, away from high

pressure and toward low pressurepressure and toward low pressure– Centripetal force is an imbalance of actual forcesCentripetal force is an imbalance of actual forces

Exists when wind has a curved pathExists when wind has a curved path Changes wind direction, not wind speedChanges wind direction, not wind speed Always directed inward toward center of rotationAlways directed inward toward center of rotation

– Coriolis Effect arises from the rotation of EarthCoriolis Effect arises from the rotation of Earth Deflects winds to the right in the Northern HemisphereDeflects winds to the right in the Northern Hemisphere Deflects winds to the left in the Southern HemisphereDeflects winds to the left in the Southern Hemisphere

– Friction acts opposite to the wind directionFriction acts opposite to the wind direction It increases with increasing surface roughnessIt increases with increasing surface roughness Slows horizontal winds within about 1000 m (3300 ft) of the surfaceSlows horizontal winds within about 1000 m (3300 ft) of the surface

– Gravity accelerates air downwardGravity accelerates air downward It does not modify horizontal windsIt does not modify horizontal winds

© AMS© AMS 1616

Wind: Joining ForcesWind: Joining Forces

Newton’s first law of motionNewton’s first law of motion– When the forces acting on a parcel of air are in balance, When the forces acting on a parcel of air are in balance,

no net force operates, and the parcel either remains no net force operates, and the parcel either remains stationary, or continues to move along a straight path at stationary, or continues to move along a straight path at a constant speeda constant speed

Interaction of forces control vertical and horizontal Interaction of forces control vertical and horizontal air flow through:air flow through:– Hydrostatic equilibriumHydrostatic equilibrium– The geostrophic windThe geostrophic wind– The gradient windThe gradient wind– Surface winds, horizontal winds within the atmospheric Surface winds, horizontal winds within the atmospheric

boundary layerboundary layer

© AMS© AMS 1717


Hydrostatic equilibriumHydrostatic equilibrium– Air pressure always declines Air pressure always declines

with altitudewith altitude– Vertical pressure gradient force Vertical pressure gradient force

is upwardis upward Were this the only force, air Were this the only force, air

would accelerate away from would accelerate away from EarthEarth

– Counteracting downward force Counteracting downward force is gravityis gravity

– Balance between the two forces Balance between the two forces is hydrostatic equilibriumis hydrostatic equilibrium

– Slight deviations from Slight deviations from hydrostatic equilibrium cause air hydrostatic equilibrium cause air parcels to accelerate verticallyparcels to accelerate vertically

© AMS© AMS 1818


Geostrophic windGeostrophic wind– Winds blowing at a large scale Winds blowing at a large scale

tend to parallel isobars with low tend to parallel isobars with low pressure on the left in the pressure on the left in the Northern HemisphereNorthern Hemisphere

– Geostrophic wind is a horizontal Geostrophic wind is a horizontal movement of air that follows a movement of air that follows a straight path at altitudes above straight path at altitudes above the atmospheric boundary layerthe atmospheric boundary layer

– Caused by a balance between Caused by a balance between the horizontal pressure gradient the horizontal pressure gradient force and the Coriolis Effectforce and the Coriolis Effect

– Develops only where the Coriolis Develops only where the Coriolis Effect is significant (i.e., in large-Effect is significant (i.e., in large-scale weather systems)scale weather systems)

© AMS© AMS 1919

Wind: Joining ForcesWind: Joining Forces Gradient WindGradient Wind

– Shares many characteristics with the Shares many characteristics with the geostrophic windgeostrophic wind Large-scale, frictionless, and blows parallel Large-scale, frictionless, and blows parallel

to the isobarsto the isobars– The path of the gradient wind is curvedThe path of the gradient wind is curved

Forces are not balanced because a net Forces are not balanced because a net centripetal force constrains air parcels to a centripetal force constrains air parcels to a curved trajectorycurved trajectory

– Occurs around high and low pressure Occurs around high and low pressure centers above the boundary layercenters above the boundary layer

– High (anticyclone) in N. HemisphereHigh (anticyclone) in N. Hemisphere Coriolis Effect is slightly greater than the Coriolis Effect is slightly greater than the

pressure gradient force giving rise to an pressure gradient force giving rise to an inward-directed centripetal forceinward-directed centripetal force

Wind is clockwiseWind is clockwise– Low (cyclone) in N. HemisphereLow (cyclone) in N. Hemisphere

Pressure gradient force is slightly greater Pressure gradient force is slightly greater than the Coriolis Effect giving rise to an than the Coriolis Effect giving rise to an inward-directed centripetal forceinward-directed centripetal force

Wind is counterclockwiseWind is counterclockwise

© AMS© AMS 2020

Wind: Joining ForcesWind: Joining Forces Surface WindsSurface Winds

– Friction slows the wind and Friction slows the wind and interacts with the other forces to interacts with the other forces to change wind directionchange wind direction

– Friction combines with the Coriolis Friction combines with the Coriolis Effect to balance the horizontal Effect to balance the horizontal pressure gradient forcepressure gradient force Friction acts directly opposite the Friction acts directly opposite the

wind direction whereas the Coriolis wind direction whereas the Coriolis Effect is always at a right angle to Effect is always at a right angle to the wind directionthe wind direction

– Winds now cross isobars at an Winds now cross isobars at an angle, which depends on angle, which depends on roughness of Earth’s surfaceroughness of Earth’s surface Angle varies from 10 degrees or Angle varies from 10 degrees or

less to 45 degreesless to 45 degrees

© AMS© AMS 2121

Surface Winds, cont.Surface Winds, cont.– The closer to the Earth’s The closer to the Earth’s

surface the winds are, the surface the winds are, the more friction comes into more friction comes into playplay

– For the same horizontal air For the same horizontal air pressure gradient, the pressure gradient, the angle between the wind angle between the wind direction and isobars direction and isobars decreases with altitude in decreases with altitude in the atmospheric boundary the atmospheric boundary layerlayer


© AMS© AMS 2222


Surface winds in the Northern Surface winds in the Northern HemisphereHemisphere– Surface winds blow clockwise Surface winds blow clockwise

and outward in a high and outward in a high (anticyclone)(anticyclone)

– Surface winds blow Surface winds blow counterclockwise and inward in counterclockwise and inward in a low (cyclone)a low (cyclone)

– In the Southern Hemisphere, In the Southern Hemisphere, surface winds in a cyclone blow surface winds in a cyclone blow clockwise and inward; in an clockwise and inward; in an anticyclone winds blow anticyclone winds blow counterclockwise and outwardcounterclockwise and outward

© AMS© AMS 2323

On a typical surface weather map, isobars exhibit clockwise curvature (ridges) and counterclockwise curvature (troughs)


© AMS© AMS 2424

Horizontal and vertical Horizontal and vertical components of the wind are components of the wind are linkedlinked– Surface winds follow Earth’s Surface winds follow Earth’s

topographytopography– Uplift occurs along frontal Uplift occurs along frontal

surfacessurfaces In a surface high, horizontal In a surface high, horizontal

winds diverge from the center. winds diverge from the center. A vacuum does not develop A vacuum does not develop because air slowly descends because air slowly descends to replace diverging air at the to replace diverging air at the surface. Aloft, horizontal winds surface. Aloft, horizontal winds converge above the center of converge above the center of the surface high.the surface high.

Anticyclones typically have Anticyclones typically have clear skies and a weak clear skies and a weak horizontal pressure gradienthorizontal pressure gradient

Continuity of WindContinuity of Wind

Warming & evaporation

H

© AMS© AMS 2525

In a surface low, In a surface low, horizontal winds converge horizontal winds converge toward the center. Air toward the center. Air does not pile up at the does not pile up at the center, but ascends in center, but ascends in response to converging response to converging surface winds and surface winds and diverging winds aloft.diverging winds aloft.

Cyclones are typically Cyclones are typically stormy weather systems stormy weather systems with cloud and with cloud and precipitation developmentprecipitation development


Cooling & Condensation

L

© AMS© AMS 2626


Surface winds accelerate and undergo horizontal divergence when blowing from 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 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 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, (e.g.’ from water to land). Divergence of surface winds causes air to descend, whereas convergence of surface winds causes air to ascend.whereas convergence of surface winds causes air to ascend.

© AMS© AMS 2727

Monitoring Wind Speed and DirectionMonitoring Wind Speed and Direction Wind velocity is a vector quantityWind velocity is a vector quantity

– Meaning it has both magnitude (speed) and Meaning it has both magnitude (speed) and directiondirection

Wind is usually distinguished between Wind is usually distinguished between horizontal and vertical componentshorizontal and vertical components– The magnitude of vertical air motion is typically The magnitude of vertical air motion is typically

only 1% to 10% of the horizontal wind speedonly 1% to 10% of the horizontal wind speed– The most common instruments only measure the The most common instruments only measure the

horizontal windhorizontal wind– A wind vane consists of a free rotating and A wind vane consists of a free rotating and

counterweighted arrow that points into the windcounterweighted arrow that points into the wind Wind direction is the direction it is coming from, not Wind direction is the direction it is coming from, not

blowing toblowing to

– A windsock stretches downwindA windsock stretches downwind– Wind speed can be estimated by its effect on Wind speed can be estimated by its effect on

water using the Beaufort scalewater using the Beaufort scale

© AMS© AMS 2828

Beaufort Scale Beaufort Scale of Wind Forceof Wind Force

© AMS© AMS 2929

Wind speed is measured using a cup anemometerWind speed is measured using a cup anemometer– The speed at which the cups spin is translated into wind speedThe speed at which the cups spin is translated into wind speed

A hot wire anemometer measures the loss of heat from a A hot wire anemometer measures the loss of heat from a heated wire, and translates that into wind speedheated wire, and translates that into wind speed

Aerovanes measure wind speed and directionAerovanes measure wind speed and direction– A 3 or 4 blade propeller spins at a rate proportional to the wind speed 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. and a fin on the back turns it into the wind, indicating direction. Electronic sensors are connected to a recording computer or digital Electronic sensors are connected to a recording computer or digital display.display.

A sonic anemometer consists of 3 arms that send and receive A sonic anemometer consists of 3 arms that send and receive ultrasonic pulses. Sound wave travel times are translated into ultrasonic pulses. Sound wave travel times are translated into wind speed and directionwind speed and direction– Scheduled to replace cup anemometers in NWS ASOSScheduled to replace cup anemometers in NWS ASOS

Instruments should be mounted at 10 m (33 ft) above the Instruments should be mounted at 10 m (33 ft) above the ground. Rooftop locations should be avoided.ground. Rooftop locations should be avoided.

Radiosondes, satellites, and wind profilers measure winds aloftRadiosondes, satellites, and wind profilers measure winds aloft

Monitoring Wind Speed and DirectionMonitoring Wind Speed and Direction

© AMS© AMS 3030

Monitoring Wind Speed and DirectionMonitoring Wind Speed and Direction

Cup AnemometerCup Anemometer

Sonic Sonic AnemometerAnemometer

Time Time variations in variations in wind speed wind speed and and direction direction over a six-over a six-hour periodhour period

aerovaneaerovane

© AMS© AMS 3131

Scales of Weather SystemsScales of Weather Systems

Planetary-scale systems: large-scale wind belts encircling the planet Planetary-scale systems: large-scale wind belts encircling the planet (e.g., midlatitude westerlies, trade winds)(e.g., midlatitude westerlies, trade winds)

Synoptic-scale systems: continental or oceanic in nature (e.g., Synoptic-scale systems: continental or oceanic in nature (e.g., migrating cyclones, hurricanes, and air masses)migrating cyclones, hurricanes, and air masses)

Mesoscale-scale systems: circulation systems that influence weather Mesoscale-scale systems: circulation systems that influence weather in part of a large city or county (e.g., thunderstorms, sea breeze)in part of a large city or county (e.g., thunderstorms, sea breeze)

Microscale systems: weather system covering a very small area such Microscale systems: weather system covering a very small area such as several city blocks (e.g., weak tornado)as several city blocks (e.g., weak tornado)

© AMS© AMS 3232

ConclusionsConclusions

Unequal rates of radiational heating and cooling Unequal rates of radiational heating and cooling within the Earth-atmosphere system are within the Earth-atmosphere system are responsible for temperature gradientsresponsible for temperature gradients

The atmosphere circulates in response and heat The atmosphere circulates in response and heat energy is converted to kinetic energyenergy is converted to kinetic energy

Various forces studied in this chapter shape Various forces studied in this chapter shape atmospheric circulation (the wind)atmospheric circulation (the wind)

This chapter built a realistic model of atmospheric This chapter built a realistic model of atmospheric motion that demonstrates why and how winds motion that demonstrates why and how winds circulate around high and low pressure systemscirculate around high and low pressure systems

Documents

© AMS 1 Chapter 8 Wind and Weather AMS Weather Studies Introduction to Atmospheric Science, 4 th Edition