Lectures 2-3 week 1-2 2009:HAS222d

Solar radiation, the greenhouse, global heat engine

http://en.wikipedia.org/

The 4 streams of this course (see syllabus) 1.Energy

forms of energyconcetrated, diluted

conservation

transmission/movement transformation

efficiency of transformationheat engines

degradation (and entropy)storage‘utilization’ by plants and animalscarbon cycle, photosynthesis

3.Humans and energy

history of energy demandand development….fossil fuels

connections with evolution

alternative energies

2.Global Environment

physical, chemical, biologicalatmosphere, ocean, land surfaceenergy, air, water, ice, carbon

the sun-atmosphere-ocean heat engine

fluid circulations in which protective‘niches’ of life develop

4.Arctic populations

natives: settlementEuropeans: explorationassimilation, exploitation

shaping of their livesby energy and foodresources in a harshenvironment

amplified global warming in the Arctic

Let’s start with the sun

diameter: 1.38 million km distance from Earth (mean): 149.6 million km (93 million miles)*

tilt of Earth’s rotation axis relative to its orbit round the sun:23.50

the orbit is an ellipse, but only about 2% different from a circle: the oribital eccentriciy**= 0.017

rotation period: 23.9 hourslength of day: 24 hoursOn July 4 this year the Earth is farthest from the sun (aphelion); on Jan 4 it was closest (perihelion); about 7% more sunlight (rate of energy falling

on Earth) in Jan than in July. As Northern Hemisphere goes, so goesclimate!

The eccentricity shifts with 100,000 year period from 0.05 to nearly zero.perihelion shifts with 21,000 year periodobliquity (tilt of axis) shifts with 41,000 year period …..all these slight changes alter the amount of sunshineand its distribution at the Earth’s surface, somehow leading to ice ages….cycles of cold and warm climate.

Averaged over the globe, sunlight falling on Earth in July (aphelion) is indeed about 7% less intense than it is in January (perihelion)." That's the good news. The bad news is it's still hot. "In fact," says Spencer, "the average temperature of Earth at aphelion is about 4o F (2.3o C) higher than it is at perihelion." Earth is actually warmer when we're farther from the Sun!

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www.cwru.edu,http://science.nasa.gov/headlines/y2001/ast03jul_1.htm

*(these two numbers together tell us how big the disc of the *(these two numbers together tell us how big the disc of the sun appears in the skysun appears in the sky…….the relationship is.the relationship is

tan tan ½½ΘΘ = = ½½ diameter/distance (see diagram above)diameter/distance (see diagram above)For small angles tanFor small angles tanΘΘ us approximately us approximately ΘΘ, measured in radians. , measured in radians. So, So, ΘΘ = 1.38/149.6 = 0.00922 radians or .00922 x 360/2= 1.38/149.6 = 0.00922 radians or .00922 x 360/2ЛЛdegrees. This is 0.53 degreesdegrees. This is 0.53 degrees…….roughly .roughly ½½ degree,almostdegree,almost the samethe sameangular size as the moon, which is why we have such perfect ecangular size as the moon, which is why we have such perfect eclipses)lipses)

========================================================================================**the eccentricity of an ellipse is defined as the ratio **the eccentricity of an ellipse is defined as the ratio √√(1(1--bb22/a/a22)) where a is the where a is the largest diameter (the major axis) and b is perpendicular to it, largest diameter (the major axis) and b is perpendicular to it, the smallestthe smallestdiameterdiameter

angle Θ

disc of thesun

Earth

The sun’s radiation reaches the Earth with an intensity of about 1368 watts per square meter…that would be like 13.68 one-hundred watt light bulbs illuminating a one-meter square surface, except that light bulbs put their 100 watts of power into other forms of heat as well as visible and infrared radiation.

Given the distance of the sun from the Earth (149.6 million km) and the diameter of the sun (1.38 million km), how much power (energy per unit time) is radiating from one square meter of the bright outer surface of the sun?

http://commons.wikimedia.org/wiki/File:Solar-cycle-data.png

Visible light has wavelengths between 400 and 700nanometers (0.4/1000 to 0.7/1000 of a millimeter) (often we will also use the length unit of “micron”… a millionthof a meter a “nanometer” is a billionth of a meter. In yet otherunits, this is 4000 to 7000 Angstroms: 1 Angstrom is about thediameter of a hydrogen atom.)

• The wavelength (l) is the distance between one peak of the wave and the next peak. It's a distance so can be measured in metres, centimetres etc. It is sometimes given the Greek letter (lambda). It's also the distance between one part of the wave and the next part which is at exactly the same stage of vibration - but 'peak-to-peak' is easier to remember.

• The frequency (f) is the number of complete waves passing a point each second. It's a 'number per second' so it's measured in /s or s-1; usually called hertz (Hz) after a German physicist.

•1 kilohertz = 1 kHz = 1000 Hz1 megahertz = 1 MHz = 1,000,000 HzFor example:100 complete sound waves enter your ear in a second (you'd hear a deep hum).f = 100 per second= 100 /s = 100 s-1= 100Hz

• The speed of a wave (v) is just what it says. It's the speed at which the vibrations in the wave move from one point to the next. Wave speed is measured in metres per second (m/s, ms-1).

•For example:speed of sound in air = 330 m/s (approximate)speed of light in space = 300,000,000 m/sec = 3 x 108 meters per second

http://www.bbc.co.uk/schools/gcsebitesize/physics/waves/waveequationsrev2.shtml

www.andor.com/image_lib

The incoming solar radiation (in watts per square meter, per micron of wavelength) outside the atmosphere (upper solid curve) and at the ground

(lower solid curve) under typical atmospheric conditions. The horizontal axis is wavelength of the light in microns (millionths of a meter).

(this comes fromPlanck’s lawfor radiation of a ‘black body’as a functionof wavelengthand temperature)

Wien’s law: peak radiation occurs at a wavelenthof 2897/T (in microns...10-6 m, and T in degrees Kelvin)

The total of this radiation, summed over all frequencies is equal to

σT4

where σ is called the Stephan Boltzmannconstant, and the peak of the curve B occurs at a wavelength which varies inversely with the temperture as you see in the figure at right. Here σ = 5.67 x 10-8watts per meter squared, per degree Kelvinwritten as w m-2K-1

To make some plots from Planck’s law (previous page) you can go to

http://csep10.phys.utk.edu/guidry/java/planck/planck.htmland select a temperature by sliding the slide-bar.

The temperature of a hot object determines the color (wavelength) of its radiation. Here the colors of various stars correspond to the peak in

these curves of radiation intensity (note that the twinkling of stars in the night sky is due to irregularities in the air temperature…refraction).

solar radiation ….

arriving at the top of theatmosphere

and at the Earth’s surface

W. Connelly HADCM3 data Wikipedia

Notice that sunlight observed at the ground has some dark lines in its spectrum… some specific colors (wavelengths)

are blocked by water vapor, ozone…

short waves long waves

Isaac Newton’s experiment with a glass prism showed sunlight splitting intoa rainbow of colors. This is because the speed of light is slower inside theglass. Wavefronts of light slow down in glass, bending the rays. The amountof slowing varies with wavelength. Hence light rays with different wavelengths (i.e.,different colors) are split apart, making a rainbow as above.

400 nm 500 nm 700 nm

http://bass2000.obspm.fr/solar_spect.php

sunlight enters the atmosphere with much energy in the visible wavelenghts

Earth radiates at a lower temperature and hence at longer wavelength…invisible infrared or ‘heat’radiation

Hartmann, Global Physical Climatology, Academic Press 1994

Infrared radiation upward from the Earth, assuming a 280K temperaturered+yellow+blue = total radiation of the earth at +7° C in the range between 400 and 1800 cm-1.

10 microns (10 millionth of a meter) appears here as 1000 cm-1blue = radiation that is absorbed by greenhouse gases.

yellow = radiation that is allowed to pass by greenhouse gases.(red = absence of an absorption spectrum due to technical reasons concerning the measurements.)

http://www.espere.net/Unitedkingdom/water/uk_watervapour.html

contributions of various gases to the greenhouse effect (other than water vapor which has the greatest effect)

solar radiation (kilowatt-hours per square meter, per day)varies with latitude and season (here neglecting the great effect of

cloudiness)

www.fao.org/DOCREP/003/X6541E/X6541E03.htm

Australia, with few mountains, cannot catch the moisture from the sea and has been experiencing severe drought. This is expected to worsen

with global warming

The air temperature on 2 Jan 1993 at the surface of the Earth. The cold (blue) air forms a dome in the Arctic, which is dense(heavy) and tends to slide southwater beneath less dense air. This leads to both the overturning circulation and the westerly winds and jet stream.

The air temperature on 2 Jan 1993 at the surface of the Earth. The cold (blue) air forms a dome in the Arctic, which is dense(heavy) and tends to slide southwater beneath less dense air. This leads to both the overturning circulation and the westerly winds and jet stream.

today in Iqaluit (the capital of Nunivut, the new Canadian territory), T = -150C (= -50F) to +50F

reflected sunshine (upper) and infrared (long-wave) ‘heat’ radiation (lower)

CERES satellite radiometer Jan 2002

clouds, snow, ice,deserts are brightabsorbing areasare dark..roughly30% of incomingradiation is reflected

heat emitted from Earth is proportionalto the 4th powerof temperature

source: Wikipedia Incoming 342 watts/m2 reduced by simplereflection (albedo ~ 0.3) to 235. Human modification due togas emissions/deforestration ~ 2.5 watts/m2

incoming solar radiation, I

some is simply reflectedback to space the rest is absorbed

by ocean and landand atmosphere yetre-radiated as infrared heat, both upward anddownward (red arrows)

Earth’s surface

I

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I2I

The net effect of the ‘blanket’ of atmosphere is to have more downward radiation towardthe Earth’s surface than just the incident sunlight.