Chapter 3

Solar and Terrestrial Radiation

Driving Question

How does energy flow into and out of the Earth-Atmosphere system?

Law of Energy Conservation – Energy cannot be created nor destroyed (First

Law of Thermodynamics)

Electromagnetic Spectrum

Earth is continuously bombarded by electromagnetic radiation from the sun All objects emit electromagnetic radiation (except

at absolute zero) Types:

Radio waves IR Visible UV X and Gamma Rays

Together these form the electromagnetic spectrum

Waves

Wavelength – the distance between successive waves crests or troughs

Frequency – the number of crests or troughs that pass a given point in a given amount of time (1 second) 1 cycle/1 second = 1 Hertz

Radiation Laws

Blackbody – an object at a constant temperature that absorbs all radiation incident on it and emits all radiation at every wavelength Perfect absorber and perfect emitter Earth and Sun are NOT blackbodies, but

they are close enough that blackbody laws can be applied

Radiation Laws

Wien’s Displacement Law The wavelength of most intense radiation

is inverse to the temperature of the object λmax = C/T

– λ: wavelength (μm)– C: constant = 2897μm K– T: Temperature (K)

The sun emits short wave radiation The earth emits long wave radiation

Radiation Laws

Stefan-Boltzman Law - Law relating the temperature of a blackbody to the amount of energy emittedE = σT4

E: Energy Emission (W/m2)σ: Stefan Boltzman Constant = 5.67e-8

W/m2K4

T: Temperature (K)Average T earth = 288KAverage T sun = 6000K

Earth’s Orbit

Earth’s orbit is slightly elliptical Closest to the sun in early January (91

million miles (perihelion) Farthest from the sun in early July (94

million miles (aphelion) Earth’s axis is tilted 23 degrees 27

minutes

Solar Altitude

The angle of the sun above the horizon Greatest = 90 Lowest = 0

Solar intensity is greatest when the solar altitude is at 90 degrees

Important Latitude Lines

Equator Tropic of Cancer: line where solar altitude is

90 degrees at summer solstice (June 21) Tropic of Capricorn: line where solar altitude

is 90 degrees at winter solstice (December 21)

Arctic Circle: At winter solstice, 24 hours of darkness northward

Antarctic Circle: At summer solstice, 24 hours of darkness southward

Equinox – “Equal Night”

First day of spring/fall

Solar altitude is 90 degrees at equator

Night and day are generally equal at 12 hours

Summer Solstice

First day of summer in NH

Solar altitude is 90 degrees at the Tropic of Cancer

24 hours of darkness south of Antarctic Circle

Winter Solstice

First day of winter in NH

Solar altitude is 90 degrees at Tropic of Capricorn

24 hours of darkness north of Arctic Circle

Why is it colder in the winter if the earth is closer to the sun? Tilt and Solar Altitude Less Daylight – decrease in amount of solar energy

Why is it colder in the winter if the earth is closer to the sun? Decreased Solar Intensity in Atmosphere

Why is it colder in the winter if the earth is closer to the sun? Decreased Solar Intensity at Surface

Solar Radiation Reflection

Occurs when radiation hitting a surface is reflected

Law of reflection: angle of incidence equals the angle of reflection

Solar Radiation

Scattering A particle (gas molecule, aerosol)

disperses solar radiation in all directions Scattering is wavelength dependent

Oxygen and Nitrogen tend to scatter blue/violet light – reason why the sky is blue

Water and ice crystals scatter light equally at all wavelengths – reason why clouds are white

Solar Radiation

Absorption Process where some of the radiation on an

object is converted to heat Different from reflection and scattering:

energy conversion and not energy redirection

Absorption by atmospheric gases varies greatly by wavelength

O3 < 0.3μm H2O > 0.8μm

Solar Radiation

Albedo

The fraction of incident radiation that is reflected by a surface

Albedo = (reflected radiation/incident radiation) Recall that 30% of solar radiation was “lost to

space” Earth’s Albedo is 30% or 0.30

Dark objects have low albedos and bright objects have high albedos

Moon’s albedo is about 7% - no atmosphere to reflect the radiation

Greenhouse Effect

Global radiative equilibrium keeps the planet’s temperature in check – emission of heat to space in the form of infrared radiation balances the solar radiation’s heating.

Solar radiation and terrestrial radiation emit at different wavelengths – allows “trapping” of radiation Recall that different gases absorb radiation at

different wavelengths

Greenhouse Effect

Without the greenhouse effect the earth’s surface temperature would be about 0oF – too cold Average temperature of earth’s surface is about

59oF Most IR radiation escapes through

atmospheric windows Gases that prevent IR radiation from entering

space are greenhouse gases Water vapor, carbon dioxide, ozone, nitrous oxide,

methane

Greenhouse Warming Examples The Gulf Coast and desert Southwest

Similar solar radiation and daytime highs, but different morning lows – why?

Reason: amount of water vapor More water vapor exists near gulf coast and it

traps IR radiation leaving the surface. Drier air in the southwest does not trap radiation allowing temperatures to drop

Clouds – generally composed of water droplets Cloudy nights are warmer – trap IR radiation Cloudy days are cooler – block solar radiation

Ozone

Unstable molecule of 3 oxygen atoms that has positive and negative effects Positive: blocks harmful UV rays in the

stratosphere from reaching the surface Negative: smog at the surface

Chemical reactions (UV) in the stratosphere account for the destruction and creation of ozone

Destruction by CFC’s (banned in US in 1979)

Global Warming

Increasing CO2 concentrations have been observed

Enhances the natural greenhouse effect

Methane and Nitrous Oxide concentrations also increasing

Global Warming Possible Effects

Shifting climate zones Melting of ice sheets and glaciers leading to an

increase in sea level Positives

Longer growing season Less energy used (warmer in winter months)

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