© AMS© AMS 11

Chapter 3Chapter 3

Solar and Terrestrial RadiationSolar and Terrestrial Radiation

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 Recurring patterns of seasons and seasonal change have been Recurring patterns of seasons and seasonal change have been

important to humans since the beginning of their existenceimportant to humans since the beginning of their existence StonehengeStonehenge

– Earliest portions date to 2950 BCEarliest portions date to 2950 BC– Aligned to summer solstice Aligned to summer solstice

and mid-winter sunsetand mid-winter sunset– Predicts solar and lunar eclipsesPredicts solar and lunar eclipses

Other locationsOther locations– Native Americans near present-day St. LouisNative Americans near present-day St. Louis

Wooden posts arranged in circles (Woodhenge calendars)Wooden posts arranged in circles (Woodhenge calendars)– Nubian Desert of southern EgyptNubian Desert of southern Egypt– Predates Stonehenge by 2000 yearsPredates Stonehenge by 2000 years

These devices predict important eventsThese devices predict important events

© AMS© AMS 33

Driving QuestionDriving Question How Does Energy Flow Into and Out of the Earth-How Does Energy Flow Into and Out of the Earth-

Atmosphere System?Atmosphere System?– Energy = the ability to do workEnergy = the ability to do work– 11stst law of thermodynamics – energy cannot be created or law of thermodynamics – energy cannot be created or

destroyed, although it can be converted from one form to destroyed, although it can be converted from one form to anotheranother Example heat energy Example heat energy → kinetic energy of winds→ kinetic energy of winds

– This chapter examines:This chapter examines: Electromagnetic radiation and laws that govern itElectromagnetic radiation and laws that govern it How this reacts with the Earth-atmosphere systemHow this reacts with the Earth-atmosphere system Conversion of solar radiation to heatConversion of solar radiation to heat Earth emission of infrared radiationEarth emission of infrared radiation The greenhouse effectThe greenhouse effect

© AMS© AMS 44

The Electromagnetic SpectrumThe Electromagnetic Spectrum TermsTerms

– Electromagnetic radiation – energy transmitted through space or Electromagnetic radiation – energy transmitted through space or materials as waves (e.g., solar radiation). It has both electric and materials as waves (e.g., solar radiation). It has both electric and magnetic propertiesmagnetic properties

– Electromagnetic spectrum – composed of radio waves, Electromagnetic spectrum – composed of radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays, and Gamma raysrays, and Gamma rays

– Wavelength – distance between successive wave crests or troughsWavelength – distance between successive wave crests or troughs– Wave frequency - number of wave crests that pass a given point Wave frequency - number of wave crests that pass a given point

per second (hertz, Hz)per second (hertz, Hz) Inversely proportional to wavelengthInversely proportional to wavelength

– Speed of electromagnetic radiation:Speed of electromagnetic radiation: 300,000 km/sec (186,000 mi/sec)300,000 km/sec (186,000 mi/sec) Commonly called the “speed of light”Commonly called the “speed of light”

– Ultraviolet – beyond violet. Short-wave radiationUltraviolet – beyond violet. Short-wave radiation– Infrared – below red. Long-wave radiationInfrared – below red. Long-wave radiation

© AMS© AMS 55

The Electromagnetic SpectrumThe Electromagnetic Spectrum

© AMS© AMS 66

WavelengthWavelengthis Inverselyis InverselyProportional Proportional

totoWaveWave

FrequencyFrequency

© AMS© AMS 77

The Electromagnetic SpectrumThe Electromagnetic Spectrum Terms, continuedTerms, continued

– Visible radiation – that portion of the spectrum Visible radiation – that portion of the spectrum perceptible to the human eyeperceptible to the human eye Violet end Violet end –– 0.40 0.40 μμmm Red end – 0.70 Red end – 0.70 μμmm

– μμm = micrometer = one millionth of a meterm = micrometer = one millionth of a meter

– Microwave radiation – wavelength = 0.1 to 1000 mmMicrowave radiation – wavelength = 0.1 to 1000 mm Microwave ovensMicrowave ovens Some used for radio communication (weather radio)Some used for radio communication (weather radio)

– Radio wavesRadio waves Wavelengths range from a fraction of a centimeter to hundreds Wavelengths range from a fraction of a centimeter to hundreds

of kilometersof kilometers– Frequency up to a billion HzFrequency up to a billion Hz– FM – 88 million to 108 million HzFM – 88 million to 108 million Hz

© AMS© AMS 88

Radiation LawsRadiation Laws Blackbody – at a constant temperature, it absorbs Blackbody – at a constant temperature, it absorbs

all radiation it receives and emits all the energy it all radiation it receives and emits all the energy it absorbsabsorbs– It is a perfect absorber and a perfect emitterIt is a perfect absorber and a perfect emitter

Surfaces of real objects may approximate Surfaces of real objects may approximate blackbodies for certain wavelengths of radiationblackbodies for certain wavelengths of radiation

To make all of the mathematical laws simple:To make all of the mathematical laws simple:– The wavelength of most intense radiation emitted by a The wavelength of most intense radiation emitted by a

blackbody is inversely proportional to its absolute blackbody is inversely proportional to its absolute temperature (Wien’s displacement law)temperature (Wien’s displacement law)

– Both Sun and Earth are nearly blackbodies. The Sun is Both Sun and Earth are nearly blackbodies. The Sun is much hotter than the Earth, therefore, its most intense much hotter than the Earth, therefore, its most intense radiation is at a much shorter wavelength than the radiation is at a much shorter wavelength than the Earth’s.Earth’s.

© AMS© AMS 99

Wien’s Displacement LawWien’s Displacement Law

λλmaxmax = C/T, where = C/T, where λλmaxmax is the wavelength of most intense radiation, is the wavelength of most intense radiation,

C is a constant of proportionality, and T is absolute temperatureC is a constant of proportionality, and T is absolute temperature

© AMS© AMS 1010

Radiation LawsRadiation Laws The total energy flux (E) emitted by a blackbody The total energy flux (E) emitted by a blackbody

across all wavelengths is proportional to the 4across all wavelengths is proportional to the 4 thth power of its absolute temperature (T), E ~ Tpower of its absolute temperature (T), E ~ T44

Flux of solar radiation at the Flux of solar radiation at the top of the atmospheretop of the atmosphere

EarthEarth

© AMS© AMS 1111

Inverse Square LawInverse Square Law

Doubling the Doubling the distance from distance from the Sun the Sun reduces solar reduces solar radiation by radiation by 1/41/4

© AMS© AMS 1212

Input of Solar RadiationInput of Solar Radiation Sun – composed of hydrogen and heliumSun – composed of hydrogen and helium

– Source of solar energy is nuclear fusion reaction Source of solar energy is nuclear fusion reaction 4 hydrogen protons fuse to form one helium nucleus4 hydrogen protons fuse to form one helium nucleus Excess mass in this fusion is converted to energy, E = mcExcess mass in this fusion is converted to energy, E = mc22

Some of this energy is used to bond the helium nucleusSome of this energy is used to bond the helium nucleus The rest is radiated off to the Sun’s surface and into spaceThe rest is radiated off to the Sun’s surface and into space

– The photosphere, or visible surface of the Sun, is cooler The photosphere, or visible surface of the Sun, is cooler than the interior and is convectivethan the interior and is convective These convective cells are called granulesThese convective cells are called granules

– Sunspots = cool areas on the Sun’s surfaceSunspots = cool areas on the Sun’s surface Accompanying bright areas are called Accompanying bright areas are called faculaefaculae changes in numbers of sunspots/faculae may affect Earth’s changes in numbers of sunspots/faculae may affect Earth’s

climateclimate

– Chromosphere – Sun’s atmosphere of superheated gases, Chromosphere – Sun’s atmosphere of superheated gases, mostly hydrogen and heliummostly hydrogen and helium Corona – the outermost portion of the Sun’s atmosphereCorona – the outermost portion of the Sun’s atmosphere

© AMS© AMS 1313

Solar radiation more Solar radiation more directly overhead directly overhead concentrates solar energy concentrates solar energy in a small areain a small area

Solar radiation that comes Solar radiation that comes in at an angle spreads the in at an angle spreads the solar energy over a larger solar energy over a larger areaarea

Concentrated energy Concentrated energy provides for more heat per provides for more heat per unit surface area = hotter unit surface area = hotter ground temperaturesground temperatures

Solar AltitudeSolar Altitude

© AMS© AMS 1414

Solar Altitude and LatitudeSolar Altitude and Latitude

The noon solar altitude The noon solar altitude always varies with latitude always varies with latitude because the Earth because the Earth presents a curved surface presents a curved surface to the incoming solar beamto the incoming solar beam

In equinox example, the In equinox example, the solar altitude is 90 degrees solar altitude is 90 degrees at the equator and at the equator and decreases with latitude decreases with latitude (towards the poles)(towards the poles)

Noon solar radiation Noon solar radiation striking horizontal surfaces striking horizontal surfaces per unit area is most per unit area is most intense at equatorintense at equator

© AMS© AMS 1515

Additionally, the Additionally, the incoming solar incoming solar radiation has more radiation has more atmosphere to pass atmosphere to pass through at low angles through at low angles of incidence.of incidence.

The atmosphere is not The atmosphere is not completely transparent completely transparent to solar radiationto solar radiation

Low angles of Low angles of incidence allow for incidence allow for more atmospheric more atmospheric scattering, reflection, scattering, reflection, and absorption of solar and absorption of solar radiationradiation

Average daily solar radiation by month

© AMS© AMS 1616

Solar AltitudeSolar Altitude Intensity of solar radiation striking local Earth Intensity of solar radiation striking local Earth

surfaces varies over the yearsurfaces varies over the year Inclination of Earth’s axis causes the Northern Inclination of Earth’s axis causes the Northern

Hemisphere to be tilted toward the Sun for part of Hemisphere to be tilted toward the Sun for part of the year, and away from the Sun for part of the the year, and away from the Sun for part of the yearyear– When the North Pole is tilted toward the Sun, the When the North Pole is tilted toward the Sun, the

Northern Hemisphere receives more solar radiationNorthern Hemisphere receives more solar radiation This is spring or summer in the Northern HemisphereThis is spring or summer in the Northern Hemisphere

– When the North Pole is tilted away from the Sun, the When the North Pole is tilted away from the Sun, the Northern Hemisphere receives less solar radiationNorthern Hemisphere receives less solar radiation This is fall or winter in the Northern HemisphereThis is fall or winter in the Northern Hemisphere

© AMS© AMS 1717

Solar Altitude & Procession of SeasonsSolar Altitude & Procession of Seasons At the June 21 Solstice the Sun is directly overhead (90At the June 21 Solstice the Sun is directly overhead (90° °

altitudealtitude) at the Tropic of Cancer) at the Tropic of Cancer– 23.523.5°° N latitude N latitude– Beginning of Northern Hemisphere SummerBeginning of Northern Hemisphere Summer

At the September 23 Equinox, the Sun is directly overhead At the September 23 Equinox, the Sun is directly overhead (90(90° ° altitudealtitude) at the equator) at the equator– 00°° latitude latitude– Beginning of Northern Hemisphere FallBeginning of Northern Hemisphere Fall

At the December 21 Solstice the Sun is directly overhead At the December 21 Solstice the Sun is directly overhead (90(90° ° altitudealtitude)) at the Tropic of Capricornat the Tropic of Capricorn– 23.523.5°° S latitude S latitude– Beginning of Northern Hemisphere WinterBeginning of Northern Hemisphere Winter

At the March 21 Equinox, the Sun is directly overhead (90At the March 21 Equinox, the Sun is directly overhead (90° °

altitudealtitude) at the equator) at the equator– 00°° latitude latitude– Beginning of Northern Hemisphere SpringBeginning of Northern Hemisphere Spring

© AMS© AMS 1818

Perihelion and AphelionPerihelion and Aphelion

© AMS© AMS 1919

Procession of the Earth Around the SunProcession of the Earth Around the Sunand the Seasonsand the Seasons

Northern Hemisphere tiltedaway fromthe Sun

Northern Hemisphere tilted towardthe Sun

© AMS© AMS 2020

Circle of Circle of IlluminationIllumination

N. Hemisphere summer solstice

Equinox

N. Hemisphere winter solstice

© AMS© AMS 2121

Path of the SunPath of the Sun

at the equatorat the equator

at N. Hemisphere midlatitudesat N. Hemisphere midlatitudes

at the North Poleat the North Pole

© AMS© AMS 2222

Variation in the length of daylight increases Variation in the length of daylight increases

with increasing latitudewith increasing latitude

© AMS© AMS 2323

The Solar ConstantThe Solar Constant

The solar constant is the The solar constant is the rate at which solar rate at which solar radiation falls on a surface radiation falls on a surface located at the outer edge located at the outer edge of the atmosphere and of the atmosphere and oriented perpendicular to oriented perpendicular to the incoming solar beam the incoming solar beam when Earth is at a mean when Earth is at a mean distance from the Sundistance from the Sun– Averages about 1.97 calories Averages about 1.97 calories

per square cm per min, or 1369 per square cm per min, or 1369 watts per square meterwatts per square meter

© AMS© AMS 2424

Solar Radiation and the AtmosphereSolar Radiation and the Atmosphere

Some solar radiation passing through the Some solar radiation passing through the Earth’s atmosphere interacts with gases and Earth’s atmosphere interacts with gases and aerosols via scattering, reflection, and aerosols via scattering, reflection, and absorptionabsorption

Law of energy conservation Law of energy conservation → Within the → Within the atmosphere, % solar radiation absorbed atmosphere, % solar radiation absorbed (absorptivity) + % scattered or reflected (absorptivity) + % scattered or reflected (albedo) + % transmitted to Earth’s surface (albedo) + % transmitted to Earth’s surface (transmissivity) = 100%(transmissivity) = 100%

© AMS© AMS 2525

Solar Radiation and the AtmosphereSolar Radiation and the Atmosphere ScatteringScattering

– Particles can disperse solar radiation Particles can disperse solar radiation – Scattering is wavelength dependentScattering is wavelength dependent– Preferential scattering of blue-violet light by Preferential scattering of blue-violet light by

oxygen and nitrogen moleculesoxygen and nitrogen molecules That is why the daytime sky is blueThat is why the daytime sky is blue

ReflectionReflection– A special case of scattering that takes place at A special case of scattering that takes place at

the interface between two media when the the interface between two media when the radiation striking that interface is redirected radiation striking that interface is redirected (backscattered)(backscattered)

– The fraction of incident radiation that is The fraction of incident radiation that is backscattered by airborne particles or backscattered by airborne particles or reflected by a surface is the albedo of that reflected by a surface is the albedo of that surfacesurface Albedo = (reflected radiation)/(incident Albedo = (reflected radiation)/(incident

radiation)radiation)

© AMS© AMS 2626

Solar Radiation and the AtmosphereSolar Radiation and the Atmosphere AbsorptionAbsorption

– Converts radiation to Converts radiation to heat energyheat energy

– UV absorbed in UV absorbed in stratosphere – chemical stratosphere – chemical reactions involved in reactions involved in formation and formation and dissociation of ozonedissociation of ozone Significantly reduces the Significantly reduces the

intensity of UV that intensity of UV that reaches Earth’s surfacereaches Earth’s surface

Causes marked warming Causes marked warming of upper stratosphereof upper stratosphere

© AMS© AMS 2727

The The Stratospheric Stratospheric Ozone ShieldOzone Shield

© AMS© AMS 2828

Why is the Southern Hemisphere Why is the Southern Hemisphere Spring Ozone Hole Over Antarctica?Spring Ozone Hole Over Antarctica? Circumpolar vortex cuts off Antarctic atmosphereCircumpolar vortex cuts off Antarctic atmosphere Loses ozone through absorption of UV radiationLoses ozone through absorption of UV radiation Circumpolar vortex weakens in springCircumpolar vortex weakens in spring

– Warmer, ozone rich air invadesWarmer, ozone rich air invades– Replenishes ozoneReplenishes ozone

Cold Antarctic stratosphere with stratospheric ice Cold Antarctic stratosphere with stratospheric ice accelerates the reaction with CFCs as a catalystaccelerates the reaction with CFCs as a catalyst– No comparable ozone hole in Arctic due to warmer No comparable ozone hole in Arctic due to warmer

temperatures and weaker circumpolar vortextemperatures and weaker circumpolar vortex The Montreal Protocol was an international The Montreal Protocol was an international

agreement to limit CFC productionagreement to limit CFC production– Violators receive economic sanctions from other signing Violators receive economic sanctions from other signing

countriescountries

© AMS© AMS 2929

The Antarctic Ozone HoleThe Antarctic Ozone Hole

© AMS© AMS 3030

Chemicals that threaten ozone layer have natural and industrial Chemicals that threaten ozone layer have natural and industrial sourcessources

They enter the stratosphere through deep tropical convective currentsThey enter the stratosphere through deep tropical convective currents

Chemicals in the Ozone LayerChemicals in the Ozone Layer

© AMS© AMS 3131

Solar Radiation and the Earth’s Solar Radiation and the Earth’s SurfaceSurface

The lighter the surface, the higher The lighter the surface, the higher the albedothe albedo

Albedo can vary with solar Albedo can vary with solar altitudealtitude– Water has highest albedo at lowest Water has highest albedo at lowest

solar altitudesolar altitude Near 100% at sunrise and sunsetNear 100% at sunrise and sunset

– This decreases rapidly as solar This decreases rapidly as solar altitude increasesaltitude increases

– Global average oceanic albedo = Global average oceanic albedo = 8%8% 92% of solar energy reaching oceans is 92% of solar energy reaching oceans is

absorbedabsorbed

© AMS© AMS 3232

Albedo of water surface asAlbedo of water surface asa function of solar altitudea function of solar altitude

Solar Radiation and the Earth’s Solar Radiation and the Earth’s SurfaceSurface

© AMS© AMS 3333

Solar Radiation and the Earth’s Solar Radiation and the Earth’s SurfaceSurface

Water absorbs red light more efficiently; more Water absorbs red light more efficiently; more green and blue light is scattered to our eyes, green and blue light is scattered to our eyes, explaining the color of the open oceanexplaining the color of the open ocean

© AMS© AMS 3434

Global Solar Radiation BudgetGlobal Solar Radiation Budget

Earth’s surface is the principal recipient of solar heating and Earth’s surface is the principal recipient of solar heating and is the main source of heat for the atmosphere, which is is the main source of heat for the atmosphere, which is evident in the vertical profile of the troposphereevident in the vertical profile of the troposphere

Global radiative equilibrium Global radiative equilibrium → solar radiational heating of → solar radiational heating of the Earth-atmosphere system is balanced by emission of the Earth-atmosphere system is balanced by emission of heat to space in the form of infrared radiationheat to space in the form of infrared radiation

© AMS© AMS 3535

The Greenhouse EffectThe Greenhouse Effect Greenhouse effect – heating of Earth’s surface and lower Greenhouse effect – heating of Earth’s surface and lower

atmosphere by strong absorption and emission of infrared atmosphere by strong absorption and emission of infrared radiation by certain atmospheric gasesradiation by certain atmospheric gases– These gases are called greenhouse gasesThese gases are called greenhouse gases

Recall that Earth emits infrared, or long-wave radiation as Recall that Earth emits infrared, or long-wave radiation as its most intense radiation, and the Sun emits ultraviolet and its most intense radiation, and the Sun emits ultraviolet and visible as its most intense radiation visible as its most intense radiation

Greenhouse gases are transparent to short-wave radiation, Greenhouse gases are transparent to short-wave radiation, but absorb long-wave radiationbut absorb long-wave radiation

This has the same net effect as a greenhouse, which lets This has the same net effect as a greenhouse, which lets in shortwave radiation through the glass, but the glass in shortwave radiation through the glass, but the glass strongly absorbs and emits infrared radiation. This helps strongly absorbs and emits infrared radiation. This helps warm the greenhouse.warm the greenhouse.– The earth is kept warm by greenhouse gasesThe earth is kept warm by greenhouse gases– Without greenhouse gases, life as we know it would not existWithout greenhouse gases, life as we know it would not exist

© AMS© AMS 3636

The Greenhouse EffectThe Greenhouse Effect

© AMS© AMS 3737

The Callendar EffectThe Callendar Effect

Theory that global climate change can Theory that global climate change can be brought about by enhancement of be brought about by enhancement of Earth’s natural greenhouse effect by Earth’s natural greenhouse effect by increased levels of atmospheric COincreased levels of atmospheric CO22

from anthropogenic sourcesfrom anthropogenic sources Systematic monitoring of carbon dioxide Systematic monitoring of carbon dioxide

began in 1957began in 1957– Keeling curve (Mauna Loa record)Keeling curve (Mauna Loa record)

© AMS© AMS 3838

Greenhouse Gases and Global Greenhouse Gases and Global Climate ChangeClimate Change

A. Mauna Loa record to date

B. Monthly mean values

© AMS© AMS 3939

Average concentration of methane, nitrous oxide, and CFC-11 and CFC-12 beginning in 1978

© AMS© AMS 4040

Possible Impacts of Global WarmingPossible Impacts of Global Warming

Climate zones may shift poleward by as much as Climate zones may shift poleward by as much as 550 km (350 mi)550 km (350 mi)– Heat and moisture stress would cut crop production in Heat and moisture stress would cut crop production in

certain areascertain areas– On the plus side, we could farm at higher latitudesOn the plus side, we could farm at higher latitudes

Rising sea levels of 9-88 cm (4-35 in.) from 1990 Rising sea levels of 9-88 cm (4-35 in.) from 1990 to 2100)to 2100)– Inundation of low islands and coastal plainsInundation of low islands and coastal plains

Many are heavily populatedMany are heavily populated

Decreased snow cover and sea-ice extentDecreased snow cover and sea-ice extent

© AMS© AMS 4141

Average Annual Average Annual Temperature Departures Temperature Departures

from the Long-Term Averagefrom the Long-Term Average

© AMS© AMS 4242

What Should We Do?What Should We Do? In spite of scientific uncertainties, many agree that In spite of scientific uncertainties, many agree that

action should be taken to head off possible action should be taken to head off possible enhanced greenhouse warmingenhanced greenhouse warming

Many agree that we should:Many agree that we should:– Sharply reduce oil and coal consumptionSharply reduce oil and coal consumption– Have greater reliance on non-fossil fuel energy sourcesHave greater reliance on non-fossil fuel energy sources– Have higher energy efficiencies (e.g, more vehicle miles Have higher energy efficiencies (e.g, more vehicle miles

per gallon)per gallon)– Massive reforestation, and a halt to deforestationMassive reforestation, and a halt to deforestation

Even if it were not for enhanced greenhouse Even if it were not for enhanced greenhouse warming, doing this would help other problem warming, doing this would help other problem areasareas– Example – cutting air pollutionExample – cutting air pollution

© AMS© AMS 4343

Monitoring RadiationMonitoring Radiation A pyranometer A pyranometer

measures the intensity measures the intensity of solar radiation that of solar radiation that strikes a surfacestrikes a surface