Earth’s Climate System Today Heated by solar energyHeated by solar energy Tropics heated more than polesTropics heated more than poles

Imbalance in heating redistributedImbalance in heating redistributed Solar heating and movement of heat by Solar heating and movement of heat by

oceans and atmosphere determines oceans and atmosphere determines distribution of:distribution of: TemperatureTemperature PrecipitationPrecipitation IceIce VegetationVegetation

Electromagnetic Spectrum Electromagnetic Electromagnetic

energy travels energy travels through spacethrough space

Energy heating Energy heating Earth mostly short-Earth mostly short-wave radiationwave radiation Visible lightVisible light Some ultraviolet Some ultraviolet

radiationradiation

Incoming Solar Radiation Radiation at top of Radiation at top of

Earth’s atmosphere Earth’s atmosphere = 1368 W m= 1368 W m-2-2

If Earth flat disk with If Earth flat disk with no atmosphere, no atmosphere, average radiation = average radiation = 1368 W m1368 W m-2-2

Earth 3-dimensional Earth 3-dimensional rotating sphere,rotating sphere, Area = 4Area = 4rr22

Average solar Average solar heating = 1368 heating = 1368 4 = 342 W m4 = 342 W m-2-2

30% Solar Energy Reflected Energy reflected by clouds, dust, surfaceEnergy reflected by clouds, dust, surface

Ave. incoming radiation 0.7 x 342 = 240 W mAve. incoming radiation 0.7 x 342 = 240 W m-2-2

Energy Budget Earth’s temperature constant ~15Earth’s temperature constant ~15CC

Energy loss must = incoming energyEnergy loss must = incoming energyEarth is constantly receiving heat Earth is constantly receiving heat from Sun, therefore must lose equal from Sun, therefore must lose equal amount of heat back to spaceamount of heat back to space

Heat loss called back radiationHeat loss called back radiation Wavelengths in the infrared (long-Wavelengths in the infrared (long-

wave radiation)wave radiation) Earth is a radiator of heatEarth is a radiator of heat

If T > 1If T > 1K, radiator of heatK, radiator of heat

Energy Budget Average Earth’s surface temperature Average Earth’s surface temperature

~15~15CC Reasonable assumptionReasonable assumption

Surface of earth radiates heat with an Surface of earth radiates heat with an average temperature of 15average temperature of 15CC

However, satellite data indicate Earth However, satellite data indicate Earth radiating heat average temperature ~-radiating heat average temperature ~-1616CC

Why the discrepancy?Why the discrepancy? What accounts for the 31What accounts for the 31C heating?C heating?

Energy Budget Greenhouse gases absorb 95% of the long-Greenhouse gases absorb 95% of the long-

wave, back radiation emitted from Earth’s wave, back radiation emitted from Earth’s surfacesurface Trapped radiation reradiated down to Trapped radiation reradiated down to

Earth’s surfaceEarth’s surface Accounts for the 31Accounts for the 31C heatingC heating Satellites don’t detect radiationSatellites don’t detect radiation Muffling effect from greenhouse gasesMuffling effect from greenhouse gases

Heat radiated back to space from elevation of Heat radiated back to space from elevation of about 5 km (top of clouds) average 240 W mabout 5 km (top of clouds) average 240 W m-2-2

Keeps Earth’s temperature in balanceKeeps Earth’s temperature in balance

Energy Balance

Greenhouse Gases

Water vapor (HWater vapor (H22OO(v)(v), 1 to 3%), 1 to 3%)

Carbon dioxide (COCarbon dioxide (CO22, 0.037%; 365 , 0.037%; 365 ppmv)ppmv)

Methane (CHMethane (CH44, 0.00018%; 1.8 ppmv), 0.00018%; 1.8 ppmv)

Nitrous oxide (NNitrous oxide (N22O, 0.00000315%; 315 O, 0.00000315%; 315 ppbv)ppbv)

Clouds also trap outgoing radiation Clouds also trap outgoing radiation

Variations in Heat Balance Incoming solar Incoming solar

radiationradiation Stronger at low Stronger at low

latitudeslatitudes Weaker at high Weaker at high

latitudeslatitudes Tropics receive Tropics receive

more solar more solar radiation per unit radiation per unit area than Polesarea than Poles

Variations in Heat Balance What else affects variation in heat What else affects variation in heat

balance?balance? Solar radiation arrives at a low angleSolar radiation arrives at a low angle Snow and ice reflect more radiation at Snow and ice reflect more radiation at

high latitudeshigh latitudes AlbedoAlbedo

Percentage of incoming solar Percentage of incoming solar radiation that is reflected rather than radiation that is reflected rather than absorbedabsorbed

Average Albedo

Sun Angle Affects Albedo All of Earth’s All of Earth’s

surfaces absorb surfaces absorb more solar more solar radiation from an radiation from an overhead sunoverhead sun

Water reflects Water reflects <5% radiation <5% radiation from an overhead from an overhead SunSun

Sun Angle Affects Albedo Water reflects a high fraction of Water reflects a high fraction of

radiation from a low-lying Sunradiation from a low-lying Sun Earth average albedo = 10%Earth average albedo = 10%

Pole-to-Equator Heat Imbalance

Incoming solar Incoming solar radiation per unit radiation per unit area higher in area higher in Tropics than PolesTropics than Poles

Sun angle higher in Sun angle higher in Poles than TropicsPoles than Tropics

Albedo higher at Albedo higher at Poles than TropicsPoles than Tropics

Variations in cloud Variations in cloud cover affect heat cover affect heat imbalanceimbalance

Seasonal Change in Solar Radiation & Albedo

Tilt of Earth’s axis results in seasonal change in Tilt of Earth’s axis results in seasonal change in Solar radiation in each hemisphereSolar radiation in each hemisphere Snow and ice cover (albedo)Snow and ice cover (albedo)

Seasonal Change in Solar Radiation

Large seasonal change in solar Large seasonal change in solar radiation between the hemispheresradiation between the hemispheres

Seasonal Change in Albedo Increases in N. hemisphere winter due mainly to snow cover Increases in N. hemisphere winter due mainly to snow cover

and to lesser degree Arctic sea iceand to lesser degree Arctic sea ice Increases in S. hemisphere winter due to sea iceIncreases in S. hemisphere winter due to sea ice

Albedo-Temperature Feedback

Water a Key to Earth’s Climate

Water has high heat capacityWater has high heat capacity Measure of ability to absorb heatMeasure of ability to absorb heat

Heat measured in caloriesHeat measured in calories 1 calorie = amount of heat required to raise 1 calorie = amount of heat required to raise

temperature of one gram of water by 1temperature of one gram of water by 1CC Heat Capacity (cal cmHeat Capacity (cal cm-3-3) = Density (g cm) = Density (g cm-2-2) )

x Specific Heat (cal gx Specific Heat (cal g-1-1)) Specific heat of water = 1Specific heat of water = 1

Ratio of heat capacity water:ice:air:land Ratio of heat capacity water:ice:air:land 60:5:2:160:5:2:1 Heat capacity of air linked to water vaporHeat capacity of air linked to water vapor

Differences in Heating Land & Oceans

Low latitude ocean major Low latitude ocean major storage tank of solar heatstorage tank of solar heat Sunlight direct, albedo Sunlight direct, albedo

low, heat capacity highlow, heat capacity high Heats surface; winds Heats surface; winds

mix heatmix heat Contrast with landContrast with land

Albedo high, heat Albedo high, heat capacity & capacity & conductance lowconductance low

Tropical/subtropical Tropical/subtropical lands become hot, but lands become hot, but don’t store heatdon’t store heat

Sensitivity of Land & Oceans to Solar Heating

Change in mean seasonal surface temperature greatest Change in mean seasonal surface temperature greatest over large landmasses and lowest over oceansover large landmasses and lowest over oceans

Thermal Response Different Large land masses Large land masses

heat and cool heat and cool quicklyquickly Extreme seasonal Extreme seasonal

temperature temperature reached 1 month reached 1 month after Solsticeafter Solstice

Upper ocean heats Upper ocean heats and cools slowlyand cools slowly Extreme seasonal Extreme seasonal

temperature temperature reached 2-3 reached 2-3 months after months after SolsticeSolstice

Redistribution of Heat Heat transfer in Earth’ atmosphereHeat transfer in Earth’ atmosphere

Sensible heatSensible heatHeat that a person directly sensesHeat that a person directly sensesSensible heat = T x specific heatSensible heat = T x specific heat

Latent heat (hidden or concealed)Latent heat (hidden or concealed)Additional heat required to change Additional heat required to change the state of a substancethe state of a substance

Sensible and latent heat affected by Sensible and latent heat affected by convectionconvection

Convection

Sensible Heat Sensible heating greatestSensible heating greatest

At low latitudeAt low latitudeOverhead SunOverhead Sun

Over landOver landLow heat conductance (air heats)Low heat conductance (air heats)

Dry regionsDry regionsLow humidityLow humidity

Sensible heat lowestSensible heat lowest Over oceanic regionsOver oceanic regions

Latent Heat Heat is temporarily hidden or latent in Heat is temporarily hidden or latent in

water vaporwater vapor Powerful process transferring heat long Powerful process transferring heat long

distancesdistances Transfer isTransfer is two step process two step process

Initial evaporation of water and Initial evaporation of water and storage of heat in vaporstorage of heat in vapor

Later release of stored heat during Later release of stored heat during condensation and precipitation condensation and precipitation (typically far from site of evaporation)(typically far from site of evaporation)

Latent Heat

0°C-100°C, 1 calorie of heat energyneeded to increase 1 g H2O by 1°C

80 cal g-1 heat required for phase transformation, ice water80 cal g-1 heat released when water freezes – latent heat of melting

H2O(l) H2O(g) requires 540 cal g-1Condensation of water releases540 cal g-1 – latent heat of vaporization

Latent Heat of Vaporization Important – evaporation occurs at any Important – evaporation occurs at any

temperature between 0-100°Ctemperature between 0-100°C Latent heat is associated with any Latent heat is associated with any

change of statechange of state Therefore, during evaporation heat is Therefore, during evaporation heat is

stored in water vapor in latent form for stored in water vapor in latent form for later releaselater release

Water Vapor Content of Air Saturation vapor densitySaturation vapor density

Warm air holds 10X more water than coldWarm air holds 10X more water than cold

Redistribution of Latent Heat Evaporation in warm equatorial regionEvaporation in warm equatorial region Stored energy carried vertically and horizontallyStored energy carried vertically and horizontally Condensation and precipitation releases energyCondensation and precipitation releases energy

Water Vapor Feedback

Unequal Heating of Tropics and Poles

Latitudes <35° have excess incoming solar Latitudes <35° have excess incoming solar radiation over outgoing back radiationradiation over outgoing back radiation

Excess heat stored in upper ocean drives Excess heat stored in upper ocean drives general circulation of oceans and atmospheregeneral circulation of oceans and atmosphere

Atmospheric Circulation Atmosphere has no distinct Atmosphere has no distinct

upper boundaryupper boundary Air becomes less dense Air becomes less dense

with increasing altitudewith increasing altitude Air is compressible Air is compressible

and subject to greater and subject to greater compression at lower compression at lower elevations, density of elevations, density of air greater at surfaceair greater at surface

Constant composition to Constant composition to 80 km80 km

What drives atmospheric What drives atmospheric circulation?circulation?

Free Convection Atmospheric mixing related to Atmospheric mixing related to

buoyancybuoyancy Localized parcel of air is heated more Localized parcel of air is heated more

than nearby airthan nearby air Warm air is less dense than cold airWarm air is less dense than cold air Warm air is therefore more buoyant Warm air is therefore more buoyant

than cold airthan cold air Warm air risesWarm air rises

Forced Convection Occurs when a fluid breaks into Occurs when a fluid breaks into

disorganized swirling motions as it disorganized swirling motions as it undergoes flowundergoes flow

Fluid flow can be laminar or turbulentFluid flow can be laminar or turbulent

Laminar vs. Turbulent Flow Whether a fluid flow is laminar or Whether a fluid flow is laminar or

turbulent depends onturbulent depends on Velocity (rate of movement)Velocity (rate of movement) Geometry (primarily depth)Geometry (primarily depth) ViscosityViscosity

Turbulent flow occurs during high Turbulent flow occurs during high velocity movement of non-viscous velocity movement of non-viscous fluids in unconfined geometriesfluids in unconfined geometries

Forced Convection in Atmosphere

Horizontally moving air undergoes Horizontally moving air undergoes turbulenceturbulence Air is forced to mix vertically through Air is forced to mix vertically through

eddy motions because ofeddy motions because ofHigh velocityHigh velocityDepth of atmosphereDepth of atmosphereLow viscosityLow viscosity

Atmospheric Circulation Force of gravity maintains a stable Force of gravity maintains a stable

atmosphereatmosphere Most of the mass of air near surfaceMost of the mass of air near surface As a result of atmospheric pressureAs a result of atmospheric pressure

Dense air at surfaceDense air at surface Air flows from high pressure to low Air flows from high pressure to low

pressurepressure Flow is turbulentFlow is turbulent Turbulent flow produces vertical Turbulent flow produces vertical

mixingmixing

Mixing by Sensible Heat Convection driven by sensible heatConvection driven by sensible heat

Air parcels rise if they become heated and Air parcels rise if they become heated and less dense than surrounding airless dense than surrounding air

As air parcels rises,As air parcels rises, Air expandsAir expands Air coolsAir cools Air becomes less denseAir becomes less dense Air parcels stop risingAir parcels stop rising

Heat transferred vertically, since air forced Heat transferred vertically, since air forced from high to low pressure, heat also moves from high to low pressure, heat also moves horizontallyhorizontally

Adibatic Process Rising and sinking air change Rising and sinking air change

temperature with no gain or loss of heattemperature with no gain or loss of heat Consider sinking parcel of airConsider sinking parcel of air

As it sinks, it contractsAs it sinks, it contracts Contraction takes workContraction takes work

Work takes (mechanical) energyWork takes (mechanical) energy Temperature of air risesTemperature of air rises

Conservation of energyConservation of energy11stst law of thermodynamics law of thermodynamics

Thermodynamics of Air First law of thermodynamicsFirst law of thermodynamics

Heat added + work done = rise in Heat added + work done = rise in TempTemp

But, adibatic process (no heat added)But, adibatic process (no heat added) Heat added + work done = rise in Heat added + work done = rise in

TempTemp Second term is not zeroSecond term is not zero

Work of compression results in a rise Work of compression results in a rise in temperature of air parcel in temperature of air parcel

Mixing by Latent Heat Water vapor is less dense than mixture of gases Water vapor is less dense than mixture of gases

composing the atmospherecomposing the atmosphere Evaporation adds water vapor to atmosphere and Evaporation adds water vapor to atmosphere and

lowers its densitylowers its density Moist air rises, expands and cools until dew point Moist air rises, expands and cools until dew point

reachedreached When air becomes fully saturatedWhen air becomes fully saturated

Condensation beginsCondensation begins Air releases latent heatAir releases latent heat

Air heats and becomes less dense causing it to Air heats and becomes less dense causing it to rise furtherrise further

Eventually water vapor lost, air parcel stops Eventually water vapor lost, air parcel stops release of latent heat and stops risingrelease of latent heat and stops rising

Which Process More Important? Atmospheric circulation driven by Atmospheric circulation driven by

adiabatic processes (sensible heat) adiabatic processes (sensible heat) redistributes about 30% heatredistributes about 30% heat

Atmospheric circulation driven by latent Atmospheric circulation driven by latent heat redistributes about 70% heatheat redistributes about 70% heat Greater amount of heat stored in Greater amount of heat stored in

waterwater Larger distances moist air parcels Larger distances moist air parcels

movemove

Dry Adibatic Lapse Rate Rising and sinking dry air parcel cools Rising and sinking dry air parcel cools

and heats at a constant rateand heats at a constant rate Dry adibatic lapse rate = 10°C kmDry adibatic lapse rate = 10°C km-1-1

Work required to lift an air parcelWork required to lift an air parcel Mix of gasesMix of gases Acceleration of gravityAcceleration of gravity

Regardless of latitude, season, altitude, Regardless of latitude, season, altitude, etc. a dry parcel of air will heat or cool etc. a dry parcel of air will heat or cool at 10°C kmat 10°C km-1-1

Dew Point Lapse Rate Consider a rising parcel of air with constant Consider a rising parcel of air with constant

humidityhumidity Dew point decreases as parcel expandsDew point decreases as parcel expands

Drop in pressure, drop in dew pointDrop in pressure, drop in dew point Lapse of dew point as parcel risesLapse of dew point as parcel rises

Dew point lapse rate 2°C kmDew point lapse rate 2°C km-1-1

Over 1 km, air cools by 10°COver 1 km, air cools by 10°C Air temperature rapidly approaches dew Air temperature rapidly approaches dew

point as parcel risespoint as parcel rises As air temperature approaches dew point, As air temperature approaches dew point,

cloud formscloud forms

Wet Adibatic Lapse Rate As wet air rises, it cools, dew point As wet air rises, it cools, dew point

reached and condensation beginsreached and condensation begins Latent heat releasedLatent heat released Decreasing rate of coolingDecreasing rate of cooling

Wet adibatic lapse rateWet adibatic lapse rate 4°C km4°C km-1-1 minimum (rapid condensation) minimum (rapid condensation) 9°C km9°C km-1-1 maximum (slow condensation) maximum (slow condensation)

Differences in temperatureDifferences in temperatureFor same amount of cooling, warm air For same amount of cooling, warm air looses more water than cold airlooses more water than cold air

Summary Once saturation reached latent heat released Once saturation reached latent heat released

as long as parcel continues to riseas long as parcel continues to rise The saturated process assumes condensation The saturated process assumes condensation

products fall out of parcel, so the parcel products fall out of parcel, so the parcel maintains 100% humiditymaintains 100% humidity

Upon decent, the parcel warms, relatively Upon decent, the parcel warms, relatively humidity falls below 100%humidity falls below 100%

After decent the parcel is warmer because After decent the parcel is warmer because latent heat was added during ascentlatent heat was added during ascent Dry adibatic process reversibleDry adibatic process reversible Wet adibatic process non-reversibleWet adibatic process non-reversible