Chapter 3

Solar and Terrestrial Radiation

Electromagnetic Spectrum

• Earth bombarded by electromagnetic radiation from the sun

• Various forms of electromagnetic radiation classified by their wavelengths – which determine how they interact with the earth and its atmosphere…make up the electromagnetic spectrum

• Electromagnetic radiation travels in the form of waves, and are classified in three ways:

– Wavelength, distance from wave crest(trough) to crest(trough)

– Frequency, number of waves passing a point in a given amount of time

– Energy – higher frequency=higher energy

Electromagnetic Spectrum

Get familiar with this!

Radiation LawsQ: All objects emit all forms of electromagnetic

radiation, but how do we know how much radiation a given object will emit?

A: Blackbody Radiation Laws

• Blackbody, an object that absorbs all radiation that is incident upon it, and emits all radiant energy it absorbs

• No perfect blackbodies, but the sun and the earth can be approximated by such a description

• The wavelength of the most intensely emitted radiation by a blackbody is inversely proportional to the absolute temperature: Wein’s Displacement Law

max = C/TC=2897, and T is in Kelvin (273.15 K = 0º C)

Radiation Laws

•Hot object’s maximum emission wavelengths are shorter, cold object’s max emission wavelengths are longer.

max = C/T

Sun

Earth

10 μm(Infrared)

0.5 μm(Green light)

• Law relating the temperature of a blackbody to the amount of energy emitted: Stefan-Boltzman Law

E~T4

Radiation Laws

E=total energy flux across all wavelengths emitted by the blackbody.

T=absolute temperature of the object

Angle of incoming radiation:•Wherever the sun is at its maximum solar angle (90 degrees) is the spot where solar rays are most concentrated.

Earth’s Motion in Space and the Seasons

Angle of incoming radiation, continued…

Earth’s Motion in Space and the Seasons

Distance from source of incoming radiation:•Inverse square law…

Earth’s Motion in Space and the Seasons

• Earth rotates once a day – 24 hours – day and night• SIDE NOTE: There are 365. 2422 days in a solar year. That is

only 11 minutes and  14 seconds less than 365. 25 days, but in 125 years the  accumulated excess in the leap years amounts to more than one  day. Therefore, in centesimal years not exactly divisible by 400 -- such as  1900, 1800 and 1700 -- there is no leap year.

• Earths orbital eccentricity – departure from circular orbit – Earth-to-sun distance varies 3.3% through the year

• Earth closest to the sun on January 3 (perihelion) and farthest from the sun (aphelion) on July 4 – earth receives 6.7% more radiation at perihelion than at aphelion. – thanks to the inverse square law.

• So…the earth is closest to the sun in OUR winter…then why is it colder here during the winter?

Earth’s Motion in Space and the Seasons

•The tilt of the Earth’s spin axis – 23 degrees 27 minutes.

•Because of the tilt, the spot on earth where maximum intensity of solar radiation is (sun is directly overhead), shifts continuously.

Sun’s direct rays at Equator

Sun’s direct rays at Tropic of Cancer.

Sun’s direct rays at Tropic of Capricorn

Sun’s direct rays at Equator

Equator

KY

NorthPole

Solar Radiation and the Atmosphere

• Solar radiation interacts with gasses and aerosols as it travels through the atmosphere…– Reflection– Scattering– Absorption

Reflection• Occurs at the interface between two

different media – like the air and a cloud

• Law of reflection: angle of incident radiation equals angle of reflected radiation

Scattering• Particles disperse solar radiation in all directions• Both molecules and aerosols scatter solar radiation• Scattering by molecules is wavelength-dependant

– In fact, the preferential scattering of blue-violet light by oxygen and nitrogen molecules is the reason the sky is blue –

Q: why are clouds white?

A: Water droplets in clouds scatter without a wavelength dependence…so clouds appear white.

Q: What color is the moon’s daytime sky? Hint – there’s no atmosphere on the moon.A: black

Absorption• Reflection and scattering only alter the direction in

which solar radiation is traveling• Absorption is actually an energy conversion process –

radiation striking the surface of a particle is converted to heat energy

• Absorption by atmospheric gasses varies greatly by wavelength – will strongly absorb some wavelengths and others little or not at all

• Absorption eliminates radiation in an associated wavelength band

• Get into this more later…

• Solar radiation is reflected without being converted to heat

• The fraction of reflected radiation in known as the albedo [albedo = (reflected radiation)/incident radiation)]

• Clouds are the most important reflectors of solar radiation

Reflection

Solar Radiation and the Atmosphere

Solar radiation and the earth’s surface

• The fraction of solar radiation that does make it to the earth’s surface is either reflected or absorbed (increasing the surface’s temperature)– Common Albedos

• Urban area: 14-18 – daytime highs warmer during sunny days

• Cirrus clouds: 40-50 – keeps nighttime temperatures warmer – daytime cooler

• Fresh snow: 75-95 – daytime temps cooler

Infrared Response and the 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.

• Remember that “Absorption by atmospheric gasses varies greatly by wavelength – will strongly absorb some wavelengths and others little or not at all”

• Greenhouse gasses are very transparent to solar radiation, but absorb infrared radiation (emitted by the earth) very well. – These gasses in turn heat, and emit infrared radiation back toward the surface = Greenhouse effect

• Principle greenhouse gasses:– Water vapor – evaporated water –

example=DESERT DAILY TEMPERATURE VARIATION

– Carbon dioxide – respiration, burning fossil fuels

– Methane – decomposition– Nitrous oxide– Ozone…

Infrared Response and the Greenhouse Effect

The Stratospheric Ozone Shield

• Ozone (O3) – 3 oxygen atoms – relatively unstable

• Near the surface – ozone is a air pollutant, and a major contibutor photochemical smog (Chapter 2)

• Ozone in the stratosphere shields us from lethal intensities of Ultraviolet (UV) radiation.

• Ozone is both created and destroyed by UV light

• Creation – UV strikes O2 atoms, causing them to split, and then the two free O attoms collide with existing )two molecules to form O3 (ozone)

• Destruction – ozone absorbs UV radiation, slitting the O3 into an O2 molecule and O atom. The free O atom then collides with an O3 atom, forming two separate O2 atoms.

The Stratospheric Ozone Shield

• CFCs – chlorofluorocarbons

• UV radiation breaks CFCs up, yielding Chlorine (Cl) gas…which reacts with and destroys ozone

The Stratospheric Ozone Shield