energy balance - udel.edu

Energy BalanceEnergy Balance

The The inputsinputs and and outputsoutputs of energy within the earth-atmosphere of energy within the earth-atmosphere system that determines the net energy available for surface system that determines the net energy available for surface processes is the processes is the Energy BalanceEnergy Balance

Electromagnetic RadiationElectromagnetic Radiation

Electromagnetic radiation can be:Electromagnetic radiation can be:

AbsorbedAbsorbed – – increases the energy content of the object, increases the energy content of the object, like a black car in the summerlike a black car in the summer

Reflected Reflected – – bounce the energy back toward the source, bounce the energy back toward the source, like a mirrorlike a mirror

TransmittedTransmitted – – pass through without absorption or pass through without absorption or reflection, like a pane of glassreflection, like a pane of glass

TransmissionTransmission – the passage of shortwave and longwave – the passage of shortwave and longwave energy either through the atmosphere or waterenergy either through the atmosphere or water

InputsInputs = shortwave radiation from the sun that passes = shortwave radiation from the sun that passes through the atmospherethrough the atmosphere

OutputsOutputs = longwave radiation from the earth’s surface and = longwave radiation from the earth’s surface and atmosphere that passes through the atmosphereatmosphere that passes through the atmosphere

Several things happen to insolation after it enters the Several things happen to insolation after it enters the atmosphere:atmosphere: Not all of the energy that enters the top of the atmosphere reaches Not all of the energy that enters the top of the atmosphere reaches

the earth’s surfacethe earth’s surface

Losses of incoming solar radiationLosses of incoming solar radiation

Refraction: Refraction: the bending of insolation as it moves through the the bending of insolation as it moves through the atmos towards earth’s surfaceatmos towards earth’s surface as insolation moves through the atmos it passes through one as insolation moves through the atmos it passes through one

medium to anothermedium to another The speed of insolation is reduced as it moves through air of The speed of insolation is reduced as it moves through air of

increasing densityincreasing density The decrease in speed as it enters thicker layers of he atmosphere The decrease in speed as it enters thicker layers of he atmosphere

causes it to shift direction or bend (refract)causes it to shift direction or bend (refract)


Reflection: Reflection: the portion of incoming solar energy that bounces the portion of incoming solar energy that bounces directly back into space without being absorbed within the directly back into space without being absorbed within the earth-atmosphere systemearth-atmosphere system

Decided by the Decided by the albedoalbedo

Albedo: Albedo: is a measure of the reflective quality of a surfaceis a measure of the reflective quality of a surface the % of insolation that is reflected by the surfacethe % of insolation that is reflected by the surface

Albedo Albedo depends on the color and texture of a surface and the depends on the color and texture of a surface and the angle of incoming solar radiation.angle of incoming solar radiation.

Dark colors absorb more energy than lighter surfaces:Dark colors absorb more energy than lighter surfaces: Fresh snow = 80-90%Fresh snow = 80-90% Grass = 25-30%Grass = 25-30% Crops = 10-25%Crops = 10-25% Asphalt = 5-10%Asphalt = 5-10% Forest = 10-20%Forest = 10-20%

Angle of insolation also affects albedoAngle of insolation also affects albedo


Scattering: Scattering: gas molecules absorb and reemit radiation, gas molecules absorb and reemit radiation, changing it’s direction but does not change it’s wavelengthchanging it’s direction but does not change it’s wavelength Some radiation absorbed by the atmos is reradiated towards Some radiation absorbed by the atmos is reradiated towards

earth’s surface while the rest of it is reradiated back to spaceearth’s surface while the rest of it is reradiated back to space

Diffuse RadiationDiffuse Radiation – the portion of insolation absorbed by the atmos and – the portion of insolation absorbed by the atmos and scattered towards the earth’s surfacescattered towards the earth’s surface

Diffuse ReflectionDiffuse Reflection – the portion of insolation absorbed by the atmos and – the portion of insolation absorbed by the atmos and scattered back to spacescattered back to space

Shorter wavelengths are scattered more easily than longer Shorter wavelengths are scattered more easily than longer wavelengths (Lord Rayleigh, 1881)wavelengths (Lord Rayleigh, 1881)

Short wavelengths are more susceptible to be scattered by small Short wavelengths are more susceptible to be scattered by small molecules in the atmosmolecules in the atmos

For small molecules in the atmos, scattering depends on 1/For small molecules in the atmos, scattering depends on 1/λλ44

The colors we see:The colors we see:

When we see a color we actually see the wavelength of When we see a color we actually see the wavelength of visible light that is reflected by that object. All other visible light that is reflected by that object. All other wavelengths are absorbed.wavelengths are absorbed.

Black happens when all visible wavelengths are absorbedBlack happens when all visible wavelengths are absorbed White happens when all visible wavelengths are reflectedWhite happens when all visible wavelengths are reflected

The greater the amount of atmos that light has to pass The greater the amount of atmos that light has to pass through to reach the surface the greater the amount of through to reach the surface the greater the amount of scattering:scattering:

That’s why the sky is blue!That’s why the sky is blue!

Absorption Absorption – the assimilation of radiation by molecules – the assimilation of radiation by molecules changing it from one form of energy to anotherchanging it from one form of energy to another

Absorption increases temperature of objectAbsorption increases temperature of object Absorbed energy is reradiated at longer wavelengths (ozone layer)Absorbed energy is reradiated at longer wavelengths (ozone layer)

CloudsClouds – are variable and greatly affect the amount of energy – are variable and greatly affect the amount of energy near the surfacenear the surface

Reflect insolation back to space Reflect insolation back to space → → cooling effectcooling effect (cloud-albedo forcing)(cloud-albedo forcing)

Absorb longwave radiation and reradiate it back to the Absorb longwave radiation and reradiate it back to the surface → surface → warming effectwarming effect (cloud- (cloud-greenhouse forcing)greenhouse forcing)

Clouds can make it cooler during the day by reflecting large amounts of Clouds can make it cooler during the day by reflecting large amounts of insolation but warmer at night absorbing and reradiating longwave insolation but warmer at night absorbing and reradiating longwave terrestrial radiationterrestrial radiation

Top of Atmosphere Energy BalanceTop of Atmosphere Energy Balance

SourceSource InputInput Output Output Sun (shortwave)Sun (shortwave) 100%100%

TotalTotal 100%100%


SourceSource InputInput Output Output Sun (shortwave)Sun (shortwave) 100%100%Reflection (shortwave)Reflection (shortwave) 21% + 6% + 4%21% + 6% + 4%

TotalTotal 100%100% 31%31%


SourceSource InputInput OutputOutput Sun (shortwave)Sun (shortwave) 100%100%Reflection (shortwave)Reflection (shortwave) 21% + 6% + 4%21% + 6% + 4%Longwave loss to spaceLongwave loss to space 63% + 6%63% + 6%

TotalTotal 100%100% 100%100%

Top of the atmosphere is balanced!Top of the atmosphere is balanced!

Surface Energy BalanceSurface Energy Balance

SourceSource InputInput Output Output Sun (shortwave)Sun (shortwave) 24% + 24% 24% + 24%

TotalTotal 48% 48%


SourceSource InputInput Output Output Sun (shortwave)Sun (shortwave) 24% + 24%24% + 24%LongwaveLongwave 97%97%

TotalTotal 145%145%


SourceSource InputInput Output Output Sun (shortwave)Sun (shortwave) 24% + 24%24% + 24%LongwaveLongwave 97%97%Longwave Longwave 107% + 6%107% + 6%

TotalTotal 145%145% 113%113%

The surface energy budget is not balanced!The surface energy budget is not balanced!

Where is the other energy to make the surface balance?Where is the other energy to make the surface balance?

Sensible HeatSensible Heat

Sensible HeatSensible Heat – heat you can feel (sense) – heat you can feel (sense)

Energy exchange between objects by flow of a fluid (atmosphere) – Energy exchange between objects by flow of a fluid (atmosphere) – ConvectionConvection

Energy exchange between objects by physical contact – Energy exchange between objects by physical contact – ConductionConduction

Convection:Convection: Energy is transferred by the Energy is transferred by the

vertical mixing of a liquid or vertical mixing of a liquid or gasgas

Ex: boiling water in a panEx: boiling water in a pan

Conduction:Conduction: As molecules warm they As molecules warm they

vibrate more rapidly, vibrate more rapidly, bumping into nearby bumping into nearby molecules, causing them molecules, causing them to vibrateto vibrate

This transfer of kinetic This transfer of kinetic energy is heat energy is heat

Ex: touching a hot pan or a Ex: touching a hot pan or a cold surfacecold surface


SourceSource InputInput Output Output Sun (shortwave)Sun (shortwave) 24% + 24%24% + 24%LongwaveLongwave 97% 97%LongwaveLongwave 107% + 6%107% + 6%Sensible heatSensible heat 10%10%

TotalTotal 145%145% 123%123%

Latent HeatLatent Heat

Latent HeatLatent Heat – energy stored by changing water from – energy stored by changing water from solid or liquid to a gassolid or liquid to a gas Latent – no change in temperature; change in phase of water Latent – no change in temperature; change in phase of water

moleculesmolecules Liquid to Gas – Liquid to Gas – EvaporationEvaporation Solid to Gas – Solid to Gas – SublimationSublimation

Energy is released during precipitation or condensationEnergy is released during precipitation or condensation


SourceSource InputInput Output Output Sun (shortwave)Sun (shortwave) 24% + 24%24% + 24%LongwaveLongwave 97% 97%LongwaveLongwave 107% + 6%107% + 6%Sensible HeatSensible Heat 10%10%Latent Heat Latent Heat 22% 22%

TotalTotal 145%145% 145%145%

The surface energy balance is now balanced!The surface energy balance is now balanced!

Atmospheric Energy BalanceAtmospheric Energy Balance

SourceSource InputInput Output OutputSW AbsorptionSW Absorption 3% + 18% 3% + 18%LongwaveLongwave 107%107%LongwaveLongwave 97% + 63%97% + 63%Sensible HeatSensible Heat 10%10%

Latent Heat Latent Heat 22% .22% .

TotalTotal 160%160% 160%160%

Global Energy BudgetGlobal Energy Budget

All three parts (top of the atmosphere, earth’s surface, and the All three parts (top of the atmosphere, earth’s surface, and the atmosphere) are now balancedatmosphere) are now balanced

Input = OutputInput = Output

therefore, the planet is in a state of equilibrium (constant temperature)therefore, the planet is in a state of equilibrium (constant temperature)

The Greenhouse EffectThe Greenhouse Effect

Like the sun, the earth emits electromagnetic radiationLike the sun, the earth emits electromagnetic radiation Unlike the sun, the earth is cooler and emits LW energy within the Unlike the sun, the earth is cooler and emits LW energy within the

infrared wavelengthsinfrared wavelengths

The earth emits The earth emits longwave energy longwave energy in all directions in all directions awayaway from from the earth’s surfacethe earth’s surface Some of this energy is radiated into spaceSome of this energy is radiated into space Some of this energy is absorbed by the atmosphereSome of this energy is absorbed by the atmosphere The energy that is absorbed by the atmosphere is reradiated by the The energy that is absorbed by the atmosphere is reradiated by the

atmosphereatmosphere Some is radiated back to spaceSome is radiated back to space The rest is reradiated back to the surgace of the earthThe rest is reradiated back to the surgace of the earth

The counterradiation of LW energy warms the troposphere The counterradiation of LW energy warms the troposphere similar to the way the inside of a greenhouse is warmedsimilar to the way the inside of a greenhouse is warmed the greenhouse effectthe greenhouse effect

Greenhouse glass transparent to SW energy from sunGreenhouse glass transparent to SW energy from sun SW energy absorbed and and reradiated as LW energySW energy absorbed and and reradiated as LW energy Glass is not transparent to longer IR wavelengthsGlass is not transparent to longer IR wavelengths LW IR wavelengths are sensed as heatLW IR wavelengths are sensed as heat

Thus the process traps heat within the greenhouse making it warmer Thus the process traps heat within the greenhouse making it warmer than outsidethan outside

The earth’s atmosphere acts like panes of a greenhouse:The earth’s atmosphere acts like panes of a greenhouse:

The atmos lets in solar radThe atmos lets in solar rad Rad is absorbed by the earth’s surfaceRad is absorbed by the earth’s surface SW energy is reradiated as LW along with terrestrial radSW energy is reradiated as LW along with terrestrial rad Some LW energy is blocked from entering space Some LW energy is blocked from entering space → → warmingwarming GH effect is GH effect is necessarynecessary…. …. Without it we would freezeWithout it we would freeze

GH is increasing due to increased COGH is increasing due to increased CO22 over past 200yrs over past 200yrs

Daily RadiationDaily Radiation

Daily RadiationDaily Radiation – radiation evaluated for a period of 24 hours – radiation evaluated for a period of 24 hours

Daily radiation received at a certain location varies with season and Daily radiation received at a certain location varies with season and latitudelatitude

It also varies with degree of cloud coverIt also varies with degree of cloud cover No insolation is received at nightNo insolation is received at night The amount of insolation peaks at solar noon (when the sun is at its The amount of insolation peaks at solar noon (when the sun is at its

highest altitude)highest altitude) Insolation decreases from solar noon to sunsetInsolation decreases from solar noon to sunset

Daily RadiationDaily Radiation

Air temperatureAir temperature responds to the changing amount of responds to the changing amount of insolation throughout the dayinsolation throughout the day Minimum daily temperature occurs just after sunriseMinimum daily temperature occurs just after sunrise Air temperature begins to increase soon after sunrise when Air temperature begins to increase soon after sunrise when

insolation starts to be absorbed by the surfaceinsolation starts to be absorbed by the surface Air temperature peaks around 3pm after solar noonAir temperature peaks around 3pm after solar noon When outputs of energy exceed the inputs, temperature begins to When outputs of energy exceed the inputs, temperature begins to

decreasedecrease

The Urban EnvironmentThe Urban Environment

Urban environmentsUrban environments differ from surrounding rural differ from surrounding rural environmentsenvironments- Urban environments generally have lower albedosUrban environments generally have lower albedos- Surfaces tend to be drier Surfaces tend to be drier

Therefore urban areas tend to be warmer b/c they absorb and retain Therefore urban areas tend to be warmer b/c they absorb and retain more energy and lower surface moisture leads to less cooling by more energy and lower surface moisture leads to less cooling by evaporation and transpirationevaporation and transpiration

Transpiration Transpiration – water is taken from the soil by plants and – water is taken from the soil by plants and released by their leavesreleased by their leaves

Water changes state from liquid to vapor cooling the plant through the Water changes state from liquid to vapor cooling the plant through the loss of latent heat of evaporationloss of latent heat of evaporation

Rural Areas:Rural Areas: Overall higher albedos than urbanOverall higher albedos than urban More plants for transpirationMore plants for transpiration Moisture is retained by the soilMoisture is retained by the soil

Solar energy absorbed by soil is used from evaporationSolar energy absorbed by soil is used from evaporation Evaporation cools surface through the loss of latent heat of evaporationEvaporation cools surface through the loss of latent heat of evaporation Forests intercept insolation before the ground, spreading insolation Forests intercept insolation before the ground, spreading insolation

over the top of the forest canopy rather than along the groundover the top of the forest canopy rather than along the ground

Urban areas:Urban areas: Lower albedosLower albedos GeometryGeometry Hunan activitiesHunan activities pollutionpollution

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